MILK DEVELOPMENT COUNCIL

Transcription

1 MILK DEVELOPMENT COUNCIL Energy Deficit in High Genetic Merit Cows Project No. 97/R2/05

2 The quantification of the energy deficit of high genetic merit cows in early lactation, to provide nutritional strategies to minimise the consequences of such on lactational performance and to establish an optimal contribution of grass silage in the diet. Summary report for Project no 97.R2.05 Funded by Milk Development Council Research team: Professor David Beever and Mr Andrew Hattan* Conducted by S.B. Cammell, A.K. Jones and D. J. Humphries *With part funding provided by South of England Agricultural Society. Centre for Dairy Research, (CEDAR-ADAS, Reading), The University of Reading, Earley Gate, Reading RG6 6AR. Report no 186 December 2002

3 (i) STUDY RECOMMENDATIONS High genetic merit Holstein cows are radically different from lower yielding British Friesians. To achieve their potential to produce large volumes of milk of acceptable fat and protein composition, management practices must recognise the competing physiological processes in the cow and the implication of these on nutrient requirements and ration composition. This study has identified a number of key issues that will allow the management of these cows to be improved in order to meet the overall objectives of high milk and milk solids output, improved fertility, controlled body condition score loss and reduced associated health issues. In early lactation, both the extent and duration of body tissue loss can be considerable. In high (>11,000kg milk) yielding cows milked 3x per day milking, body tissue mobilisation occurred until week 14 of lactation, equivalent to 60kg body fat. By week 30 of lactation, less than 50% of this tissue had been replaced. Measured body tissue loss was closely related to changes in body condition score, but measurement of body weight change, which indicated cessation of weight loss by lactation week 5, did not concur with measured body tissue mobilisation When cows were mobilising body tissue in early lactation, milk composition was invariably compromised, especially milk protein content. Providing adequate transition management from 3 to 4 weeks before calving led to significant improvements in feed intake, milk yield and milk solids over the first 20 weeks of lactation. Cows receiving no transition management had lower peak milk yields but increased lactation persistency. Feeding high levels of dietary protein in early lactation increased body tissue mobilisation to support increased milk secretion and this practice should be discontinued. Feeding higher levels of starch in the ration stimulated feed intake and milk yield but also promoted body tissue repletion (or reduced body tissue mobilisation). Measured ME intakes of 300MJ/d were achieved with some evidence of diet metabolisability being compromised at high feed intakes. Inclusion of increased amounts of grass silage in the ration caused marked reductions in total feed intake and lower milk yields. Milk fat output was not compromised and milk fat content increased. Milk protein yield fell and body tissue repletion was compromised. A maximum grass silage inclusion rate on a dry matter basis of 12% in the total ration or 25% of the total forage is recommended. Achievement of high feed intakes is important and increasing dietary starch levels at the expense of dietary protein or fibre should be considered, provided it is cost effective. i

4 (ii) EXECUTIVE SUMMARY A series of studies were conducted with high genetic merit Holstein:Friesian cows at the Centre for Dairy Research to; quantify the energy deficit of high genetic merit cows in early lactation using respiration calorimetry. provide nutritional strategies to minimise the consequences of such on lactational performance. establish an optimal contribution of grass silage in the diet of such cows make recommendations for rationing to minimise the extent and duration of body energy mobilisation and optimise milk constituent synthesis. Year 1. Studies 1 and 2. A study (study 1) with supporting respiration calorimetry (study 2) was undertaken to determine the lactational performance of high- compared with average-yielding cows (HYC and AYC respectively) and to quantify energy utilisation during the first 20 weeks of lactation. Rations based on maize silage and concentrates with limited amounts of grass silage were fed adlibitum. HYC had an average intake of 23.4kg dry matter (DM)/d compared with 21.6kg/d for AYC and resultant mean milk yields were approximately 30% higher for HYC (42.2kg/d) than AYC (32.5kg/d). However, milk fat and protein contents were compromised (HYC, 38.9 & 31.3g/kg: AYC, 43.6 & 34.9g/kg) such that milk solids output was increased by only 18% for HYC compared with AYC. HYC had a lower mean body condition score than AYC (1.89 v 2.26) with higher non-esterified fatty acid (NEFA) and β-hydroxy butyrate (BHB) levels than AYC although only the BHB effects were statistically (p<0.05) significant. Based on measured estimates of ration metabolisable energy (ME) content, overall energy balances were computed. Between calving and week 20 of lactation, HYC consumed an additional 21MJ ME/d (275 v 254MJ/d), produced an additional 21MJ/d milk (128 v 107MJ/d) and had a higher daily heat production (143 v 134 MJ/d). Net tissue gain was however lower on HYC (-6.5) compared with AYC (3.7MJ/d). Net energy content (MJ/kgDM) was unaffected by either cow type or ration composition (mean, 5.6MJ/kgDM) but HYC partitioned more net energy towards milk (5.47MJ/kgDM) compared with AYC (4.95MJ/kgDM) and less to body tissue reserves (-0.28 v 0.17MJ/kgDM). Calorimetric studies at 6 week intervals from week 6 to week 30 of lactation with HYC fed the same ration showed measured ME intake to increase progressively to week 18 (298MJ/d) compared with 279MJ/d at week 6, before declining to 277MJ/d by week 30. Peak milk yield noted during a measurement period was 51.7kg/d at week 6, with a milk energy output of 144MJ/d accompanied by total heat production of 157MJ/d. Consequently, the cows were ii

