V. Cow age and stage of oestrus at insemination

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Abstract
Records were obtained for 7616 inseminations with 0. 5 ml of Caprogen, containing catalase and at least 2.5 million total sperm, in 63 Taranaki dairy herds. Herd owners recorded at which milking or milkings each cow was observed in oestrus. Only about 21.6% of the inseminated cows were observed in oestrus at two consecutive milkings. Even though the interval between the p. m. and a. m. milkings was 13.2 h, 24.1% of the cows recorded in oestrus at a p. m. milking were still in oestrus next morning, whereas the comparable figure for cows first seen at an a. m. milking was 19.5%. These averages did not differ between 2-year-old, 3-year-old, and mature cows. The average oestrous period is estimated to be 14 to 15 h. The average conception rate (49-day % non-return rate) for 2-year-old cows was 64.3%, compared with 69.5% and 67.2% for 3-year-old and mature cows respectively. There was a higher proportion of short return intervals (1 to 17 days) among the 2-year-old cows. For cows seen in oestrus at two consecutive milkings the proportions of short and long (25 to 49 days) return intervals were 3.2% and 4.8% respectively, compared with 6.0% and 6.6% for cows observed only at a single milking. Adjusted conception rates for early-, mid-, late-, and post-oestrous inseminations were 66.9%, 69.8%, 74.7%, and 71.9% respectively. These results reflect differences in fertilisation rate which directly influenced the proportion and distribution of normal (18 to 24 days) return intervals.