Influence of genotype and yield and composition of milk on interval to first postpartum ovulation in milked beef and dairy cows

Our objectives were to investigate the effects of genotype, yield and composition of milk, and changes in BW and body condition on the initiation of cyclic ovarian activity in nonsuckled beef and dairy cows milked twice daily. Nulliparous heifers were selected from three breeds, 12 Angus, 13 Simmentals, and 26 Holsteins, based on their EBV for milk yield. Cows were machine-milked twice daily, and daily milk yield was recorded; composite samples of milk were collected twice weekly for analyses of milk components. Blood was collected from all cows thrice weekly for 75 d postpartum, and concentrations of progesterone in serum measured by RIA were used to estimate day of first postpartum ovulation. Holstein cows produced more (P 0.05) milk (unadjusted or 3.5% fat-corrected) than Angus or Simmental cows during the first 30 d of lactation. Holstein cows had higher (P 0.01) peak yield of milk than Simmental and Angus cows. Days to peak milk yield were similar for Simmental and Holstein cows, and both were more (P 0.05) than those for Angus cows. Percentages of fat, protein, and total solids in milk were highest (P 0.05) for Simmental cows, whereas milk of Holstein cows had the highest (P 0.05) percentage of lactose and lowest (P 0.05) concentration of somatic cells. Average BW at ovulation differed (P 0.05) among breeds. Estimated daily changes in BW from calving to first ovulation were different (P 0.05) for Holstein and Simmental cows but were similar to those of Angus cows. Holstein cows had poorer (P 0.05) body condition at ovulation than Angus and Simmental cows. Angus cows (36 +/- 4 d) had longer (P 0.05) intervals to first ovulation than Holstein (20 +/- 3 d) and Simmental cows (22 +/- 4 d). At 30 d after calving, 50, 88, and 92% of the cows had initiated ovarian cyclicity in the Angus, Simmental, and Holstein breeds, respectively. We conclude that, in the absence of suckling, genotype, but not changes in BW, body condition, or yield and composition of milk, is a major factor influencing postpartum interval to first ovulation in nonsuckled, milked Angus, Holstein, and Simmental cows