5 mobilising 22MJ/d, (more than 1kg body fat) at this time to support all energy needs (milk production and maintenance). By week 12, tissue energy mobilisation had reduced to 6MJ/d and cows were in positive energy balance by week 18, with progressive increases thereafter. Comparison of this data with weekly body weight change in the parallel production study provided no relationship. In contrast, changes in body condition score (BCS) closely paralleled the calorimetric measurements of body tissue depletion/repletion and changes in BCS is suggested as a more useful index to assess body energy change than body weight. Year 2. Studies 3 and 4. A lactation study (study 3) was undertaken with only HYC supported by a calorimetry study (study 4) using similar cows. The lactation study examined the effects of pre-calving transition management followed by either high starch or high protein lactation rations. No transition management significantly (p<0.05) reduced total DM intake (21.7 v 23.1kg/d) and milk yield (39.9 v 44.0kg/d) but milk composition was unaffected. High protein feeding during lactation promoted milk fat content and yield compared with high starch rations whilst milk protein content and yield were unaffected by either treatment compared with the control. Plasma NEFA levels were higher in non-transition cows (257µmoles/l) compared with all other cows (mean, 153µmoles/l) but lower than all other treatments (237 v 346µmoles/l) during lactation. Energy balances were computed for all treatments. Cows receiving no transition management had a mean ME intake of 255MJ/d with respective milk and heat outputs of 111 and 144MJ/d, resulting in no overall tissue energy retention. Mean ME intake was higher for treatments which included transition management (271MJ/d) along with higher milk energy (124MJ/d) and heat (153MJ/d) outputs. Overall net energy was unaffected by treatment (mean, 5.64MJ/kgDM) but partition of net energy between milk and body energy was changed by high protein feeding. On the control and high starch rations, milk energy represented 859kJ/MJ net energy with 141kJ/MJ deposited in body tissue. In contrast on high protein rations, milk energy represented 964kJ/MJ net energy whilst body tissue repletion accounted for only 36kJ/MJ. Supporting calorimetric studies examined the high protein and high starch rations only. Energy balance measurements at 6 week intervals from lactation week 6 to week 24 indicated no overall increase in ME intake on the high protein compared with the high starch ration, associated with an increased output of milk energy (140 v 129MJ/d, protein v starch), relatively little change in heat production (mean 159MJ/d), but major effects on tissue energy retention. Overall means were broadly similar (starch, -4.0; protein, -12.6MJ/d) but the depth of energy loss was much greater on the protein ration at week 6 and also at week 12. By week 18 however, though stimulation of ME intake, cows on the high protein ration were in positive energy balance compared with those fed high starch which were still losing a small amount of body energy. iii

6 Year 3. Studies 5 and 6. A lactation study (study 5) was undertaken with HYC to determine the effects of increasing the starch content of the concentrate or the grass silage content of the forage component on lactational performance from calving to week 20. Increasing the level of dietary starch from 27.7 to 31.4% resulted in the highest mean milk output (42.9kg/d) with the highest protein yield (1.40kg/d) whilst milk fat content was significantly reduced (34.0g/kg). In contrast including a limited amount of grass silage in the ration increased total milk fat yield (1.74kg/d) and content (44.1g/kg), with no negative effects on total feed intake although milk protein output was reduced (1.28kg/d). Further replacement of maize silage with grass silage led to marked reductions in feed intake, milk yield (36.6kg/d) and milk protein output (1.17kg/d), although milk fat content (41.3g/kg) was still higher than those rations containing no grass silage whilst yield (1.50kg/d) was similar to that noted on the high starch ration. Based on these data a maximum level of grass silage inclusion in the ration of high yielding cows is recommended. Furthermore, it is postulated that these effects were more related to changes in ME and starch intake, and less to direct changes in dietary fibre intake which were not sufficient to account for the observed changes in milk constituent output. Feeding high levels of starch in all rations, possibly with lower overall milk yields related in part to 2x compared with 3x milking as adopted in the earlier studies, resulted in significantly less body condition score loss. All cows had regained their immediately post-calving BCS by lactation week 8 to 10 and marked improvements in the overall fertility of these cows with respect to days open and services required per successful pregnancy were noted. It is suggested that these effects may in part have been due to the improved energy status of these cows. In study 6, the partition of dietary energy in high yielding cows was examined, using the previously described respiration calorimetry facilities. All 5 experimental treatments were fed and partition of energy and nitrogen was determined in a replicated crossover design. The lactation responses were almost identical to those reported in study 5 with compromised milk volumes and milk protein yields being noted as a consequence of the level of grass silage inclusion being increased. Metabolisable energy intake was highest for cows receiving the high starch-containing ration and this resulted in the highest milk energy output. However only of total ME was partitioned to milk energy with the remainder being used to support body tissue repletion. In contrast, on rations containing increasing amounts of grass silage, body tissue energy repletion was markedly reduced. This data demonstrates clearly how changing the ration composition can have a major effect on the way in which cows use available energy to support different productive functions and establishes that rations containing high levels of grass silage are likely to compromise the overall productivity of high genetic merit cows. From this a maximum inclusion level of grass silage of 12% of the total ration or 25% of the forage component is suggested. iv

