Review: Feed intake, milk production and milk composition of dairy cows fed flaxseed

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1 Review: Feed intake, milk production and milk composition of dairy cows fed flaxseed H. V. Petit Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, C.P. 90 Succ Lennoxville, Sherbrooke, Quebec, Canada J1M 1Z3. Contribution number 1032 from the Dairy and Swine Research and Development Centre. Received 19 May 2009, accepted 19 January Can. J. Anim. Sci. Downloaded from by on 11/30/17 Petit, H. V Review: Feed intake, milk production and milk composition of dairy cows fed flaxseed. Can. J. Anim. Sci. 90: Flaxseed contains approximately 55%of total fatty acids of the oil as a-linolenic acid and is rich in lignans, which are strong antioxidants. Diets rich in omega-3 fatty acids and antioxidants are known to have beneficial effects on human health such as a decrease in the incidence of cancer, cardiovascular diseases, hypertension, and arthritis. Flaxseed could then be an interesting natural feed to consider for changing milk composition. Cyanogenic glycosides (linustatin and neolinustatin) are present in flaxseed, but the concentration of hydrocyanic acid is very low in milk and ruminal fluid of cows fed flaxseed products. In general, feeding up to 15%of the total dry matter as whole flaxseed has a limited effect on dry matter intake. Heat treatments such as micronization and extrusion have no effect on dry matter intake and the effect of formaldehyde treatment on feed intake is unclear. The effects of flaxseed supplementation on milk production of dairy cows in the early stage of lactation have been neutral. Diet supplementation with whole flaxseed has had no effect on milk yield and composition of dairy cows in the mid or late stages of lactation. Physical processing of flaxseed increased milk production although heat treatment did not. Results on the effect of flaxseed processing on overall milk fat concentration have been controversial, but heat and formaldehyde treatments had no effect. Flaxseed supplementation had no effect on milk fat and protein concentrations, and processing of flaxseed had little effect. The extent of change in the concentration of fatty acids in milk is generally proportional to the level of inclusion of flaxseed in the diet. In conclusion, feeding flaxseed does not affect milk production or composition in the large majority of studies, but its long-term effects on health of cows and productivity still need to be determined. Key words: Review, flax, dairy Petit, H. V Revue: Ingestion, production de lait et composition du lait de la vache laitie` re recevant du lin. Can. J. Anim. Sci. 90: Le lin contient environ 55%d acides gras totaux sous forme d acide a-linole nique et il est aussi riche en lignanes, qui sont des antioxydants puissants. Les rations riches en acides gras ome ga 3 et en antioxydants sont reconnues pour avoir des effets be ne fiques sur la sante humaine tels qu une baisse de l incidence de cancer, de maladies cardiovasculaires, hypertension, et arthrite. Le lin est donc un aliment naturel intéressant a` considérer pour modifier la composition du lait. En ge ne ral, l addition de 15%de lin dans la matie` re se` che a des effets limite s sur l ingestion de matie` re se` che. Les traitements thermiques tels que la micronisation et l extrusion n ont pas d effet sur l ingestion de matie` re sèche et l effet du traitement à la formalde hyde sur l ingestion n est pas clair. Les effets d une supple mentation en lin sur la production de lait des vaches en de but de lactation sont neutres. La supple mentation en lin graine entière n a pas d effet sur la production et la composition du lait des vaches en milieu et fin de lactation. Les traitements physique et thermique du lin n augmentent pas la production de lait. Les re sultats sur l effet du traitement du lin sur la concentration en matie` res grasses du lait sont controverse s mais la chaleur et la formaldéhyde n ont aucun effet. L apport de lin n a pas d effet sur la concentration en prote ine et en matie` res grasses du lait et le traitement du lin n a aucun effet. L importance du changement de la concentration en acides gras du lait est ge ne ralement proportionnelle a` la concentration en lin de la ration. En conclusion, le lin n affect epas la production laitie` re ni la composition du lait dans la majorite des e tudes mais ses effets a` long terme sur la santé et la productivite de la vache restent encore a` pre ciser. Mots clés: Revue de litte rature, lin, laitier Health-conscious consumers are interested in buying dairy products that are rich in mono- and polyunsaturated fatty acids. Dietary saturated fatty acids are perceived to be less healthy than polyunsaturated fatty acids. For example, myristic (C 14:0 ) and palmitic (C 16:0 ) acids have been demonstrated to have undesirable hypercholesterolemic effects and to increase the risk of coronary heart disease (Berner 1993). On the other hand, conjugated linoleic acid and the long-chain polyunsaturated fatty acids, especially linoleic (C 18:2n6 ), 115 and a-linolenic (C 18:3n3 ) have anticarcinogenic (Parodi 1997) and potentially cardioprotective roles in humans (Massaro et al. 1999). As a result there has been a great deal of interest in manipulating the fatty acid profile of milk fat to respond to consumers concerns. Flaxseed contains a high oil level (40%of total seed weight), with a-linolenic acid constituting approximately Abbreviations: SCC, somatic cell count

2 116 CANADIAN JOURNAL OF ANIMAL SCIENCE 55%of oil s total fatty acids (Mustafa et al. 2002; Petit 2002, 2003). Research has shown several health benefits of omega-3 fatty acids (including a-linolenic acid) to humans including a decrease in the incidence of cancer, cardiovascular diseases, hypertension, and arthritis and an improvement in visual acuity (Simopoulos 1996; Wright et al. 1998). Moreover, diets rich in omega-3 fatty acids (including a-linolenic acid) reduce platelet aggregation, blood triglycerides and cholesterol levels and the occurrence of blood clots, and show both antithrombotic and anti-inflammatory effects (Nash et al. 1995; Leeson and Caston 1996; Simopoulos 1996). Reducing concentrations of C 12:0 to C 16:0, and replacing them with mono- and polyunsaturated fatty acids, particularly C 18:3n3, could be beneficial for consumer acceptance of milk fat. Feeding cows a supplement such as flaxseed could therefore be a natural feeding option to consider when looking at modification of milk fatty acid profile. FLAXSEED COMPOSITION Flaxseed (Linum usitatissimum) contains about 40%oil, 20%protein, and 30%neutral detergent fibre (Petit 2002, 2003), which makes it an interesting feed ingredient