Genetic Parameters for the Milk Coagulation Properties and Prevalence of Noncoagulating Milk in Finnish Dairy Cows

Transcription

1 Genetic Parameters for the Milk Coagulation Properties and Prevalence of Noncoagulating Milk in Finnish Dairy Cows T. IKONEN, K. AHLFORS, R. KEMPE, M. OJALA, and O. RUOTTINEN Department of Animal Science, University of Helsinki, PO Box 28, 14 Helsinki University, Finland ABSTRACT The genetic parameters were estimated for milk coagulation properties and milk production traits, and the prevalence of noncoagulating milk in the Finnish dairy cattle population was investigated. Data were included for 789 Finnish Ayrshire cows and 86 Finnish Friesian cows from 51 herds. The animal model used for estimation included fixed effects for parity, stage of lactation, breed, and herd. Further, effects of milk protein genotypes on phenotypic and genetic variation in the studied traits were examined. Heritability estimates for the milk coagulation properties were moderately high. The k-casein B allele was associated with the best phenotypic and genetic values for curd firmness, and the A and E alleles were associated with the poorest. About 24% of the additive genetic variation in the curd firmness was due to milk protein polymorphism. About 8% of the Finnish Ayrshire cows in the present study produced noncoagulating milk. Because of the occurrence of the noncoagulating milk and a possibly unfavorable genetic trend in the milk coagulation properties, it would be important to improve these traits in the Finnish Ayrshire breed. Milk coagulation properties could be improved directly by selecting for these traits or indirectly by favoring the k-casein B allele or by selecting against genetic markers associated with poorly coagulating or noncoagulating milk. ( Key words: milk coagulation, noncoagulation, genetic parameters, Finnish dairy cows) Abbreviation key: FAy = Finnish Ayrshire, FFr = Finnish Friesian, NC = noncoagulating. INTRODUCTION The importance of cheese production has increased during the past 15 yr in Finland, and currently about Received May 11, Accepted September 9, % of the milk produced there is used for cheese production. The coagulation ability of milk is essential in cheese making. Milk with favorable coagulation properties (i.e., short coagulation and curdfirming times and a firm curd) is expected to give more cheese with desirable composition than milk with unfavorable properties (3, 6). There are no published data on variation or trend in the milk coagulation properties over the past decades in Finland. However, according to the observations made in Finnish dairies, the average coagulation ability of milk has been deteriorating during the past 2 to 3 yr. Consequently, to be able to produce the same amount of cheese, more milk is needed today than in recent decades. In addition, two Finnish data sets collected in 198s (29) and 199s (12) showed that 3.6% of 168 Finnish Ayrshire ( FAy) cows and 1.2% of 59 FAy cows, respectively, produced noncoagulating ( NC) milk. Because of the importance of cheese production in Finland, there is an interest in halting the undesirable trend in milk coagulation properties and in improving these traits. Consequently, reasons for the unfavorable changes and variation in the coagulation properties have to be identified. Should a reasonable part of the variation in the milk coagulation properties be genetic, these characteristics could be improved by selection. In addition, it is important to find out whether the relatively high frequency of NC milk that has been reported in (12, 29) was by chance or is a common phenomenon in the Finnish dairy cattle population. Heritability estimates for the milk coagulation properties have been estimated in only a few studies (15, 25, 29), none of which used an animal model to account for all known genetic relationships among animals. Because of the small data sets and statistical procedures assumed, some heritability estimates were not reliable. The results of the previous studies implied, however, that a moderate part of the variation in the milk coagulation properties could be due to additive genetic effects J Dairy Sci 82:

2 26 IKONEN ET AL. The lack of suitable equipment for routine determination of the milk coagulation properties in the national cow population would restrict possibilities of direct selection for these traits. An indirect way of improving the milk coagulation properties might possibly be to favor the k-cn B allele or alleles at other loci that are associated with favorable coagulation properties. The k-cn B allele is known to be associated with more desirable coagulation properties (1, 4, 12, 19, 26, 28, 29, 3) and protein composition of milk (12, 19, 22, 28) than is the A allele, but no reliable estimates exist of the effect of the k-cn E allele on these traits. The k-cn E allele is rather common (3%) in the FAy (13), the main dairy breed in Finland. Another indirect way of improving the milk coagulation properties might be to breed for routinely recorded dairy traits that correlate favorably with the milk coagulation properties. Genetic correlations between the milk coagulation properties and milk production traits have thus far been estimated from only a few, small data sets (15, 25). Reliable estimates for the genetic correlations between the milk coagulation properties and milk production traits are therefore needed. The objectives of this study were to estimate genetic parameters (heritabilities and genetic correlations) for milk coagulation properties and milk production traits and to estimate the prevalence of NC milk in the Finnish dairy cattle population. Data MATERIALS AND METHODS A total of 789 FAy and 86 Finnish Friesian ( FFr) cows from 51 herds in southern Finland were sampled once during the period from February to April The milk samples were a mixture of evening and morning milkings; samples were analyzed for milk coagulation properties, ph, milk yield, fat percentage, protein percentage, SCC, and major milk protein genotypes. Background information about the cows and herds was obtained from national milk recording data sets from Agricultural Data Processing Centre (Vantaa, Finland). It was not possible to get detailed information about the feeding of the cows, although feeding procedures can affect milk coagulation properties (1, 8, 19). Laboratory Analyses The milk coagulation properties were determined at 32 C using a Formagraph (Foss Electric A/S, Hillerød, Denmark) in the laboratory of the Food Research Institute of Agricultural Research Centre (Jokioinen, Finland). Rennet (Renco Calf Rennet Liquid; New Zealand Rennet Company Ltd., Eltham, New Zealand) was diluted in.7 M sodium acetate buffer (1:1, vol/vol; ph 5.5). The ph of milk was determined (PHM82 Standard ph Meter; Radiometer, Copenhagen, Denmark). The milk samples were allowed to coagulate for 3 min because, during the cheese-making process, curd is usually cut 3 min after the addition of rennet to the milk. The three milk coagulation properties determined were milk coagulation time in minutes, curdfirming time in minutes, and firmness of the curd in millimeters. Milk coagulation time was the time from the addition of rennet to milk to the beginning of coagulation. Curd-firming time was the time from the beginning of coagulation to the moment the width of the curve was 2 mm. Firmness of the curd was the width of the curve 3 min after the addition of rennet. The milk samples that did not form curd in 3 min (i.e., a straight line on the output paper) were defined as NC samples. Fat and protein percentages were determined using a Milko Scan 65 (Foss Electric A/S), and SCC was determined by means of a Fossomatic 36 (Foss Electric A/S) in local dairy laboratories. The frequency distribution for SCC was not normal, so SCC were logarithmically transformed. Genotypes for the a s1 -CN, b-cn, k-cn, and b-lg were determined in Finnish Animal Breeding Association laboratory (Vantaa, Finland) using isoelectric focusing as described by Erhardt (5). Statistical Analyses Univariate and bivariate models. Heritabilities for the studied traits and genetic correlations between the milk coagulation properties and milk production traits were first estimated using an univariate and a bivariate model: y ijklmn = m + parity i + stage j + breed k + herd l + anim m + e ijklmn, [1] where y ijklmn = milk coagulation or milk production trait, m = mean, parity i = fixed effect of parity i (i = 1 to 4), stage j = fixed effect for stage of lactation j (j = 1 to 6), breed k = fixed effect of breed k (k = 1 to 2),