7 (iii) INTRODUCTION The introduction of improved genetics in the UK Holstein herd has brought unprecedented increases in the potential of dairy cows to produce large quantities of milk. Many cows in the National herd are capable of producing in excess of 10,000kg per 305d lactation and with adequate management many are achieving such levels of output. Equally many 2 year old heifers will produce over 9000 kg milk in their first lactation which is quite remarkable given that these animals have not achieved mature size and still have to complete their growth, as well as being expected to support a further pregnancy. All of these changes occurred over a relatively short time frame after several years when genetic gain in the UK herd had been relatively modest, leaving us behind when compared with the type of cows being bred by farmers in certain parts of Europe (e.g. Holland) and North America. The rapid introduction of these improved genetics into the UK however brought a quite different cow which needed to be understood and managed accordingly. Whilst some useful technical advice could be developed from research work that had been undertaken overseas, there were several unique features of UK dairying that led to serious omissions in our knowledge necessary to manage these cows in order to meet their potential. Driven primarily by milk yield considerations, farmers have continued with this type of cows but it has not been without some cost. Excessive loss of body condition score during early lactation has become a notable feature of many of these cows as their demands for nutrients to support milk production as well as obligatory maintenance costs could not be met from consumed feed alone. This unavoidably leads to mobilisation of body tissue, often in excessive amounts, in order to meet this shortfall. From this it would appear that such cows have an overwhelming priority for milk production that they will go to extraordinary lengths to maintain even if it is at the cost of body reserves. The second issue noted by many who are milking high genetic merit cows is that milk composition can be seriously compromised especially during early lactation when it is recognised that both milk fat and especially milk protein levels are usually reduced. In the first study from this laboratory with high genetic merit cows, Beever et al, (1998) reported low milk protein levels at week 5 of lactation, with individual cows having an average content of less than 25g protein/kg milk. Even by week 20 when the study was completed, milk protein levels had not been satisfactorily restored to acceptable levels, whilst milk fat levels, which were initially low, did recover as lactation progressed with levels in excess of 40g/kg being noted by week 15 of lactation. This again suggests that the modern high genetic cow has an overriding priority to produce milk even if there is an inadequate supply of nutrients to support the production of milk of acceptable composition with respect to fat and protein. From a consideration of the biochemistry involved, it follows that in such situations it is the synthesis of lactose which attains v

8 a high priority as this is the main osmoregulator of milk volume and thus has the major influence on milk volume. In line with these issues relating to prioritisation of nutrient use in the high genetic merit dairy cow, the third issue which is of increasing concern to UK dairy farmers, although the problem is not unique to them, is declining fertility rates. This has been exemplified in several studies over recent years and it has been postulated by some that if the current downward trend is not halted then in less than 20 years the modern Holstein will have become extinct due to its failure to breed sufficient replacements. The fertility of the cow, as in any mammalian species is complex and has been the subject of a parallel MDC project undertaken by The Royal Veterinary College in conjunction with CEDAR. Both failure to ovulate (ie cycle) and to conceive even when cycling have been advanced as possible causes for this impaired fertility and it has been suggested that compromised nutritional status at this time, especially in relation to energy availability may be a major causative factor. Collectively these issues are having serious negative effects on the ability of dairy farmers with high genetic merit cows to achieve acceptable margins over costs and improved strategies for the management of these animals are required. Whilst potential welfare issues, as perceived through increased body condition loss, may not lead immediately to financial losses, they can not be considered desirable when the public scrutiny of our industry has never been greater. Compromised milk composition will for most buyers result in poorer milk returns, which is clearly not desirable when the overall price of milk is so vulnerable to market forces. Finally health issues such as increased infertility as well as increased lameness and mastitis which have not been discussed but are more evident in higher producing cows, can all lead to premature culling of the cow. The full costs of this are not readily appreciated by most dairy farmers but assuming an average cost of producing a lactating cow, less the realised OTMS cull cow price, of 575 this amounts to an overhead cost of 3p/kg milk if a lifetime milk production of only 19000kg (ie 2 lactations) is achieved. Achieving one more lactation could increase lifetime milk production to 30000kg and reduce the cost of replacement to less than 2p/kg milk whilst a total production of 40000kg will reduce the overhead to manageable proportions (1.44p/kg). This project was undertaken to examine the lactational performance of high yielding cows compared with average yielding cows and to provide reliable quantitative data on energy metabolism, including the extent and duration of tissue energy mobilisation in early lactation together with the rate at which mobilised tissues are replenished in later lactation. Data on energy metabolism in high yielding dairy cows was considered to be limiting, almost non existent, and prior to the study of Beever et al (1998), undertaken with a limited number of cows, the seminal studies of Dr Flatt at Beltsville with his famous cow Lorna yielding 90lbs (41kg/d), were the vi

9 only available reference point. The study was designed to make use of the unique respiration calorimetry facilities available for dairy cows at CEDAR in which full energy balances can be determined. Subsequent studies were then proposed to determine how the extent and duration of energy mobilisation in early lactation could be controlled within manageable boundaries whilst at the same time addressing the important issue of compromised milk composition. Of central importance to all of this was the issue of optimising feed dry matter intake as the most suitable means by which adequate levels of metabolisable energy can be consumed. In this respect studies to optimise the level of grass silage in the ration were also proposed for whilst grass silage is known in many situations to be a serious limitation to cows achieving their maximum intake of feed, there are still many farms with high yielding cows where grass silage comprises a significant proportion of total forage stocks. Prior to this study only limited advice on the feeding of high yielding dairy cows for optimal production was available. Little of it was based on actual scientific research and opinions had been formed on the basis of extrapolation from existing practices. The recommendation that cessation of body weight loss was synonymous with the cows being suitable for re-breeding was one issue that could not be reconciled, when simple estimates of energy balance were determined on the basis of known feed inputs and milk secretion. The six studies comprising of 3 lactational studies and 3 supporting respiration calorimetry studies are presented in the following sections, with outline details of the studies and the results obtained, followed by a discussion of the implications of the data. Finally a short resume of the main findings is presented which should form the basis of practical recommendations to dairy farmers who are striving to improve the management of high genetic merit cows. vii

