The relation of the post-partum breeding interval to reproductive efficiency in the dairy cow.

F OR profitable milk production the dairy cow must reproduce regularly. An interval of i~ to 13 months between calves usually is considered satisfactory. Cows calving every i~ months or less had higher yields per day for the interval between calves than did cows that required more than ~ months before reproducing in studies by Gaines and Palfrey (i931) and Chapman and Casida (I935). This observation suggests that optimum milk-producing efficiency would be obtained by breeding cows as soon as possible after calving. Such a practice fails to consider the other possible physiological aspects of the problem. Casida and Venzke (I936) found that approximately 26 days were required following calving for the uterus to return to normal position, tonus, and size. Hofstad (194I) in a study of 309 conceptions in a dairy herd in New York reported that 34 cows bred before the sixtieth day following calving required a higher average number of services per conception than did cows first bred more than 60 days after calving. The percentage of abortions and cases of metritis, dystocia, and retained placenta were much higher in the animals bred at less than 6o days post-partum than in those first bred later than that date. In a study of ~9I range cattle, Lasley and Bogart (t943) found that of 35 cows bred ro to 40 days after calving 48.6 percent settled with one insemination, but a maximum fertility of 75 percent conception with one insemination was obtained in 12 cows bred i6i to 19 o days after calving. Erb and Shaw (I948) reported in a summary of a survey of breeding failures in Washington that there was an apparent improvement in breeding efficiency by waiting at/east 50 days after calving before rebreeding. Similar improved results for delayed breeding have been reported by Jennings (I94I) in breeding mares less than 9 days following foaling as compared with allowing more rest after foaling before rebreeding. While the literature indicates that lowered reproductive efficiency may result from breeding too soon after calving, the evidence is limited. Records in one of the artificial breeding units in Illinois show that go to ~5 percent of the cows are being serviced in a post-partum breeding interval of less than 60 days. This practice and the limited evidence available concerning