for inclusion in lactating dairy cows rations as a source of both energy and protein. Flaxseed has attracted attention as a lipid supplement for dairy cattle due to its high concentration of a-linolenic acid, an essential fatty acid that is not synthesized by mammals. Cyanogenic glycosides are the main anti-nutritional factors in flaxseed, which on enzymatic hydrolysis release hydrocyanic or prussic acid (Feng et al. 2003). The concentration of cyanogenic glycosides (linustatin and neolinustatin) varied from 365 to 550 mg 100 g 1 in 10 Canadian cultivars of flaxseed, which were grown over a 3-yr period in three different Western provinces of Canada (Oomah et al. 1992). Rumen microflora are capable of hydrolyzing cyanogenic glycosides of flaxseed to yield free hydrocyanic acid (Coop and Blakley 1949). According to Conn (1979), oral lethal doses for hydrocyanic acid in cattle are 2.0 mg kg 1 of body weight, which is equivalent to approximately 1300 mg for a 650-kg cow. Feeding 10%flaxseed in the diet of dairy cows and using an average concentration of 0.16 g of hydrocyanic acid per kilogram of seed (Brimer et al. 1983), consumption of hydrocyanic acid will average 352 mg d 1, which is far below lethal levels. Although long-term consumption of flaxseed and its effects on health have not been studied in dairy cattle, it is unlikely that diets of 10%or less of flaxseed will have negative effects on cow productivity due to anti-nutritional factors. Excretion of hydrocyanic acid into milk has not been investigated in depth but the few available studies suggest that cyanogenic compounds may be transferred to milk (Takeba et al. 1982). However, our results (Petit et al. unpublished) have shown that concentration of hydrocyanic acid was below 0.03 mg L 1 of milk and ruminal fluid when cows were fed 20%flaxseed hulls in the dietary dry matter. Considering that the lowest lethal dose reported in human is between 0.5 and 3.5 mg kg 1 of body weight (Seawright 1995), it is very unlikely that consumption of milk produced from cows fed flaxseed products may lead to any toxic effect. Indeed, daily oral doses of 2.9 and 4.7 mg of cyanide are consider as safe for human (US Environmental Protection Agency 1976). Flaxseed is one of the richest sources of the plant lignan precursor secoisolariciresinol diglucoside (Axelson et al. 1982). The content of secoisolariciresinol diglucoside in flaxseed has been reported to vary from 6.1 to 25.9 mg g 1 (Johnsson et al. 2000; Eliasson et al. 2003). Lignans are classified as phytoestrogens because of their structural similarities with natural and synthetic estrogens and antiestrogens, but they are also able to compete with endogenous estradiol for estrogen receptors. Although phytoestrogens may decrease fertility, as shown in cattle fed red clover and alfalfa (Adams 1995), flaxseed lignans do not interfere with reproduction, as diets of 9.0%rolled (Ambrose et al. 2006) and 10.8% whole flaxseed have been shown to enhance fertility of dairy cows. Lignans are a group of polyphenolic compounds present in plants as glycoside conjugates. Following ingestion, microbial enzymes convert plant lignan precursors to mammalian lignans, mainly enterodiol and enterolactone (Setchell et al. 1980). A recent study (Coˆ rtes et al. 2008) has shown that the main mammalian lignan metabolite produced from flax products by ruminal microbiota is enterolactone while fecal microbiota leads mainly to the net production of enterodiol. Moreover, enterolactone is the main mammalian lignan present in the milk of dairy cows fed flax products, while enterodiol is below the detection level (Petit and Gagnon 2009; Petit et al. 2009). Secoisolariciresinol diglucoside and its mammalian lignan metabolites have a high antioxidant activity (Kitts et al. 1999). Many studies have demonstrated that dietary phytoestrogens can scavenge free radicals (Amarowicz et al. 1993; Prasad 1997). The production of mammalian lignans from plant lignans is clearly linked to anticarcinogenic effects including antiestrogenic, anticarcinogenic, and antioxidant activities (Yuan et al. 1999). This suggests that supplementation with flaxseed products may contribute to favourable changes in milk composition for better human health by enhancing mammalian lignan enterolactone concentration. FEED INTAKE AND DIGESTIBILITY A number of studies have been conducted to determine the influence of feeding flaxseed on feed intake. In general, whole flaxseed is readily accepted by dairy cows, and feeding up to 15%of the total dietary dry matter as flaxseed had no effect on dry matter intake of dairy cows in the early (Petit 2002), mid (Kennelly and Khorasani 1992; Secchiari et al. 2003), or late

3 (Martin et al. 2008) stages of lactation. Two experiments on flaxseed supplementation have been carried out during the transition period. In the first experiment, feeding diets of whole flaxseed had no effect on prepartum dry matter intake (5.2 to 5.9%flaxseed in the dry matter) but increased postpartum intake (10.8 to 12.0%flaxseed in the dry matter) of cows by 9% compared with diets containing 4.9 to 5.2%calcium salts of palm oil or 14.0 to 15.4%micronized soybeans (Petit and Benchaar 2007). In the second experiment, feeding diets of whole flaxseed had no effect on feed intake during the transition period (from week 6 prepartum to week 4 postpartum); however, multiparous cows fed postpartum diets of 11%whole flaxseed compared with those fed a control diet with no added fat or a diet of 3.5%saturated and rumen inert fat sources had the highest and lowest liver concentrations of glycogen and triglycerides, respectively (Petit et al. 2007a). These results suggest that flaxseed supplementation may prevent the development of fatty liver in the transition dairy cow. Nevertheless, this may be true only for multiparous cows as liver concentrations of glycogen and triglycerides were similar among diets in primiparous cows. The addition to feeds of liquid formalin, a commercial 40%solution of formaldehyde, at a rate of 2% formaldehyde by weight of protein (McDonald and Scott 1977) has been used to protect fat supplements rich in polyunsaturated fatty acids against biohydrogenation by rumen microbes. Feeding ruminally inert fat sources such as calcium salts of fatty acids has been reported to improve fibre digestion (Schauff and Clark 1992), which in turn could increase feed intake. Feeding formaldehyde-treated whole flaxseed, on a dry matter basis, at 6.7%in a diet containing 58.8%ryegrass silage (Petit et al. 2002a) or at 17%in a diet containing 83% ryegrass silage (Petit et al. 2001) had no effect on dry matter intake of dairy cows in the early stage of lactation. Similarly, feeding formaldehyde-treated ground flaxseed