10 (iv) CONTENTS page (i) STUDY RECOMMENDATIONS i (ii) EXECUTIVE SUMMARY ii (iii) INTRODUCTION v (iv) CONTENTS ix STUDY 1 A comparison of the lactational performance of high and average yielding cows and assessment of energy metabolism based on gross inputs and outputs of energy OBJECTIVE EXPERIMENTAL APPROACH Cows Management Measurements Feeds and diet RESULTS Lactational performance Liveweight and condition score Blood metabolites Energy status DISCUSSION 9 STUDY 2 An evaluation of energy metabolism in high yielding dairy cows OBJECTIVE EXPERIMENTAL APPROACH Cows and Management Measurements and technique RESULTS Lactational performance and dry matter intake Energy utilisation DISCUSSION 14 STUDY 3 A comparison of the effect of pre-calving transition management and post-calving dietary starch and protein manipulations on the lactational performance of high yielding cows OBJECTIVE EXPERIMENTAL APPROACH Cows, diet and management (pre calving) Cows, diet and management (post calving) Measurements RESULTS Lactational performance Blood metabolites Energy status DISCUSSION 26 viii

11 page STUDY 4 An evaluation of energy metabolism in high yielding dairy cows receiving high-starch or high-protein containing mixed rations OBJECTIVE EXPERIMENTAL APPROACH Cows diet and management Measurements RESULTS Lactational performance and dry matter intake Energy metabolism DISCUSSION 32 STUDY 5 A comparison of the effect of increasing the starch content in the concentrate portion or the grass silage content in the forage component of high starch containing rations for high yielding dairy cows on lactational performance and body condition and 34 bodyweight change. 5.1 OBJECTIVE EXPERIMENTAL APPROACHES Cows Treatments Measurements Diets RESULTS Performance Blood metabolites DISCUSSION 41 STUDY 6 An evaluation of the effect of increasing the starch content in the concentrate portion or the grass silage content in the forage component of high starch containing rations on energy and nitrogen utilisation by high yielding dairy cows OBJECTIVES EXPERIMENTAL APPROACH Cows Management Measurements Feeds and diet RESULTS Lactational performance Feed intake and diet digestibility Neutral and acid detergent fibre intake and digestibility Dietary nitrogen intake, digestibility and utilisation Energy digestion and partition of digestible energy Metabolisable energy utilisation DISCUSSION CONCLUSIONS 57 ix

12 page 8.1 REFERENCES APPENDIX (figures 1-22b) 62 x

13 STUDY 1 A comparison of the lactational performance of high and average yielding cows and assessment of energy metabolism based on gross inputs and outputs of energy. 1.1 OBJECTIVE The purpose of this initial study was to examine the lactational performance of cows of high and average genetic merit. Systems specific for the management of these 2 types of animals were established with marginal differences in ration formulation but a major difference in milking practice with the high genetic cows being milked 3X per day compared with only 2X per day for the other group of cows. Previous studies had indicated that increased outputs of milk with high genetic merit animals may lead to significant reductions in milk fat and protein content, thus compromising to some extent the higher milk volumes when output was recognised in terms of milk fat and protein yield. There had also been earlier suggestions that changes in bodyweight with stage of lactation were not synonymous with changes in body condition score and thus may not be a suitable indicator of the cessation of body tissue loss, when it is often considered to be a suitable time to commence rebreeding of the cow. This production study aimed to describe these events and to establish where the high milk yields of high genetic merit cows may be compromising other physiological events. 1.2 EXPERIMENTAL APPROACH Cows A total of 40 multiparous Holstein cows were recruited to the study from the CEDAR herd on the basis of previous lactational yield, satisfactory conformation and suitable calving dates. The experimental design comprised a continuous lactation study carried out over the first 24 weeks of lactation with separate management regimes for high and average genetic merit cows. The cows comprised 20 high genetic merit cows, with known propensity to yield >10,000litres milk in 305d lactation (high yielding cows, HYC) and 20 average merit cows with production levels of approximately 7500 litres per 305d lactation (average yielding cows, AYC). Pedigree indices (PTA 95 ) were calculated for each cow and the cows were described as follows; previous 305d lactation yield (HYC, 8337; AYC, 7051), PTA 95 milk and milk fat + protein, (HYC, +400kg, +26.6kg; AYC; +302kg, +21.5kg). 1

