Analyses of the yield of milk, milk component, and energy in a Holstein, Ayrshire and Holstein-Ayrshire cross dairy cattle population.

A total of 238 records (101 Holstein, 63 Ayrshire and 74 crossbred) made by cows calving in a two year period from August 1967 to July 1969 inclusive at the Oyster River Research Farm were used to study: a) lactation yields of; milk, milk fat, protein, lactose, total organic solids, and energy; b) lactation average percent; milk fat, protein lactose and total organic solids; c) percent milk energy as; milk fat, protein and lactose; d) the ratio of fat production to protein production, and e) the energy concentration of the milk. Statistical analyses were by the least squares method of Harvey (16). Constants were fitted for the three breed groups, for the two years included in this study and for the season of lactation start. Constants were also fitted for all interactions. Number of days milked, age in days of the cow at parturition, and number of days open were used as co-variables. A significance level of 5 percent was used throughout. Breed group and the co-variables of days milked and age were significant for yield traits. The Holsteins and crossbreds did not differ significantly from each other but both groups had significantly higher yields than the Ayrshires. The least squares overall means for yields of; milk, milk fat, protein, lactose, total organic solids and energy were; 14,914 ± 177, 548 ± 7.3, 510.9 ± 6.1, 738.4 ± 9.2, 1,797.6 ± 21.2 lbs., and 4,946 ± 58.9 mcal respectively. The least squares constants for each breed for the above listed traits (in the same order) were: 1) Holstein; 1,239 ± 244, 14.2 ± 10.0, 27.2 ± 8.4 60.2 ± 12.7 lbs., and 238.6 ± 81.2 mcal; 2) crossbred 688 ± 240, 21.6 ± 9.9, 26.2 ± 8.2, 37.2 ± 12.5 lbs., and 225.6 ± 80.1 mcals; and 3) Ayrshire; -1927 ± 256, -35.8 ± 10.5, -53.4 ± 8.8, -97 ± 13.3, -186.5 ± 30.7 lbs., and -464 ± 85.1 mcals. The partial regression coefficients of all yield traits on number of days milked and on age at parturition were significant and positive. The overall least squares mean percent protein was 3.44 ± 0.015. The breed least squares constants for Holstein, crossbred and Ayrshire were; -0.11 ± 0.021, ).02 ± 0.021, and 0.09 ± 0.022 respectively. Differences between crossbred and Ayrshire means were non significant but both breed group means were significantly higher than the Holstein. Breed, season, breed by season and year by season were significant sources of variation for percent milk fat and percent total organic solids. Ayrshire means were higher than Holstein for all seasons. For winter calving, crossbred means did not differ significantly from Holstein; for summer calving, crossbred means did not differ significantly from Ayrshire. Differences among Holstein season means were non significant. Crossbreds and Ayrshires calving in the summer tended to have higher percentages than winter or spring calvers. Means of winter calving cows were higher in the second year than in the first year. Percent lactose increased from 4.88 ± 0.019 in the first year to 5.02 ± 0.017 in the second year. Significant standard partial regression coefficients for days milked were; percent milk fat 0.2 7 ± 0.06, percent lactose 0.2 8 ± 0.06, and percent total organic 0.31 ± 0.06. Breed, season, breed by season, year by season and days milked were all significant effects on percent milk energy as milk fat, the ratio of milk fat to protein and the energy concentration of the milk. Since percent milk fat is highly correlated with the above traits (correlation coefficients were 0.96, 0.86, and 0.97 respectively) the results were very similar to those obtained for percent milk fat. Season, year, breed by season and days milked were significant effects on percent milk energy as protein. Higher values were associated with winter calving than summer calving and the effect of season was greater on crossbred than on the other two breed groups. The standard partial regression coefficient for days milked was -0.2 8 ± 0.07. The means for the first year was 27.8 ± 0.20 and for the second was 27.2±0.25. Breed, season, year by season and age were significant for percent energy as lactose. Breed means, which were all significantly different from each other were 27.7 ± 0.22, 26.8 ± 0.20 and 25.6 ± 0.24 for Holstein, crossbred and Ayrshire respectively. Higher values were associated with winter calving and differences were greater in the first year than the second year. The standard partial regression coefficient for age was -.20 ± 0.06.Land and Food Systems, Faculty ofGraduat