to dairy cows in the mid stage of lactation at either 2.4 or 14.7%of the dietary dry matter had no effect on dry matter intake of cows (Goodridge et al. 2001). On the other hand, feeding a higher concentration of formaldehyde-treated whole flaxseed (11.4%of the dry matter) increased dry matter intake of dairy cows in the mid stage of lactation from 3.06 to 3.37%of body weight compared with cows fed untreated whole flaxseed (Petit 2003). Processing of oilseeds could increase the availability of oil in the rumen, thus affecting digestibility of nutrients and feed intake. However, physical breakdown of flaxseed has had no effect on feed intake as shown by similar consumption of dairy cows in the early stage of lactation fed diets containing either 10%dry rolled flaxseed or 10%whole flaxseed (Khorasani and Kennelly 1994). The results of studies concerning total tract digestibility of flaxseed-supplemented diets have been inconclusive. Apparent total tract digestibility of PETIT * DAIRY COWS FED FLAXSEED 117 diets containing from 10 to 12%whole flaxseed compared with digestibility of control diets with no flaxseed was lower (Petit 2002; Petit et al. 2005; Martin et al. 2008) or similar in other cases (Petit 2003; Petit et al. 2004). Feeding diets with 12%ground compared with diets with 12%whole flaxseed increased total tract digestibilities of dietary crude protein and ether extract, but decreased digestibility of dietary acid detergent fibre when fed to dairy cows in the early stage of lactation (da Silva et al. 2007). Similarly, feeding rolled compared with whole flaxseed at 10%of the dry matter increased total tract digestibility of ether extract with no effect on total tract digestibility of dry matter and neutral detergent fibre of the diet (Oba et al. 2009). Reduced fibre digestibility is associated with processes such as grinding (Scott et al. 1991) and is consistent with the increased release of oil from the seed into the rumen (Murphy et al. 1990), which contributes to increase digestibility of fat (da Silva et al. 2007). Gonthier et al. (2004) reported an enhanced dry matter and neutral detergent fibre digestibility with ground flaxseed added at 12.6%of the dry matter, which they explained by a higher digestibility of the flaxseed fibre constituents compared with those of the basal diet. According to Doreau et al. (2009a), the effect of flaxseed on digestion depends on the amount of flaxseed supply, resulting in greater negative effects of flaxseed supplementation on digestion in cows fed at low compared with high feeding levels due to a greater retention time of digesta in the rumen at low intake. Heat treatments are commonly used to protect oilseeds from microbial degradation in the rumen (Schingoethe et al. 1996; Mustafa et al. 2002) in order to shift the digestion postruminally. Heat treatments such as micronization and extrusion have had no effect on dry matter intake of dairy cows in the late stage of lactation fed ground flaxseed at 12%of the dry matter compared with those fed a control diet with no flaxseed (Gonthier et al. 2005). Moreover, feeding 1%micronized or 1%raw flaxseed resulted in dry matter intake similar to feeding no flaxseed (Soita et al. 2003). This agrees with the similar digestibility and ruminal fermentation observed when cows were fed a control diet with no flaxseed or diets containing 7.5%extruded or rolled flaxseed (Doreau et al. 2009a). Most experiments carried out on the effect of feeding flaxseed on feed intake of dairy cows were short term using designs of less than 2 mo (Kennelly and Khorasani 1992; Khorasani and Kennelly 1994; Goodridge et al. 2001; Petit et al. 2002a; Secchiari et al. 2003; Soita et al. 2003; Gonthier et al. 2005). There is little information on feeding flaxseed for more than 2 mo. In two cases, compared with calcium salts of palm oil or micronized soybeans, feeding whole flaxseed for the first 16 wk of lactation had no effect on postpartum dry matter intake of dairy cows (Petit 2002), but increased dry matter intake by 1.6 kg d 1 when flaxseed feeding was initiated 6 wk before calving (Petit and Benchaar 2007).

4 118 CANADIAN JOURNAL OF ANIMAL SCIENCE Moreover, feeding diets of 10%rolled flaxseed compared with 10%rolled sunflower seed from day 55 of lactation and for a minimum of 8 wk tended to increase intake of dry matter (Ambrose et al. 2006). However, feeding whole flaxseed between weeks 20 and 30 of lactation showed that cows fed no flaxseed had greater dry matter intake than those fed flaxseed (20.1 vs kg d 1 ; Petit et al. 2005). Cows fed sunflower seeds had lower dry matter intake than those fed flaxseeds (Petit et al. 2003), which may explain the lower feed intake in the first experiment (Ambrose et al. 2006), while there was no difference in dry matter intake when flaxseed was compared with a diet with no additional fat supplement (Petit et al. 2005). In general, the effects of level and type of fat supplement (both saturated and non-saturated) on dry matter intake are negligible when total dietary fat concentration is below 6%of the dry matter (Schingoethe and Casper 1991; Dhiman et al. 2000; Kennelly 1996; Petit et al. 2002a). Kennelly (1996) suggested that the addition of fat to ruminant diets in the form of oilseeds will have less detrimental effects on dry matter intake due to a slower release of oil from the seed than if a similar amount was fed as free oil, thus resulting in no decrease in dry matter intake when feeding whole flaxseed. In fact, declines in dry matter intake with fat-supplemented diets appear to be related to negative effects of fats. Feeding whole (Petit et al. 2002a), rolled or extruded (Doreau et al. 2009a) flaxseed has been shown to have no effect on ruminal concentrations of ammonia N, and total and individual volatile fatty acids, thus explaining the general lack of effect on dry matter intake. According to Benson et al. (2001), long-chain fatty acids are utilized differently in the early compared with the mid stage of lactation: in the early stage of lactation, the long-chain fatty acids may be preferentially oxidized, whereas in the mid stage of lactation, when cows are in a more positive energy balance, the supplemental longchain fatty acids may be secreted in milk or used for body adipose tissue. This may suggest that the negative effect of lipid supplementation on dry matter intake would be more important as lactation progressed as observed by Petit et al. (2005). Different responses to supplemental fats may also depend on the main forage in the basal diet. For example, Onetti and Grummer (2004) reported that feeding tallow with diets high in corn silage resulted in higher decreases in feed intake than when feeding diets high in alfalfa