Milk progesterone profiles and factors affecting post partum ovarian activity in beef cows

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Abstract
Progesterone concentration was measured in milk samples collected twice or three times weekly from 157 suckling beef cows in three herds during the period from parturition until the resumption of ovarian cycles. The length of the post partum acyclic period varied between cows from 11 to 125 days. The length of the acyclic period did not differ between cows suckling one or two calves or between the two breeds of cow studied. There were significant negative correlations between the length of the acyclic period and 1. body weight at calving (r = −0·394; P<0·001) and 2. days from calving to the next longest day of the year (r=−0·782; P<0·001). Cows calving between February and April were acyclic significantly longer (83 days) than those calving between August and December (44 days; s.e. of difference = 3·5; P < 0·001). In one herd there was a significant negative correlation between the length of the acyclic period and order of calving both in spring (r=−0·379; P<0·05)- and autumn/winter (r=− 0·349; P < 0·05)-calving cows. Of the 40 spring-calving cows in the same herd, 17 were served naturally prior to any rise in milk progesterone concentrations and 11 of these conceived to this service.