14 1.2.2 Management All cows were dried off at target body condition score (BCS) of 2.5 to 3.0 and housed in a straw bedded yard until calving, receiving a ration based in part on their subsequent lactation ration, balanced for dietary cation/anion ratio, from 3 weeks prior to calving. Post-calving all cows were cubicle housed and fed individually one of the 2 total mixed rations ad libitum through Calan Broadbent gates. The HYC were milked 3x per day at 0530, 1430 and 2000h whilst the AYC were milked only 2x per day (0630 and 1530h) Measurements Feed intake and milk yield were recorded daily for each cow and milk samples were taken 6 and 4x per week (HYC and AYC) for analysis of protein, fat and lactose contents. All cows were weighed and body condition scored weekly by the same person. In addition, blood samples were obtained weekly from all cows from 3 weeks prior to expected calving until week 20 of lactation and the resultant plasma samples were analysed for respective contents of non-esterified fatty acids (NEFA), β-oh butyrate (BHB), glucose and urea Feeds and diet Composition of the two total mixed rations are presented in table 1 whilst the chemical composition of the diets is presented in table 2. The maize silage was harvested in mid September and had dry matter (DM) and starch contents of 348g/kg fresh weight and 259g/kgDM respectively, with ph 3.79 and a laboratory based ME estimate of 11.7MJ/kgDM. In contrast, the grass silage, prepared from a primary growth perennial ryegrass harvested in mid May and clamped after a partial field wilt without additive, had DM and crude protein contents of 320g/kg fresh weight and 137g crude protein/kg DM respectively, with ph 3.97 and an estimated ME content of 11.0MJ/kgDM. 2

15 Table 1.1 Total mixed ration (TMR) composition (g/kg DM) as fed to high and average yielding cows HYC AYC Maize silage Dried lucerne Grass silage Hay Total forage Crimped Whole Oilseed rape Potato starch Fish meal Hipro soya Cracked wheat Molassed sugarbeet pulp Rapeseed meal Crimped peas Minerals Regumaize Megalac Dicalcium phosphate Megalac, Volac Ltd Regumaize, Intermol, UK Provimi High-UDP fish 4 James and Co Both rations contained approximately 560g forage DM/kg total ration DM with maize silage as the predominant forage. However, whilst the AYC ration had only maize silage and grass silage in a 3:1 ratio, the HYC ration included maize and grass silage (approx ratio, 4.9:1) at 678g/kg total forage (DM basis) with dried Lucerne and chopped hay (approximate 4:1 ratio) providing the remainder of the forage component. The concentrate portion of the AYC ration was comprised principally of cracked wheat and rapeseed meal whilst in the HYC ration, significant amounts of HiPro soya, potato starch and molassed sugar beet feed were included, together with crimped peas and crimped whole oil seed rape. Both rations were formulated to 12MJ/kgDM and >170g crude protein/kgdm, with the ingredients of the HYC diet based in part on the result of discussions with farmers milking high genetic merit cows. 3

16 Table 1.2 Chemical composition of the total mixed rations (g/kg volatile corrected dry matter (VCDM) unless stated HYC TMR AYC TMR VCDM (g/kg fresh) Ash NDF ADF Starch Water soluble carbohydrates Oil Crude Protein ME MJ/kg VCDM Total VFA Lactic acid Ethanol Ammonia-N g / kg total N RESULTS Lactational performance Data in table 1.3 relate to the lactational performance of the cows from calving to week 24 of lactation when the study was terminated. Overall DM intakes were satisfactory on both management regimes with the HYCs consuming an additional 1.8kgDM/d (p<0.001) compared with AYC. In relation to mean body weight (BWT) of the cows, total mean DM intakes of 37.4 (HYC) and 33.7 (AYC)g/kg BWT were achieved throughout the study, with HYC being marginally below 40g/kg BWT (or 4% BWT) which is often used as an industry benchmark for such cows. Patterns of feed intake with stage of lactation (Appendix fig 1) indicate that both groups of cows showed a typical rise in feed intake post calving until peak intakes were achieved at approximately week 5 for both treatments. Thereafter the differences between the 2 groups of cows were maintained, with the notable exception of a pronounced rise in DM intake for the AYC between weeks 22 and 24. Overall milk yield was approx 10kg/d higher for HYC compared with AYC (p<0.001) with HYC having a mean peak yield of 48kg/d at week 5/6 compared with 35.7kg/d for AYC (p<0.001) noted at week 5. Milk yield declined thereafter (Appendix fig. 2) with a significant week x treatment interaction indicating weekly milk yield declines of 0.45 (AYC) and 0.65 (HYC) kg/d (p<0.01) respectively. Mean milk composition for the AYCs was 43.6g fat and 34.9g protein/kg, which were considered to be most satisfactory for modest genetic cows yielding approximately 5500kg during the first 168 days of lactation. In relation to stage of lactation (Appendix fig. 3) milk fat content for AYC showed considerable variation but with an overall decline until week 9 4