silage. However, there is no information available on feeding flaxseed over the whole lactation. MILK PRODUCTION The effects of flaxseed supplementation in the diet on milk production of dairy cows in the early stage of lactation seem to be neutral (Table 1). Feeding 10.4% whole flaxseed in the dry matter yielded an average of 35.7 kg of milk per day for the first 16 wk of lactation, which was similar to the milk yield of cows fed 17.7 to 18.4%micronized soybeans (34.4 kg d 1 ), but higher than that of those fed 3.8 to 4.0%calcium salts of palm oil (33.5 kg d 1 ; Petit 2002). Similarly, Soita et al. (2003) found no difference in milk yield of cows in the early stage of lactation fed a diet of 1.0%whole flaxseed and those fed no flaxseed. Moreover, supplementation after calving with whole flaxseed at 9.7%of dietary dry matter compared with a control diet with no added fat increased milk production of dairy cows in the early stage of lactation by 29%(Petit et al. 2004). In contrast, supplementation of the diet initiated 6 wk before calving (5.2 to 5.9%whole flaxseed, 2.4 to 2.7%calcium salts of palm oil or 6.2 to 9.4%micronized soybeans in the precalving diets and 10.8 to 12.0%whole flaxseed, 4.9 to 5.2%calcium salts of palm oil or 14.0 to 15.4% micronized soybeans in the postcalving diets) resulted in similar milk production, which averaged 31 kg d 1 for the first 16 wk of lactation (Petit and Benchaar 2007). Similarly, dairy cows in the early stage of lactation fed diets of 10%rolled flaxseed and those fed 10%rolled sunflower seed had similar milk yield (Ambrose et al. 2006). On the other hand, dairy cows in the early stage of lactation fed 9.6%of whole flaxseed in the dietary dry matter had a 8.1%decrease in milk yield compared with those fed a control diet with no flaxseed (Khorasani and Kennelly 1994). According to Onetti and Grummer (2004), there is an interaction between stage of lactation and amount of supplemental fat observed, with supplemental fat increasing milk production of dairy cows in the early stage of lactation, but not of cows in the mid stage lactation cows, where milk fat depression occurred. Discrepancies among experiments on the effect of whole flaxseed supplementation on milk production in early lactation might result from differences in diet composition and length of the experiment. Feeding whole flaxseed in short-term experiments with less than 5 wk significantly increased (Petit et al. 2004) or decreased (Khorasani and Kennelly 1996) milk production of cows in the early stage of lactation although there was no difference in long-term experiments (Petit 2002; Petit and Benchaar 2007). Feeding tallow with diets high in corn silage had no effect on milk production, although a moderate positive milk production response was observed when tallow was fed with alfalfa-based diets or diets with corn silage and alfalfa in similar proportions (Onetti and Grummer 2004). Moreover, Onetti and Grummer (2004) reported a significant positive milk yield response to supplementation with calcium salts of palm fatty acids when cows were fed diets high in corn silage, while there was no effect when cows were fed alfalfa-based diets or diets with alfalfa and corn silage in similar proportions. Diet supplementation with whole flaxseed has had generally little effect on milk production of cows in the mid or late stages of lactation. Dairy cows in the mid stage of lactation and fed 0, 5, 10 and 15%whole flaxseed in the diet averaged 26.9 kg of milk per day and

5 PETIT * DAIRY COWS FED FLAXSEED 119 Table 1. Effect of dietary concentration and processing of flaxseed on milk production of dairy cows Flaxseed %flaxseed (dry matter basis) Stage of lactation Change in milk yield, %of control diet (no flaxseed) Forage to concentrate ratio (DM basis) Reference Can. J. Anim. Sci. Downloaded from by on 11/30/17 Whole, untreated 1.0 Early :50 Soita et al. (2003) 9.6 Early 8.1* 40:60 Khorasani and Kennelly (1994) 9.7 Early 29.0* 52:48 Petit et al. (2004) 10.4 Early :56 Petit (2002) 11.4 Early :46 Petit and Benchaar (2007) 5.0 Mid :60 Kennelly and Khorasani (1992) 5.0 Mid :37 Petit and Gagnon (2009) 10.0 Mid :60 Kennelly and Khorasani (1992) 10.0 Mid :32 Petit and Gagnon (2009) 11.1 Mid :40 Petit et al. (2009) 11.8 Mid :45 Petit et al. (2005) 15.0 Mid :60 Kennelly and Khorasani (1992) 15.0 Mid :27 Petit and Gagnon (2009) 1.8 Late :40 Secchiari et al. (2003) 12.4 Late :35 Martin et al. (2008) Rolled, untreated 10.0 Early :60 Khorasani and Kennelly (1994) Whole, formaldehyde 17.0 Early 6.1* 83:17 Petit et al. (2001) 6.7 Mid :41 Petit et al. (2002a) Ground, untreated 5.1 Mid :20 Collomb et al. (2004) 7.5 Mid :20 Collomb et al. (2004) 12.5 Late :45 Gonthier et al. (2005) Ground, formaldehyde 2.4 Mid :49 Goodridge et al. (2001) 4.7 Mid :49 Goodridge et al. (2001) Extruded 12.7 Late :45 Gonthier et al. (2005) 14.8 Late :35 Martin et al. (2008) Micronized 1.0 Early :50 Soita et al. (2003) 12.7 Late :45 Gonthier et al. (2005) *PB0.01. the flaxseed concentration in the diet had no effect on milk production (Kennelly and Khorasani 1992). Similar results have been reported when dairy cows in the mid (Petit and Gagnon 2009; Petit et al. 2009) and late (Martin et al. 2008) stages of lactation were fed from 0 to 15%whole flaxseed in the diet in experiments of less than 5 wk duration. Secchiari et al. (2003) found no difference in milk yield of cows fed 1.8%whole flaxseed in the total dry matter and those fed full fat extruded soybeans. However, some results suggest that dietary crude protein concentration of whole flaxseed-based diets may be important. Dairy cows in the mid stage of lactation fed whole flaxseed tended (P 0.06) to have higher milk yield (24.9 vs kg d 1 ) with higher than lower (18.4 vs. 16.0%) crude protein concentration in the diet (Petit et al. 2005), which may result from the positive effects of protein on digestibility and feed intake (Cowan et al. 1981). Physical breakdown of the flaxseed prior to feeding has generally increased milk production. Cows fed 10% ground flaxseed had a 6.5%increase (1.2 kg d 1 )in milk production compared with those fed 10%whole flaxseed (da Silva et al. 2007). Similarly, feeding rolled compared with whole untreated flaxseed at 10%of the dry matter increased milk production of dairy cows in the early stage of lactation by 10%(Khorasani and Kennelly 1994). However, feeding lower concentrations of ground flaxseed (1.0 or 1.4 kg, representing 5.1 and 7.5%of the dry matter, respectively) compared with a control diet with no added fat had no effect on milk yield of dairy cows in the mid stage of lactation fed a high-forage diet (Collomb et al. 2004) and feeding a diet of 12.5%ground flaxseed or a control diet with no added fat to