3 GENETIC PARAMETERS FOR MILK COAGULATION 27 herd l = fixed effect of herd l (l = 1 to 51), anim m = random additive genetic effect of animal 2 m, N(, As a ), and e ijklmn = random residual effect, N(, 2 Ise ). A variance-covariance structure between studied traits 1 and 2 for a bivariate model was assumed, where s a1a2 and s e1e2 are additive genetic and residual covariances between traits 1 and 2: Var = a 1 a 2 e 1 e 2 2 As a1 As a1a2 As a1a2 2 As a2 2 Is e1 Is e1e2 Is e1e2 2 Is e2 Parity was grouped in four classes: first, second, third, and fourth to ninth parities; stage of lactation was grouped into six classes: 5 to 3 d, 31 to 6 d, 61 to 12 d, 121 to 18 d, 181 to 24 d, and >24 d after calving; and breed was grouped into two classes: FAy and FFr. Herd was treated as a fixed effect because the herds were a group selected from a certain area, and differences in the milk coagulation properties between them were of interest. Of the 51 herds, 33 were pure FAy herds, 1 was a pure FFr herd, and 17 were mixed herds. The number of animals per herd ranged from 6 to 47. The 789 FAy and 86 FFr cows with records were daughters of 246 FAy and 41 FFr sires, respectively. The average number of daughters per sire was only 3 but ranged from 1 to 79. The pedigree information consisted of parents and grandparents of the cows with records, and the total number of animals in the statistical analyses was Variance and covariance components for the random effects were estimated from the data using the REML VCE package (1). Solutions for the fixed and random effects in the models were obtained using the PEST package (9), and statistical significance of the fixed effects was tested using the F test provided by the PEST package (9). Multivariate models. In addition to univariate and bivariate analyses, heritabilities for the studied traits and genetic correlations between the milk coagulation properties and milk production traits were estimated using a corresponding multivariate model, in which a milk coagulation characteristic was analyzed simultaneously with ph, milk yield, fat percentage, protein percentage, and SCC. The multivariate model was used to determine whether more reliable and more accurate estimates for the genetic parameters would be obtained when information on phenotypic and genetic association between all studied traits was available. Modification of the univariate model. In order to estimate the effects of b-cn, k-cn, and b-lg genotypes on the milk coagulation properties and to study how their inclusion in the model affects additive genetic variation in the coagulation properties, the following model was assumed: y ijklmnop = m + parity i + stage j + breed k + herd l + b-k-cn m + b-lg n + anim o + e ijklmnop [2] where b-k-cn m = fixed effect of b-k-cn genotype class m (m = 1 to 9), and b-lg n = fixed effect of b-lg genotype (n = 1 to 3). The b-cn and k-cn were included in Model [2] as composite b-k-cn grouped into nine genotype classes (Figure 1). Because the k-cn B allele was rare (.8), the AB, BB, and BE genotypes of k-cn were combined within each b-cn genotype. Being almost monomorphic, a s1 -CN was not considered in the formation of the composite casein genotypes. The b-lg polymorphism was grouped into three genotype classes: AA (n = 88), AB (n = 366), and BB (n = 421). Other effects, assumptions, and statistical procedures used with Model [2] were identical to those used with Model [1]. Means and Variation RESULTS AND DISCUSSION The milk coagulation properties varied considerably (Table 1). About 8% of the milk samples did not coagulate in 3 min (curd firmness =. mm), and, thus, the distribution for the curd firmness was skewed toward the lowest (i.e., the most undesirable) values. In addition, every third milk sample did not reach a curd firmness of 2 mm in 3 min. The number of samples for the coagulation time and, especially, for the curd-firming time was, therefore, lower than for curd firmness. The curd-firming time was thus excluded from further statistical analyses. Means and variation for the milk coagulation and milk production traits (Table 1) were of the same magnitude as those reported by Ikonen et al. (12). Factors Affecting the Milk Coagulation Properties Parity. Milk yield, SCC, and ph increased with parity (Table 2). Both SCC and ph usually have an

