Embryo mortality and early post-oestrous cycle embryonic death estimated from oestrous cycle lengths and milk progesterone analysis

An estimate of embryo mortality (cycles longer than 28 days) was obtained from milk progesterone analysis and delayed return rate in two dairy herds. Oestrus cycle lengths were measured, and cycles grouped according to whether artificial insemination (AI) had, or had not, been performed. Early post-oestrous cycle embryonic death (cycles extended beyond the average of 21 days, yet less than 28 days in length) was estimated by examining the changes in distribution of the two groups of oestrous cycle lengths. Embryo mortality occurring between 28 and 75 days after AI was measured at 15,2% using progesterone analysis and 21,6% using delayed returns. A 23,3% increase (P < 0,001) in the number of extended cycles (22 28 days) in the mated group of cows was found. This change in distribution of the length of mated and unmated groups of cycles was significantly different, and was assumed to represent early post-oestrous cycle embryonic death. Some implications of these findings are discussed.