dairy cows in the late stage of lactation led to similar milk yield (Gonthier et al. 2005). Differences in the stage of lactation may explain the discrepancies among experiments, as cows in the early stage of lactation had a positive milk response to flaxseed, while cows in the mid or late stage of lactation did not. Formaldehyde treatment has been used to protect fat supplements against biohydrogenation (Tymchuk et al. 1998) and protein degradation by rumen microbes (Kempton et al. 1979). However, cows fed a diet of 17%of formaldehyde-treated whole flaxseed between weeks 9 and 19 of lactation had a 6.1%decrease in milk yield compared with those fed a diet with no flaxseed (Petit et al. 2001), although feeding 6.7%formaldehydetreated whole flaxseed to dairy cows in the mid stage of lactation had no effect on milk yield compared with those fed a mixture of fish oil and formaldehyde-treated whole flaxseed, calcium salts of palm oil or flaxseed oil infused in the abomasum (Petit et al. 2002a). Moreover, feeding a diet of 2.4 and 4.7%of formaldehyde-treated ground flaxseed had no effect on milk production of

6 120 CANADIAN JOURNAL OF ANIMAL SCIENCE cows in the mid stage of lactation (Goodridge et al. 2001). Formaldehyde treatment of whole flaxseed fed at 11.4%of the dry matter increased, by an average of 11.2%(2.65 kg d 1 ), milk production of dairy cows between weeks 25 and 35 of lactation due to greater dry matter intake when treating seeds with formaldehyde (Petit 2003). The greater milk production with formaldehyde-treated flaxseed could be a result of an increase in the availability of dietary amino acids for absorption by the animal. In fact, formaldehyde treatment of casein has increased dry matter intake of lambs (Kempton and Leng 1979) and digesta flow of dietary amino acids at the duodenal level (Kempton et al. 1979), which would contribute in improving animal production. Micronization is a dry heat treatment in which infrared gas generators heat feedstuffs from the inside out to approximately 110 to 115 o C. It has been used with cereal grains (Harbers 1975), full fat soybeans (Petit et al. 1999), and canola seed (Wang et al. 1999). Mustafa et al. (2002) have shown that micronization of flaxseed decreases ruminal dry matter and crude protein degradability of flaxseed by 14 and 21%, respectively, and that micronization increased intestinal digestibilities of ruminal undegraded amino acids of flaxseed. Although micronization was an effective heat treatment to improve postruminal supply of amino acids from flaxseed (Gonthier et al. 2004), Soita et al. (2003) reported that feeding micronized flaxseed resulted in similar milk yield compared with feeding nonmicronized flaxseed (38.4 vs kg d 1 ) and there was no difference in milk yield of cows fed flaxseed and those fed no flaxseed (38.3 kg d 1 ). Micronization of flaxseed fed at 12.7%of the dry matter had no effect on milk yield of dairy cows in the mid stage of lactation compared with a control diet with no flaxseed (Gonthier et al. 2005). Heat is used to decrease crude protein degradability, but the response to heat treatment is variable (Mir et al. 1984), and severe heat treatments (Moshtagi Nia and Ingalls 1992) often have to be used to decrease crude protein degradability, resulting in decreased total tract digestibility (Plaisance et al. 1997). Protection of protein is usually paralleled that of fat as the protein-rich matrix surrounds the fat droplets of oilseeds (Khorasani et al. 1992). However, micronization had little effect on in vitro biohydrogenation of fatty acids of flaxseed (Petit et al. 2002b) and was ineffective in increasing the post-ruminal supply of amino acids from flaxseed (Mustafa et al. 2003). Extrusion is a heat treatment commonly used to protect dietary protein from microbial degradation in the rumen. Extrusion of oilseeds such as canola seed increases milk production in dairy cows (Ingalls and Grumpelt 1987) and fat digestion in dairy calves (Sharma et al. 1986). Moreover, extrusion has the potential to reduce crude protein degradability in the rumen as shown in lupin seeds (Cros et al. 1992), which could improve the nutritive value of seed as a source of undegraded protein. Khorasani et al. (1992) suggested that reducing degradability of the protein-rich matrix surrounding the fat droplets of oilseeds could overcome the negative effects of supplemental fat on ruminal digestion (Devendra and Lewis 1974). Extrusion was ineffective in increasing the post-ruminal supply of amino acids from flaxseed-based diets (Mustafa et al. 2003) as a result of an increase in ruminal crude protein digestibility and a reduction in the amount of crude protein available for digestion post-ruminally for cows fed diets of extruded flaxseeds (Gonthier et al. 2004). Extrusion of flaxseed fed at 12.7%of the dry matter had no effect on milk yield of dairy cows in the mid stage of lactation (Gonthier et al. 2005) and similar results were observed when extruded flaxseed was fed at 14.8%of the dietary dry matter to dairy cows in the late stage of lactation compared with cows fed a control diet with no flaxseed (Martin et al. 2008). However, the effect of extrusion on ruminal nutrient degradability may vary according to parameters used for heating (e.g., temperature of extrusion and resident time), which may modify the response to feeding extruded flaxseed. MILK COMPOSITION AND YIELD OF MILK COMPONENTS Fat Inclusion of whole flaxseed in the diet of dairy cows has generally no effect on milk fat concentration and yield of milk fat (Table 2). Feeding 10.4%of whole flaxseed to dairy cows in the early stage of lactation (Petit 2002) and 1.8%of whole flaxseed to those in the late stage of lactation (Secchiari et al. 2003) had no effect on milk fat concentration and milk fat yield compared with cows fed a diet with no flaxseed. Milk fat concentration of dairy cows in the early stage of lactation fed diets of 9.7%whole flaxseed was similar to that of cows fed a control diet with no added fat but 4%fat-corrected milk yield and milk fat yield were higher for cows fed flaxseed due to higher milk yield (Petit et al. 2004). Inclusion of whole flaxseed in the diet of dairy cows in the mid stage of lactation at levels ranging from 5 to 15%(Kennelly and Khorasani 1992) and at 11.1%(Petit et al. 2009) of the total dry matter had no effect on fat percentage (range 3.4 to 3.6%and 3.96 to 4.06%, respectively) and milk fat yield. Similar results were observed for dairy cows in the early stage of lactation fed diets of 1.0% (Soita et al. 2003), 10.0%(Khorasani and Kennelly 1994), and 11.4%(Petit and Benchaar 2009) whole flaxseed and for cows in the late stage of lactation fed diets of 12.4%whole flaxseed (Martin et al. 2008) compared with cows fed a control diet with no flaxseed. In contrast, dairy cows in the mid stage of lactation fed diets of 5, 10 and 15%of whole flaxseed had higher milk fat concentrations than those fed a control diet with no flaxseed, although there was no difference in milk fat yield (Petit and Gagnon 2009). In general, physical processing of flaxseed has no effect on milk fat concentration. Flaxseed fed at 10%of