4 28 IKONEN ET AL. Figure 1. Estimates of effect of the b-k-cn genotypes on the milk coagulation properties. 1 AB (n = 5) + BB (n = 1) + BE (n = 26); 2 AB (n =8)+BB(n =2) +BE(n =6); 3AB (n = 13) + BB (n = 1). unfavorable effect on the milk coagulation properties [e.g., (26, 27)]. Parity had, however, no statistically significant ( P =.423) effect on the milk coagulation properties. Similar results were observed also in some other studies (4, 15, 26), but, in the study of Schaar et al. (28), the milk coagulation properties improved with parity. Stage of lactation. The milk coagulation properties were at their best during the 1st mo of lactation (5 to 3 d after calving) and again from the 9th mo (>24 d after calving) onward (Table 3). The changes in the curd firmness over the course of lactation were parallel to those in protein and fat percentages and in SCC and were almost opposite to the changes in milk yield over lactation (Table 3). This result agrees with those reported in (12, 14), but, in others (4, 24), the milk coagulation properties deteriorated as lactation proceeded. In (15, 26, 28), stage of lactation had no effect on the milk coagulation properties. Confounded effects for stage of lactation and season in some of the previous studies may partially explain the contradictory results for the effect of stage of lactation on the milk coagulation properties. Breed. The milk coagulation properties were on average better for the FFr than for the FAy cows (Table 4), which was in part because NC milk was found only in the FAy. Also a difference between the breeds in ph of milk explained some of the differences TABLE 1. Means and variation in the milk coagulation and milk production traits. 1Number of cows and observations. 2Coefficient of variation, expressed as a percentage. 3Milk samples that coagulated in 3 min. n 1 X Minimum Maximum CV 2 Coagulation time, 3 min Curd-firming time, min Curd firmness, mm Curd firmness, 3 mm ph Daily milk yield, kg Fat content, % Protein content, % Log-transformed SCC

5 GENETIC PARAMETERS FOR MILK COAGULATION 29 TABLE 2. Estimates (Est.) of effect of parity relative to the first parity class on the milk coagulation and milk production traits. Parity to 9 (n = 313) (n = 24) (n = 148) (n = 174) P Est. SE Est. SE Est. SE Coagulation time, 1 min Curd firmness, mm ph <.1 Daily milk yield, kg <.1 Fat content, % Protein content, % <.1 Log-transformed SCC <.1 1Milk samples that coagulated in 3 min. in the coagulation properties between the breeds. The fat content of milk was higher for the FAy, whereas there was no difference in protein content of milk between the breeds. The k-cn B allele, which had a favorable effect on the milk coagulation properties (Figure 1), was more frequent (chi-square test, P <.1) in FFr (.14) than in FAy (.7). Casein polymorphism explained only a negligible part of the differences in the milk coagulation properties between the breeds because the differences remained statistically significant after the records were adjusted for the milk protein genotype effects (Table 4). In the study of Macheboeuf et al. (19), the more favorable milk coagulation properties of Montbéliarde and Tarentaise cows compared with those of Holstein cows were mostly due to differences between the breeds in the distribution of the k- CN alleles and in the casein content of milk. Herd. There was variation in the milk coagulation time ( P <.5), milk yield, ph, fat percentage, and protein percentage ( P <.1 for each previous trait) and in the frequency of the k-cn B allele between the herds (data not shown). As with the breed effect, differences in the previous traits between the herds were not due to k-cn polymorphism because the herd effect on these traits was statistically significant after the records were adjusted for the effects of milk protein genotypes. Erhardt et al. ( 6 ) found no difference in the protein percentage of milk between the herds with a high frequency (25.8%) of the k-cn B allele and those herds with a low frequency (9.7%), but fat percentage was higher in the herds with a high frequency. The β-κ-cn genotypes. Within each b-cn genotype, the k-cn B allele was associated with the most favorable coagulation properties (Figure 1), as has been observed in numerous studies (1, 4, 12, 26, 28, 29, 3). The k-cn E allele did not appear in combination with the b-cn A 2 A 2 genotype, but, within the other two b-cn genotypes, the k-cn E allele was TABLE 3. Estimates (Est.) of effect for stage of lactation relative to the fourth stage of lactation class on the milk coagulation and milk production traits. Days after calving 5 to 3 31 to 6 61 to to to 24 >24 (n = 56) (n = 84) (n = 192) (n = 195) (n = 191) (n = 157) P Est. SE Est. SE Est. SE Est. SE Est. SE Coagulation time, 1 min <.1 Curd firmness, mm <.1 ph <.1 Daily milk yield, kg <.1 Fat content, % <.1 Protein content, % <.1 Log-transformed SCC <.1 1Milk samples that coagulated in 3 min.

