The average conception rate (CR) to first insemination in 1969- 70 was 60.6% compared to 67.7% in 1982-83. CR declined with increasing herd size in the latter study (from 70.5% in herds of <101 cows to 66.0% in herds of >300 cows), but this trend was substantially less than the former study (from 62.8% to 52.3%). The change in herd size effects was mainly due to improved accuracy in the detection of oestrus in larger herds probably associated with the use of tail paint. On average, 50% of cows in 35 Waikato herds had calved by the end of a period of 18.3 d from the date of planned start of calving. The next 25% of cows calved over a period of 17.5 d, and the last 25% calved over 36.3 d. Consequently, the average calving covered 71.9 d from the planned start compared to 46 d in the Ruakura No.2 herd. A comparison with results from a 1972 survey showed that calving patterns had become more concentrated because of improved breeding management and the use of induced calving.
Proceedings of the New Zealand Society of Animal Production, Volume 44, , 57-60, 1984
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