7 Table 2. Effect of dietary concentration and processing of flaxseed on milk fat and protein concentrations of dairy cows Flaxseed %flaxseed (dry matter basis) Stage of lactation Change in milk fat, %of control diet (no flaxseed) Change in milk fat yield, %of control diet (no flaxseed) Change in milk protein, %of control diet (no flaxseed) Change in milk protein yield % of control diet (no flaxseed) Reference Whole, untreated 1.0 Early Soita et al. (2003) 9.6 Early Khorasani and Kennelly (1994) 9.7 Early * * Petit et al. (2004) 10.4 Early * 10.5* Petit (2002) 11.4 Early Petit and Benchaar (2007) 5.0 Mid Kennelly and Khorasani (1992) 5.0 Mid 14.2* Petit and Gagnon (2009) 10.0 Mid Kennelly and Khorasani (1992) 10.0 Mid 28.7* Petit and Gagnon (2009) 11.1 Mid Petit et al. (2009) 11.8 Mid * 14.5 Petit et al. (2005) 15.0 Mid Kennelly and Khorasani (1992) 15.0 Mid 12.8* Petit and Gagnon (2009) 1.8 Late Secchiari et al. (2003) 12.4 Late 10.5 ND z 1.7 ND Martin et al. (2008) Rolled, untreated 10.0 Early Khorasani and Kennelly (1994) Whole, formaldehyde 17.0 Early * 4.7* 1.7 Petit et al. (2001) 6.7 Mid Petit et al. (2002a) Ground, untreated 5.1 Mid 4.2 ND 0.6 ND Collomb et al. (2004) 7.5 Mid 4.0 ND 1.8 ND Collomb et al. (2004) 12.7 Late Gonthier et al. (2005) Ground, formaldehyde 2.4 Mid 11.2 ND 0.6 ND Goodridge et al. (2001) 4.7 Mid 15.4 ND 0.6 ND Goodridge et al. (2001) Extruded 12.7 Late Gonthier et al. (2005) 14.8 Late 14.1* ND 2.1 ND Martin et al. (2008) Micronized 1.0 Early Soita et al. (2003) 12.7 Late Gonthier et al. (2005) z NDnot determined. *PB0.01. PETIT * DAIRY COWS FED FLAXSEED 121

8 122 CANADIAN JOURNAL OF ANIMAL SCIENCE the diet resulted in a similar milk fat concentration and milk fat yield when supplemented as ground, rolled, and whole flaxseed (Khorasani and Kennelly 1994; Kennelly 1996; Oba et al. 2009). Moreover, feeding 1.0 or 1.4 kg (5.1 and 7.5%of the DM, respectively) of ground flaxseed had no effect on the concentration of milk fat of cows fed a high forage diet compared with a control diet with no flaxseed (Collomb et al. 2004). On the other hand, milk fat concentration tended (P 0.11) to decrease from 4.1 to 3.8%with grinding of flaxseed fed at 12%of the dietary dry matter (da Silva et al. 2007), which may indicate a greater release of oil in the rumen (Mohamed et al. 1988) due to grinding. The higher proportion of forage in the experiment of da Silva et al. (2007) than in the experiment of Kennelly (1996) on the effect of flaxseed processing on milk composition may have decreased the rumen outflow rate and increased retention time of flaxseed and release of oil in the rumen, decreasing milk fat concentration (Banks et al. 1980). Formaldehyde treatment has had no effect on the milk fat concentration and milk fat yield (Table 2) of cows fed whole flaxseed and the milk fat concentration and milk fat yield of cows fed ground flaxseed was similar to those fed a control diet. Feeding either formaldehyde-treated flaxseed at 17%of the dietary dry matter or calcium salts of palm oil resulted in similar milk fat concentrations, although yield of milk fat was lower for cows fed the former diet as a result of lower milk yield (Petit et al. 2001). Ground formaldehyde-treated flaxseed fed at 2.4 or 4.7%of the dietary dry matter had no effect on milk concentrations of fat compared with a control diet with no flaxseed (Goodridge et al. 2001). Generally, oils effectively protected against ruminal biohydrogenation increase milk fat yield (Ashes et al. 1992) and feeding free flaxseed oil decreases milk fat concentration due to the lesser dry matter intake and the lesser digestibility of fibre (Martin et al. 2008). Moreover, higher concentrations of some trans fatty acids such as trans-10 18:1 and trans-10, cis-12 conjugated linoleic acid in ruminal fluid of cows fed free oils are positively related to milk fat depression (Griinari and Bauman 1999). However, ineffective protection of whole flaxseed against biohydrogenation (Petit et al. 2002a) or low level of added fat (Tymchuk et al. 1998) would result in no effect on milk fat yield. In general, reductions in milk fat concentrations seem to be due to the highly unsaturated fatty acids affecting rumen fermentation. Formaldehydetreated flaxseed has been shown to have little effect on ruminal fermentation compared with a control diet with no flaxseed (Petit et al. 2002a), thus having no effect on milk fat concentration. Feeding fat through oilseeds maintains or increases milk fat concentration (Dhiman et al. 2000). However, when oilseeds and calcium salts of fatty acids are compared, milk fat concentration is higher (Petit et al. 2001) or similar (Petit et al. 2002a) for cows fed calcium salts of fatty acids of palm oil than those fed formaldehyde-treated flaxseed. Higher milk fat concentration when feeding diets of calcium salts of palm oil may be due to their lower ability to effect production of trans fatty acids compared to the feeding of polyunsaturated fats as there is an association between milk fat depression and some trans fatty acids in milk (Griinari and Bauman 1999). Heat treatment of flaxseed has resulted in variable effects on milk fat concentration. Feeding micronized flaxseed at a rate of 1 kg d 1 (Soita et al. 2003) or diets of 10%raw, micronized or extruded flaxseed (Gonthier et al. 2005) led to similar milk fat concentrations and milk fat yields compared with a control diet with no flaxseed. In contrast, extrusion of flaxseed fed at 14.8% of the diet decreased fat concentration from 4.11 to 3.53%in milk of dairy cows in the late stage of lactation compared with a control diet (Martin et al. 2008). According to Chilliard et al. (2009), extrusion is a heat treatment increasing the rate of oil release from seeds into rumen fluid compared with whole flaxseed, which results in abundant formation of trans fatty acids in milk and lower milk fat concentration. Protein Milk protein concentration and milk protein yield (Table 2) do not seem to be affected by flaxseed supplementation, as only three or fewer studies reported an increase. Feeding diets of 7 to 12.4%whole untreated flaxseed to dairy cows in the early stage of lactation had no effect on milk protein concentration compared with feeding a control diet with no flaxseed (Khorasani and Kennelly 1994; Petit et al. 