6 21 IKONEN ET AL. associated with the poorest milk coagulation properties. In addition, the frequency of the k-casein E allele was somewhat higher among the cows with a noncoagulating milk sample (.36) than among other cows (.29), but the difference between the groups was not significant. The unfavorable effect of the k- CN E allele compared with that of the B and A alleles was in agreement with the preliminary results obtained for the FAy and FFr cows (12) and with those reported by Lodes et al. (16) and by Oloffs et al. (25). Milk production, ph, fat percentage, protein percentage, or SCC were not affected by the b-k-cn genotypes, which agreed with the results reported by Ikonen et al. (12). The favorable effect of the k-cn B allele on protein percentage has been observed in some studies (2, 11, 2) but not in others (7, 18, 23). In addition to the favorable effect on milk coagulation properties, the k-cn B allele was associated with a high casein content (12, 28), with a high k-cn content (1, 12), and with small casein micelle size (17). Hence, the total protein content of milk may not give an accurate enough estimate of casein content or of casein composition of milk. It is, however, the casein that coagulates in the cheese-making process and together with fat forms the major part of the dry matter in cheese. The β-lg genotypes. The milk coagulation time was shortest for the b-lg AA genotype ( P <.1); the curd firmness was not affected by the b-lg genotypes (data not shown). The b-lg AA genotype had a favorable effect on some or all milk coagulation properties in some studies (19, 21, 3), but, in the study of Pagnacco and Caroli (26), the genotypes had no effect on these properties. Genetic Parameters for the Milk Coagulation Properties There was a negligible or no difference between the estimates of heritabilities and genetic correlations obtained using a univariate or bivariate model and a multivariate model. Standard errors of the estimates were somewhat lower with a multivariate model than with other models. Thus, estimates of heritabilities of the traits and genetic correlations between a milk coagulation trait and a milk production trait that were obtained using a multivariate model are presented (Table 5). The genetic correlation between the coagulation time and the curd firmness was estimated using a bivariate model because it was not considered necessary to include these highly correlated traits in a multivariate analysis. Heritability estimates. Heritability estimates for the milk coagulation properties were moderately high and were of the same magnitude as those estimated for the milk production traits (Table 5). The heritability estimate for the coagulation time obtained in this study was somewhat lower than those reported in other studies (15, 25), and the estimate for the curd firmness was of the same magnitude as those reported by Oloffs et al. (25) but was somewhat higher than that reported by Tervala et al. (29). When the records were adjusted for the b-k-cn and b-lg genotype effects, estimates of additive genetic variance for the coagulation time and the curd firmness decreased by 2 and 24%, respectively. Thus, additive genetic variation in these milk coagulation properties was partially due to milk protein polymorphism. This result was parallel to the distinct effect of the b-k-cn genotypes on the milk coagulation time TABLE 4. Estimates (Est.) of differences between the Finnish Ayrshire (FAy) (n = 789) and the Finnish Friesian (FFr) (n = 86) cows in milk coagulation and milk production traits. 1No milk protein genotypes in the model. 2b-k-CN and b-lg genotypes in the model. 3Milk samples that coagulated in 3 min. Model [1] 1 Model [2] 2 FAy FFr P FAy FFr P Est. SE Est. SE Coagulation time, 3 min < Curd firmness, mm < ph Daily milk yield, kg Fat content, % Protein content, % Log-transformed SCC

