Plasma progesterone concentrations from before puberty to after parturition in British Friesian heifers reared on high planes of nutrition and inseminated at their first oestrus

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Abstract
Five autumn-born British Friesian heifers were given a high-fat milk substitute ad libitum to 12 weeks of age followed by skim milk ad libitum to 20 weeks of age and were weaned at 26 weeks of age (Treatment 1). Nine heifers were offered the high-fat milk substitute to 8 weeks of age and were weaned at 12 weeks of age (Treatment 2). First oestrus was detected by a study of cervical secretions and plasma progesterone concentrations.Heifers on Treatment 1 were significantly heavier at 12 weeks of age, but there was no difference between treatments in the mean age or weight of the heifers at first oestrus. Ten heifers conceived at their first or second oestrus (Group A) and four heifers required four to nine inseminations to establish pregnancy (Group B).Age at first oestrus was inversely related to pre-pubertal progesterone concentrations, which tended to be higher in Group B heifers. All heifers showed a small peak in progesterone concentration before first oestrus, the height being significantly greater in Group B than in group A heifers.Progesterone concentrations in cycles preceding a successful insemination tended to be lower in the 2 days before oestrus. Progesterone concentrations during pregnancy were significantly lower for the Group B heifers, and were significantly lower for all heifers during the middle 4 months of gestation than during the preceding or succeeding 2 months, with a marked dip at 140 to 160 days of gestation. Post-partum basal progesterone concentrations did not differ from prepubertal concentrations; small peaks of progesterone tended to occur before the first oestrus post partum.