17 after calving whilst less variation was observed with milk protein content (Appendix fig. 4), with a minimal value of 33.5g/kg being achieved at week 5. Thereafter, milk protein contents increased progressively until week 12 before stabilising for the remainder of the study. In contrast, mean milk fat and protein contents for HYC were significantly (p<0.001) lower (38.9 and 31.3g/kg respectively) compared with AYC, with the lowest fat (37.1g/kg) and protein (29.0g/kg) levels being noted at week 6. Thereafter milk protein contents increased progressively to reach approximately 33g/kg by week 22 whilst milk fat contents showed only marginal improvement as lactation progressed. Data are also included in table 1.3 for mean milk lactose content which was significantly (p<0.001) reduced on HYC (47.0g/kg) compared with AYC (48.1g/kg). Table 1.3 The effect of treatment and stage of lactation on feed intake, milk production and milk composition at weeks 6, 12, 18 and 24 of lactation (g/kg unless stated) Lactation week HYC AYC Treat P < Mean Mean Week P < 2 T x W P < 3 VCDM intake (kg/day) (sem) (0.51) (0.54) (0.55) (0.53) (0.37) (0.56) (0.56) (0.57) (0.59) (0.40) Milk yield (kg/day) (sem) (1.11) (1.15) (1.17) (1.16) (0.92) (1.22) (1.22) (1.25) (1.27) (1.00) Milk composition (g/kg) Fat (sem) (1.33) (1.43) (1.44) (1.40) (0.79) (1.46) (1.43) (1.47) (1.50) (0.86) Protein (sem) (0.56) (0.58) (0.59) (0.59) (0.48) (0.62) (0.62) (0.63) (0.63) (0.52) Lactose (sem) (0.30) (0.32) (0.32) (0.31) (0.22) (0.32) (0.33) (0.33) (0.34) (0.24) 1 Probability of no overall treatment effect 2 Probability of no effect of week 3 Probability of no treatment X week interaction As a consequence of these changes, AYC had a combined mean daily output of milk fat and protein of 2.55kg, with a fat:protein ration of 1.26:1 whilst mean output for HYC was 16% higher (2.96kg) but with only a marginally lower fat:protein ratio (1.24:1) (Table 1.4). Together with lactose, mean daily outputs of milk solids were 20% higher for HYC (4.95kg) compared with AYC (4.12kg), this response being less than that noted for milk volume (+29%) with HYC due to compromised milk composition, mainly fat and protein (Table 1.4). 5

18 Table 1.4 The effect of treatment and stage of lactation on milk constituent yield, liveweight and body condition score at weeks 6, 12, 18 and 24 of lactation (kg/day unless stated) Lactation week HYC AYC Treat P < Mean Mean Week P < 2 T x W P < 3 Solids yields (kg/d) Fat (sem) (0.061) (0.065) (0.066) (0.064) (0.043) (0.067) (0.066) (0.068) (0.069) (0.046) Protein (sem) (0.034) (0.036) (0.037) (0.036) (0.028) (0.038) (0.038) (0.039) (0.039) (0.030) Lactose (sem) (0.054) (0.056) (0.057) (0.057) (0.045) (0.060) (0.060) (0.061) (0.062) (0.048) Total milk solids (sem) (0.135) (0.140) (0.143) (0.141) (0.109) (0.148) (0.147) (0.150) (0.152) (0.117) Mean LWT (kg) (sem) (10.9) (11.0) (11.0) (10.9) (10.2) (11.8) (11.8) (11.9) (11.9) (11.1) Mean BCS (sem) (0.096) (0.100) (0.101) (0.098) (0.08) (0.105) (0.104) (0.106) (0.108) (0.08) 1 Probability of no overall treatment effect 2 Probability of no effect of week 3 Probability of no treatment X week interaction Liveweight and condition score Mean cow BWT is also included in table 1.4 indicating that AYC on average were 15kg heavier than HYC, whilst overall BCS was approx. 0.4 units lower for HYC (p<0.01). Mean post-partum bodyweights did not differ between the 2 treatments (mean, 629kg) but whereas BWT only declined until week 2 for AYC, an increased loss was noted for HYC until week 5, with net losses of 9 and 19kg for AYC and HYC respectively. Thereafter, AYC gained at a higher rate than HYC although bodyweights achieved by week 24 were not significantly different (Appendix fig. 5). In relation to BCS (Appendix fig. 6), HYC showed a marked loss until week 12 by which time a mean BCS of 1.7 was noted, before stabilising at BCS 1.8. In contrast, AYC had higher post-calving BCS, and this remained relatively stable until week 12 before increasing to week 24. By the end of the study, AYC had a BCS of 2.34, which was significantly (p<0.001) higher than HYC (1.81). Regression of body weight change with changes in BCS for AYC during weeks 3-24 when BWT gains were noted, revealed that a one point gain in BCS was associated with a BWT gain of 74kg. In contrast, for HYC where BWT gains were noted from week 5 of lactation, a one point gain in BCS was associated with a much smaller BWT gain of only 20kg liveweight gain. Furthermore, during the period of BWT loss in HYC (weeks 1-5), one point loss of BCS was 6

19 equivalent to a BWT loss of 26kg. However, these results do need to be interpreted with considerable caution as the errors associated with the overall means were quite large, although these were unlikely to be as great as those recorded in the on-farm situation Blood metabolites Data in table 1.5 summarise the mean plasma concentrations of major metabolites, showing that HYC had higher BHB (p<0.01), NEFA, glucose and urea levels compared with AYC. BHB levels were high in HYC at calving whilst peak values were observed at week 4, before these declined progressively until week 10. AYC tended to have lower values at all times, with a significant (p<0.05) treatment effect being noted between weeks 1-6 and (HYC>AYC). NEFA levels declined in both groups of cows during weeks 1-4 of lactation with levels tending to be higher in HYC compared with AYC. Subsequently, levels stabilised in both groups by week 15 of lactation. Post-calving blood glucose levels increased in both treatments but the effects were not statistically significant. Levels then appeared to stabilise in AYC by week 7 of lactation whilst this did not occur in HYC until week 11. A significant (p<0.05) treatment effect was noted between weeks 16 and 20 (HYC>AYC). Finally with respect to blood urea, levels rose in both treatments post-calving and whilst higher values were noted in HYC, this effect was not statistically significant. Table 1.5 Mean plasma concentrations of non esterified fatty acids (NEFA), β hydroxy butyrate (β-ohb), glucose and urea in average and high yielding cows during weeks 1-24 of lactation. HYC (s.e.m.) AYC (s.e.m.) Treat Week T x W P < 1 P < 2 P < 3 NEFA (µmol/l) 138 (11.2) 119 (12.9) β - OHB (µmol/l) 766 (33.7) 631 (42.3) Glucose (mmol/l) 2.98 (0.06) 2.82 (0.07) Urea (mmol/l) 6.51 (0.27) 6.18 (0.31) Probability of no overall treatment effect 2 Probability of no effect of week 3 Probability of no treatment X week interaction Energy status Based on the lactational performance of the cows presented earlier, the data presented in table 1.6 represents an attempt to provide an overall energy balance for the two treatments. Estimates of ME intake were based on in vivo derived determinations of the ME content of the two rations (see study 2) with milk gross energy contents determined according to milk solids composition and the equation as proposed by Tyrrell and Reid (1965). ME requirements (and hence heat 7