7 GENETIC PARAMETERS FOR MILK COAGULATION 211 TABLE 5. Heritability estimates (Est.) for the milk coagulation and milk production traits (on diagonal) and genetic correlations between the traits (upper triangle) Est. SE Est. SE Est. SE Est. SE Est. SE Est. SE Est. SE 1. Coagulation time, 1 min Curd firmness, mm ph Daily milk yield, kg Fat content, % Protein content, % Log-transformed SCC.9.3 1Milk samples that coagulated in 3 min. and the curd firmness and to that of the b-lg genotypes on the coagulation time. Estimates of additive genetic variation in the other traits changed little, if any, when the records were adjusted for the milk protein genotype effects, which agreed with the negligible effect of the genotypes on them. In the study by Oloffs et al. (25), additive genetic variation in the milk coagulation properties increased because of the inclusion of the milk protein genotypes in the model, which disagreed with the clear effect of the genotypes on these properties. Genetic correlations. The high genetic correlation observed between the coagulation time and the curd firmness (Table 5) was expected because these parameters describe the consecutive steps of the milk coagulation process. Except for ph and protein percentage, estimates for the genetic correlations between the milk coagulation properties and the milk production traits were not reliable because of high standard errors. The unfavorable association between the milk coagulation properties and high ph (Table 5) agreed with some reported results (15, 25). The positive correlation between protein percentage and coagulation time and the negative correlation between protein percentage and curd firmness (Table 5) were somewhat unexpected. The parallel changes in curd firmness and protein percentage of milk with stage of lactation (Table 3) implied an association between high protein percentage and favorable milk coagulation properties. Conversely, differences in the milk coagulation properties and protein percentage between parity (Table 2), breed (Table 4), or b-k-cn genotype classes (data not shown for protein percentage) were more or less divergent. In addition, there was no clear association between extreme breeding values for curd firmness and breeding values for protein percentage (Table 7). In the study of Oloffs et al. (25), genetic correlations between the milk coagulation properties and protein percentage could not be reliably estimated. High casein percentage, however, was associated with favorable milk coagulation properties (25), which also emphasized the importance of casein in milk coagulation. In contrast to the present results, Lindström et al. (15) reported that high protein and fat percentages were correlated with short milk coagulation times. NC Milk The NC milk found (12, 29) in the FAy breed in the small Finnish data sets was observed also in the data of the current study, in which 66 FAy cows (i.e., 8%) produced NC milk. Extremely poorly coagulating or NC milk, usually occurring in late lactation, had been observed among Holstein (24) and Friesian cows (4). In the data of this study, there were 1 evaluated FAy bulls that had at least 15 daughters and that formed three families based on their mutual sire, grandsire, or both (Table 7). In Family 1, the 2 bulls with the largest daughter groups in the data were closely related. A relatively large proportion of these daughters produced NC milk. Among the bulls in family 2, the proportions of daughters producing NC milk were equal to or less than those among the bulls in family 1; in family 3, there was only 1 bull having 2 daughters with NC milk. Consequently, 31 of the 66 cows with NC milk were descendants of the previous 7 bulls, which implies that genetic factors were partially responsible for the occurrence of NC milk. The rest of the cows with NC milk were daughters of 31 other bulls. These bulls were on average younger than the previous 7 bulls and had fewer daughters being milked at the time of the sample collection.

