II. Interactions between conception rate and submission rate on the proportion of the herd reported in calf to A.B.

Abstract

Ninety-seven herds in the Wellington-Hawke's Bay Herd Improvement Association in which at least 90% of the cows were artificially inseminated, and which used A.B. exclusively for at least 7 weeks, were divided into three submission rate (S.R.) categories: LOW S.R. — less than 80% of the cows inseminated during (he first 4 weeks of mating (28 herds); AVERAGE S.R. — at least 80% but less than 90% inseminated (46 herds); and HIGH S.R. — at least 90% inseminated (23 herds). Within each category herds were reclassified into three conception rate (C.R.) groups: LOW C.R. with a 49-day non-return rate for all first inseminations of less than 57%; AVERAGE C.R. with a non-return rate of at least 57% but less than 63%; and HIGH C.R. with a non-return rate of at least 63%.

The proportions of each herd recorded as in calf to A.B. at the end of the seventh week of mating (7-wcek H.I.R.) in the three LOW S.R. groups were: LOW C.R. 66.4%; AVERAGE C.R. 66.6%; and HIGH C.R. 78.2%. In the AVERAGE S.R. groups the comparable results were 69.2. 72.8, and 75.1%, and in the HIGH S.R. groups they were 74.3. 76.3, and 79.2% respectively. The effect of C.R. on the H.I.R. was most pronounced in herds with a low S.R. Management techniques aimed at obtaining a high S.R. are important if any unpredictable drop in C.R. is not to affect the 7-week H.I.R.

The results show that over 90% of the cows in a herd should be artificially inseminated during a 7-week mating period if all future replacement heifers are to be A.B. progeny. If any allowance is to be made for binomial variation in the within-herd sex ratio. there is no possibility of selecting only particular cows to inseminate for future replacement stock. The simple recommendation is to inseminate all cows as quickly as possible during an A.B. mating period of at least 7 weeks.