2004; Martin et al. 2008) or increased by 3.8%(Petit 2002) milk protein concentration in one study and decreased milk protein concentration by 9.1%in another study (Petit et al. 2005). Moreover, feeding diets of 9.7%whole flaxseed compared with calcium salts of palm oil resulted in a 5.2%increase in milk protein concentration (Petit et al. 2004). Milk protein yield increased significantly when cows in the early stage of lactation were fed a diet of 9.7 to 10.4%whole flaxseed as a result of higher milk yield (Petit et al. 2004) and higher milk protein concentration (Petit 2002). Feeding diets of 5 to 15%whole flaxseed to cows in the mid stage of lactation had no effect on milk protein concentration and milk protein yield (Petit and Gagnon 2009; Petit et al. 2009) and similar results were obtained when cows in the early stage of lactation were fed diets of 1% (Soita et a. 2003) and 11.4%(Petit and Benchaar 2007) whole flaxseed. However, although there was no difference in milk protein yield, whole flaxseed inclusion in the diet of dairy cows in the mid stage of lactation at levels ranging from 5 to 15%of the total dry matter resulted in a linear decrease in milk protein concentration (3.21%for control animals compared with 3.13% for cows fed 15%flaxseed) with increasing level of flaxseed inclusion (Kennelly and Khorasani 1992). Moreover, feeding diets of 11.8%whole flaxseed

9 between weeks 20 and 30 of lactation in diets containing 16 or 18%crude protein had no effect on milk protein concentration and milk protein yield compared with feeding no flaxseed (Petit et al. 2005). Differences in stage of lactation and milk production are likely factors that moderate the response of cows to added dietary fat as a result of energy not being limited for dairy cows in the mid stage of lactation and those with low milk yield. According to Schingoethe et al. (1996) the effect of fat supplementation on milk protein depends on the source of fatty acids being fed. It has been shown that feeding diets of 12.5 to 12.7%flaxseed reduces microbial crude protein flow to the duodenum and microbial efficiencies (true and apparent) in dairy cows in the late stage of lactation, thus decreasing the amount of microbial protein supply for milk protein synthesis (Gonthier et al. 2004). Differences in the basal diets may also explain the discrepancies among studies as different sources of forage were used and various ratios of forage to concentrate. Processing of flaxseed has had little effect on milk protein concentration and milk protein yield. Feeding diets of rolled flaxseed to dairy cows in the early stage of lactation (Khorasani and Kennelly 1994) and ground flaxseed to dairy cows in the mid (Collomb et al. 2004) or late (Gonthier et al. 2005) stages of lactation resulted in similar milk protein concentrations compared with cows fed a control diet with no flaxseed (Table 2). Moreover, cows fed diets of whole or rolled flaxseed added at 10%of the dry matter had similar milk protein concentrations (Oba et al. 2009). Micronization (Soita et al. 2003; Gonthier et al. 2005) and extrusion (Gonthier et al. 2005; Martin et al. 2008) treatments have had no effect on milk protein concentration when flaxseed was added at less than of 10%of the dry matter. Feeding either no flaxseed or 12.5%ground, 12.7%micronized or 12.7%extruded flaxseed resulted in similar milk protein concentrations and percentages of true protein, casein N, nonprotein N, and whey in milk (Gonthier et al. 2005) although urea N in milk was increased when feeding ground flaxseed (da Silva et al. 2007). Formaldehyde treatment of whole (Petit et al. 2001) and ground (Goodridge et al. 2001) flaxseed has increased milk protein concentration significantly only in one experiment (Table 2). Compared with a control diet with no flaxseed feeding formaldehyde-treated flaxseed fed at 17%of the dry matter increases by 4.7%protein concentration in milk of dairy cows in the early stage of lactation (Petit et al. 2001), suggesting a better N utilization on the flax diet. A depression in milk protein concentration is frequently associated with dietary fat supplementation; however, daily protein production may be unchanged as supplemental fat tends to have a positive effect on milk yield (Kennelly 1996). Increased milk protein concentration for cows fed formaldehyde-treated compared with untreated feeds is usually a result of greater bypass of protein due to the formaldehyde treatment (Crawford and Hoover 1984), PETIT * DAIRY COWS FED FLAXSEED 123 which may result from a reduction in microbial protein production (Mahadevan et al. 1983) and would increase amino acids availability at the intestine level. Therefore, the formaldehyde treatment applied to flaxseed could have prevented ruminal protein degradability and increased the amount of protein secreted in milk. Lactose The effect of feeding flaxseed on milk lactose concentration is unclear. Feeding diets of whole flaxseed at 7 to 10%of the dry matter to dairy cows in the early stage of lactation (Khorasani and Kennelly 1994), and up to 15%to dairy cows in the mid (Kennelly and Khorasani 1992), or late stage of lactation (Secchiari et al. 2003; Martin et al. 2008) had no effect on milk lactose concentration compared with cows fed a control diet with no flaxseed. In contrast, milk lactose concentration decreased when feeding diets of 11.8%whole flaxseed to dairy cows in the mid stage of lactation (Petit et al. 2005) and increased compared with feeding calcium salts of palm oil, although there was no difference between feeding whole flaxseed and micronized soybeans (Petit 2002). Milk lactose concentration was not affected by grinding (Collomb et al. 2004; da Silva et al. 2007) and rolling (Khorasani and Kennelly 1994; Oba et al. 2009) when flaxseed was fed in diets at 7 to 10%of the dry matter. Similarly, extrusion has no effect (Martin et al. 2008). Formaldehyde treatment of whole flaxseed tended (P 0.11) to increase lactose concentration in milk of dairy cows in the mid stage of lactation (Petit 2003) but feeding formaldehyde-treated whole flaxseed decreased lactose concentration in milk compared with feeding calcium salts of palm oil or infusing flaxseed oil in the abomasum of cows (Petit et al. 2002a). On the other hand, feeding diets of calcium salts of palm oil and of 17%formaldehyde-treated whole flaxseed (Petit et al. 2001) or diets of whole flaxseed and calcium salts of soybean oil (Cavalieri et al. 2005) resulted in similar milk lactose concentrations. Somatic Cell Count Milk somatic cell count (SCC) has generally not been affected by flaxseed supplementation although there are few data published on that topic. Feeding diets of whole flaxseed at around 11.8%of the dry matter resulted in similar milk somatic cell score (log 10 SCC) compared with diets of sunflower seeds in the mid stage of lactation (Petit et al. 2005) and to diets of calcium salts of soybean oil (Cavalieri et al. 2005) or calcium salts of palm oil and micronized soybeans in the early stage of lactation (Petit and Benchaar 2007). However, feeding diets of 10%whole flaxseed compared with energy booster, a saturated fat (Petit et al. 2007a) or compared with diets with no fat (Petit et al. 2007b) decreased milk SCC of dairy cows in the early stage of lactation. There is some evidence that feeding plant oil rich in omega-3 fatty acids alters the production of cytokines and