8 212 IKONEN ET AL. Five of the previous 7 bulls were homozygous for the k-cn locus (Table 7). The bulls genotypes were inferred from the genotypes of their daughters that were included in this study and in the study of Ikonen et al. (13). Among the daughters of the homozygous bulls, the k-cn A, B, and E alleles were present; there was no major difference in k-cn genotype frequencies between the daughters producing NC milk and the other daughters. This result implies that it was not the k-cn gene but rather some other gene or genes near the k-cn gene that were causing NC milk. Because there was great variation in the curd firmness within the daughter groups of the 7 bulls (Table 7), these bulls may be heterozygous for some of the potential genes causing NC milk. Additive Genetic Values for the Curd Firmness About 6% of the cows producing NC milk were primiparous cows; their proportion in the whole data set was 35%. Standardized (mean = 1, variation = 1) additive genetic values (EBV) for the curd firmness of the 875 cows with records were examined to determine whether there was any genetic trend in that trait over the cow birth years 1983 to The EBV estimated with Model [1] were used. Curd firmness was chosen because it was the coagulation trait for which each cow had an observation and for which the heritability estimate was highest. In addition to curd firmness, EBV for ph, milk yield, fat percentage, protein percentage, and SCC were also studied. The exact accuracies of the EBV were not calculated. Based on the heritability estimate for curd firmness, the accuracy of the EBV for that trait would be around.6 but was assumed to be somewhat higher because of information on parents and grandparents of the cows with records. The accuracy of the EBV for the other traits was of similar magnitude as for the curd firmness. No change was found in EBV for the curd firmness for cows born during the period from 1983 to 1991, but the cows born in 1992 and 1993 had, on average, the lowest breeding values for the trait. In addition, TABLE 6. The 1 evaluated Finnish Ayrshire (FAy) bulls with at least 15 daughters. 1 Bull genotype for k-cn 1Mutual pedigree (indicated by numbers) and information on curd firmness (CF) values of daughters milk. 2Paternal grandsire. 3Maternal grandsire. 4Number of daughters. 5Proportion of daughters with a noncoagulating milk sample. Pedigree Information on daughters PGS 2 Mean Range Sire MGS 3 n 4 %NC 5 of CF of CF Family to 4. EE to 48. AA 1 Family to 41. AE to 4. AA to 45. AA to 43. AE 3 Family to 34. AA to 44. AA to 49. AA to 39. AA

9 GENETIC PARAMETERS FOR MILK COAGULATION 213 these cows had high breeding values for ph, low values for fat percentage, but medium values for protein percentage. All daughters of bull and majority of the daughters of bull 3755 were born in 1992 or Consequently, one-third of the cows born in these years were daughters of the 2 previous bulls; the proportion of other half-sib groups was 5%. Frequent use of these 2 bulls, which had several daughters producing NC milk, was one likely explanation for the unfavorable genetic trend in the curd firmness for the period from 1992 to 1993 in the present data. Bulls and 3755 were extensively used in the entire FAy population during the 199s. Thus, frequent use of these bulls, their sons, and other relatives may have had an undesirable impact on the genetic level of the milk coagulation properties in the entire FAy population. Breeding for Favorable Milk Coagulation Properties Based on the moderately high heritability estimates for the milk coagulation properties there is clear potential for genetic improvement of these properties. Out of the 2 cows with the highest EBV for curd firmness, 17 had the k-cn AB, BB, or BE genotype; the cows with the lowest EBV had the AA, AE, or EE genotype (Table 7). Because determination of the milk coagulation properties in the entire cow population is not practical, these properties could be improved indirectly by favoring the k-cn B allele. One disadvantage of favoring the k-cn B allele might be its association with low milk and protein yields. Even though the k-cn B allele has no strong effect on milk and protein yields in the FAy (11), it occurs in linkage disequilibrium with the b-cn A 1 allele, which has been strongly associated with low milk and protein yields (11). However, in this study, there was no clear difference in the breeding values for milk yield between the cows with the highest breeding values for the curd firmness and carrying the k-cn B allele and the cows with the lowest breeding values for the curd firmness and carrying the k- CN A or E allele (Table 7). Based on the estimates of the genetic correlations between the milk coagulation properties and milk production traits obtained in this study and in others (15, 25), it is unclear whether the milk coagulation properties would be improved by selecting for any routinely recorded milk production trait. CONCLUSIONS Because of occurrence of NC milk in the FAy population and the probability of an unfavorable trend in TABLE 7. Breeding values for the 2 cows with the highest and the 2 cows with the lowest EBV for curd firmness (CF). 1Daily milk yield. 2F% = Fat percentage; P% = protein percentage. 3Phenotypic value for CF. 4k-CN Genotype. Highest EBV Lowest EBV Cow CF ph DMY 1 F% 2 P% SCC CFP 3 k-cn 4 Cow CF ph DMY F% P% SCC CFP k-cn AB AA AB AE AB AA AB AE BE AE AB AA BE AE BE AE AB AE AB EE AA EE BE AE AB AA AB EE EE AE AB AA AA AA BB EE BE AA AB AE

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