20 production) for milk production and energy gain or loss associated with tissue depletion or repletion were calculated according to AFRC, (1993). Table 1.6 Mean metabolisable energy intake and milk energy output in average and high yielding cows during weeks 1-24 of lactation, with calculated estimates of heat production and tissue energy retention (MJ/d unless stated). Lactation week HYC AYC Treat P < Mean Mean Week P < 2 T x W P < 3 MEI (s.e.m.) (6.09) (6.46) (6.56) (6.43) (4.40) (6.70) (6.65) (6.82) (7.06) (4.76) Milk (s.e.m.) (3.67) (3.85) (3.89) (3.82) (2.88) (4.03) (4.00) (4.08) (4.20) (3.13) Heat (s.e.m.) (2.39) (2.51) (2.55) (2.50) (1.86) (2.63) (2.61) (2.67) (2.74) (2.02) Retained (s.e.m.) (5.20) (5.57) (5.64) (5.49) (3.41) (5.72) (5.64) (5.80) (6.06) (3.69) 1 Probability of no overall treatment effect 2 Probability of no effect of week 3 Probability of no treatment X week interaction HYC had an estimated ME intake during weeks 1-24 of 275MJ/d, approximately 8% higher than AYC (p<0.01) but milk energy content was significantly (p<0.001) reduced on these cows due to lower milk constituent contents. Overall however, milk energy output was 19% higher for HYC (p<0.001) whilst estimated heat production was also increased (HYC, +6%; p<0.01). This resulted in a small negative energy balance being noted on HYC (-6.5MJ/d) compared with a significantly (p<0.05) higher overall retention (+3.7MJ/d) for AYC. With respect to stage of lactation effects, ME intake was at all times higher for HYC with maximum level (288MJ/d) being noted at week 10 compared with AYC where an initial peak (268MJ/d) was observed at week 11, followed by small further increases through to week 21. In both treatments, milk gross energy content fell after calving with minimum values being recorded at week 8 (HYC) and 9 (AYC) respectively. As expected milk energy output increased post-calving on both treatments, with peak output being noted at week 5. Heat production, which was the summation of calculated maintenance energy costs, the costs associated with milk production and either the mobilisation or repletion of body tissue, showed a similar relationship with respect to stage of lactation and total feed intake. Marked increases in heat production were noted for both groups of cows until week 5 of lactation, with relatively 8

21 stable outputs thereafter until week 14 (AYC) and 16 (HYC) respectively, followed by further small increases. With respect to body energy retention, regression of weekly means (Appendix fig 7) showed AYC achieved energy balance at week 7, whilst this was not observed until week 12 for HYC. This contrasts with the bodyweight change data as presented earlier, suggesting that BWT changes in dairy cows are unlikely to be a useful indicator of body energy status. 1.4 DISCUSSION This study was designed to examine the impact of increased genetic merit on lactational performance from calving to week 24 of lactation. The AYC used in this study could be considered representative of many cows in the national herd and based on the data obtained were estimated to have 305d lactation yields of approx 8000kgs. Mean intakes of over 21kg DM/d for a TMR containing 12MJ metabolisable energy and 170g crude protein/kg were considered satisfactory and overall milk composition was above the UK national average for Holsteins, according to recent data from National Milk records. As expected, the AYC lost a significant amount of BCS post-calving but this was associated with minimal bodyweight loss, whilst as lactation progressed, both BCS and BWT showed sustained improvements. Overall, HYC did not show major differences in bodyweight compared with AYC although BWT loss was increased in both extent and duration during the early stages of lactation. However changes in BCS were most noticeable, providing clear evidence that HYC mobilised more body tissue in early lactation compared with AYC, whilst being unable to replete these tissues at the same rate as AYC. In response to their apparent potential to produce more milk, HYC consumed an additional 1.8kg feed DM/d equivalent to an estimated extra 21MJ/d. This increase was less than had been anticipated on the basis of the cows and the type of ration formulated for them and given the obvious need to increase total feed intake in such cows, this is an area of concern needing to be addressed. Failure to achieve ME intakes approaching 300MJ/d in HYC appeared to have several effects. Whilst there was no apparent reduction in milk volume secretion, milk composition was at times quite seriously compromised with the 29% improvement in milk yield being manifested in only a 20% improvement in milk solids output, and even less in terms of milk fat and protein. Again this is an area of concern in terms of realisable milk price, whilst the occurrence of reduced milk protein contents is generally considered to be indicative of compromised energy metabolism. Estimation of key blood metabolites indicated raised levels of NEFA and BHB in HYC, both indicative of increased body tissue catabolism and possible incomplete utilisation of available substrates, but only BHB levels were statistically elevated, suggesting that neither of these 9