10 124 CANADIAN JOURNAL OF ANIMAL SCIENCE the functional properties of macrophages, lymphocytes, and other immunocompetent cells as shown a reduced proliferative response of activated peripheral blood mononuclear cells of dairy cows fed whole flaxseed (Lessard et al. 2003), which could help to maintain integrity of the embryo. Grinding of flaxseed had no effect on milk SCC of dairy cows in the early stage of lactation (da Silva et al. 2007). MILK FATTY ACID PROFILE According to Kennelly (1996) the extent to which oilseed feeding will alter the fatty acid composition of milk depends on a number of factors including: (1) the degree of ruminal biohydrogenation; (2) composition of the non-lipid component of the diet; (3) influence of the lipid source on microbial fatty acid synthesis and de novo synthesis of fatty acids in the mammary gland; (4) stage of lactation; and (5) intestinal and mammary gland desaturase activity. Compared with cows fed a control diet, cows fed diets of flaxseed oil treated with formaldehyde and resistant to microbial attack produced milk fat containing, as a percentage of total fatty acids, 20% a-linolenic acid (McDonald and Scott 1977) and cows infused with flaxseed oil in the abomasum produced milk fat with 14% a-linolenic acid (Petit et al. 2002). In contrast, milk from cows fed no flaxseed product contained usually less than 1% a-linolenic acid (Glasser et al. 2008). These data would then suggest that the ability of the mammary gland to secrete a-linolenic acid in milk is not a limiting factor in feeding strategies designed to alter milk composition, but that protection against biohydrogenation by rumen microbes is the critical point for the transfer of a-linolenic acid from the diet in milk. In general, milk fatty acid profile is slightly improved for better human health by feeding flaxseed, as shown by higher concentrations of fatty acids (e.g., omega 3) recognized as being beneficial to reduce the incidence of cancer, cardiovascular diseases, hypertension, and arthritis and an improvement of visual acuity (Simopoulos 1996; Wright et al. 1998). As there is an excellent review that has been published recently that provides quantitative estimates of the impact of flaxseed and other oilseed supplements on milk fatty acid profile (Glasser et al. 2008), only a summary will be presented here. Moreover, all studies but one cited in Tables 1 and 2 provide details on milk fatty acid profile. The metaanalysis of the response of cow milk fatty acid composition to oilseed lipid supplements performed by Glasser et al. (2008) has shown that the inclusion of flaxseed in the diet has caused a reduction in the concentration of short-chain (C4 to C12) fatty acids and C16:0 while the concentration of monounsaturated and long-chain fatty acids has increased compared with feeding a control diet. However, concentrations of polyunsaturated fatty acids in milk of cows fed flaxseed usually do not exceed 3 to 4%of total fatty acids (Kennelly 1996). The extent of change in the concentration of fatty acids in milk is proportional to the level of inclusion of flaxseed in the diet (Kennelly 1996; Petit and Gagnon 2009). Percentages of trans-18:1 and total conjugated linolenic acid in milk fat increase linearly with increased amounts of flaxseed in the diet (Glasser et al. 2008). Greater trans-18:1 relative percentage in milk fat also has been reported previously when diets of flaxseed oil were fed to dairy cows compared with when diets of raw cracked soybeans were fed (Dhiman et al. 2000). Feeding diets of flaxseed also increases omega 3 fatty acid concentrations in milk fat, although experiments with flaxseed supplements reported increases of no more than 1%of total fatty acids (Glasser et al. 2008). Diets with flaxseed oil given as extruded, micronized, or ground seeds decreased milk fat concentrations of C6 to C8, C10 to C14, and 16:0, and increased those of total C18, trans-18:1, conjugated linoleic acid, and linolenic acid more than did whole seeds (Glasser et al. 2008). Physical breakdown of flaxseed may contribute to increase partial ruminal biohydrogenation of linolenic acid as shown by enhanced cis9, trans11-18:2 concentration in milk fat of cows fed diets with linseed fat given as extruded, micronized, or ground seeds (Glasser et al. 2008). Small changes in the concentration of linolenic acid in milk of cows fed diets of flaxseed may indicate that extensive biohydrogenation of linolenic acid occurs in the rumen as previously reported by Doreau et al. (2009b) for cows fed diets of extruded or rolled linseed. Lee et al. (2009) have shown that a multitude of conjugated linoleic acid isomers originates from linolenic acid biohydrogenation. Concerning the effect of the main forage of the basal diet on the response to flaxseed supplements on milk fatty acid profile, only milk cis- 18:1 percentage was increased, with the greatest increase being observed with alfalfa-based diets, followed by corn silage, grass hay, and then grass silage (Glasser et al. 2008). CONCLUSIONS Although cyanogenic glycosides (linustatin and neolinustatin) are present in flaxseed, the concentration of hydrocyanic acid is very low in milk and ruminal fluid of cows fed flaxseed products in the dietary dry matter. Feeding up to 15%of the total dry matter as whole flaxseed has a limited effect on dry matter intake, and heat treatments of flaxseed, such as micronization and extrusion, have no effect. The effects of flaxseed supplementation on milk production of dairy cows have been neutral. Physical processing of flaxseed increased milk production, although heat treatment did not. Results on the effect of flaxseed processing on overall milk fat concentration have been controversial, but heat and formaldehyde treatments had no effect. Flaxseed supplementation had no effect on milk fat and protein concentrations and processing of flaxseed had little effect. Although long-term consumption of flaxseed and its effects on health have not been studied in dairy cattle, it is unlikely that diets of 10%or less of flaxseed