22 parameters are likely to be useful indicators of subtle changes in the energy status of high yielding dairy cows. 10

23 STUDY 2 An evaluation of energy metabolism in high yielding dairy cows 2.1 OBJECTIVE This study was undertaken to determine energy metabolism in high yielding cows fed the same ration as formulated for study 1. Respiration calorimetry was used to provide precise determinations of energy digestion within the alimentary tract, quantification of metabolisable energy supply and the partition of ME use between milk energy output, heat production and tissue energy mobilisation or repletion. Measurements were obtained at 6 week intervals between lactation weeks 6 and 30 in order to determine stage of lactation effects on ME intake and utilisation, with emphasis on identifying the extent and duration of tissue mobilisation during early lactation. 2.2 EXPERIMENTAL APPROACH Cows and Management A total of 8 cows of similar parity and genetic merit to the HYC used in the lactation study described earlier were selected for this study and received the same pre-partum management. Immediately post-calving the cows were housed in individual tie stalls and fed ad libitum with the same total mixed ration for HYC as described earlier. Subsequently, all cows were subjected to determinations of energy metabolism from weeks 6 30, at 6 week intervals using open circuit calorimetry Measurements and technique The calorimetry technique comprises the measurement of faecal, urine and milk energy output by total collection together with determination of heat production by indirect means, with measurements of oxygen consumption and carbon dioxide and methane production being used in the equation proposed by Brouwer (1965). Prior to any calorimetry measurements, the cows were subjected to periods of familiarisation in the respiration chambers, followed by consecutive measurements of faecal, urine and milk output (5 days) and gaseous exchange (3 days). The data were then calculated in relation to ME intake, dietary ME concentration, milk energy output and heat production, with body energy gain or loss being determined by difference. 11

24 2.3 RESULTS Lactational performance and dry matter intake Data relating to lactational performance and dry matter intake are presented in table 2.1. Overall DM intake averaged 24.3kg/d for the weeks of experimental measurement with peak intakes being noted between weeks 12 and 18. At week 6, which according to the earlier study must have been close to peak milk production, milk yield averaged almost 52kg/d with a subsequent decline thereafter to 35kg/d by week 30, equivalent to just over 1% reduction per week. Milk fat and protein contents were characteristically reduced in early lactation, with compromised levels being noted until week 24 for fat, whilst protein only achieved 32g/kg by week 30. In relation to milk fat and protein yield, the week 6 combined output averaged 3.17kg/d, followed by an average weekly decline thereafter of 0.032kg, to achieve a daily output of 2.43kg by week 30, whilst fat: protein ratio showed minimal variation between 1.12 and 1.18:1. Table 2.1 The effect of stage of lactation on milk yield, milk composition and solids yield, with measurements of water intake and cow liveweight and body condition score Lactation week s.e.m. (range) Pr > F Milk yield (kg/d) ( ) Milk composition (g/kg) Fat ( ) Protein ( ) Lactose ( ) Solids yields (kg/d) Fat ( ) Protein ( ) Lactose ( ) Water intake (kg/d) ( ) Mean liveweight (kg) ( ) BCS ( ) Energy utilisation Data relating to energy utilisation are presented in table 2.2. Measured ME intake at week 6 was 279MJ/d, whilst by week 18 an increase of almost 20MJ/d gave a mean intake approaching 300MJ/d. Milk energy output on the other hand was highest in week 6, in line with the highest output of fat and protein being noted at this time, and estimated to be 52% of total ME intake. Thereafter milk energy output declined progressively with lactation stage to 105MJ/d by week 30. In contrast, heat energy was largely unaffected by stage of lactation, (mean, 159MJ/d) and 12

25 considerably higher than values reported earlier for lower yielding British Friesian cows by Sutton et al, (1991) fed grass silage and concentrates. Body energy deficit was estimated to be 22MJ/d at week 6, equivalent to more than 1kg body tissue loss/d, whilst by week 12 this had reduced to 6MJ/d. Thereafter, small positive tissue energy retentions were estimated, and further examination of the data led to the conclusion that zero body energy balance was most likely to have been achieved by week 14 of lactation, at least 6 weeks earlier than reported in previous studies by Beever et al (1998) from this laboratory. It is also of interest to note that achievement of zero energy balance by week 14 was more or less coincident with the time at which minimal BCS was noted in the production study reported previously. Extrapolating these data to the first 30 weeks of lactation provides estimates of the likely changes in total body energy content and assuming most of this was related to body fat stores, estimates of changes in body fat content were computed. By lactation week 10, cows had lost 2.15GJ of body energy equivalent to 60kg body fat. This equates to over 25% of the likely total body energy of dairy cows at calving (Gibb et al, 1992). During lactation weeks 11-20, a small overall net body energy gain of 0.16GJ, equivalent to 4.5kg body fat, was estimated, indicating a period of relatively stable body energy reserves whilst the largest repletion occurred during weeks 21-30, with body tissue showing a net gain of 0.98GJ, equivalent to almost 28kg body fat. Nevertheless, based on these calculations, it appears that even at week 30, when the cows had only another 90 to 100 days left in milk to achieve a 305d lactation, full replacement of those body fat reserves that existed at calving and were used to support milk production during early lactation had not occurred. Furthermore, on the basis of total milk energy secretion from calving to week 10 (9.6GJ), a total energy cost of producing this milk can be determined (15.4GJ), of which approximately 14% was derived from mobilised body tissue, assuming for the purpose of this calculation that all maintenance costs were derived from the feed. 13