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Twelve Hereford Friesian cows were allocated to either a high plane (HP) or a low plane of nutrition (LP) post-partum. HP animals received rations supplying sufficient energy and protein for maintenance plus 10 kg/day milk yield. LP cows were offered approximately 70% of their maintenance needs for 63 days post-partum. Thereafter they received the same ration as the HP animals. A mineral vitamin supplement was included in all concentrates fed. Calves were only permitted to suckle twice daily and all animals were weighed once weekly. Blood samples were taken once daily and the levels of serum progesterone, luteinizing hormone (LH) and prolactin (PRL) estimated. Once weekly blood samples and diurnal samples (once fortnightly) were measured for their content of insulin, non esterified fatty acids (NEFA), glucose, total protein, albumin and urea. Oestrus was detected by visual observation and by intra vaginal electrical resistivity and ovulation was confirmed by rectal palpation. After 110 days all animals were synchronized with two injections of prostaglandin. This was followed by fixed time insemination at 72 and 96 hours. Time from parturition to first ovulation was: HP; 39 +/- 8.7 days; LP, 65,3 +/- 33,2 days. All HP animals showed regular cyclic ovarian activity whereas only 3 of the LP cows were normal (LPR). The ovarian activity of the remaining LP cows (LPI) was almost completely suppressed. Of the animals that cycled normally there was no difference in progesterone concentrations between treatments. Average LH concentrations did not differ between HP and LP or between LPR and LPI cows. HP cows maintained their body weight post-partum whilst the LP lost 21.4 +/- 4.40% by day 63. Milk yield of the LPI animals was consistently higher than that of the LPR and it was found that the nine regularly cyclic animals had a lower milk yield to body weight ratio than those that failed to cycle properly (P less than 0.05). Differences between the HP and LP cows were found in relation to insulin, PRL, glucose, NEFA, urea and albumin concentrations. No variation in these parameters was observed that could explain the divergence in ovarian activity of the LPR and LPI groups.

The effect of undernutrition of beef cows on blood hormone and metabolite concentrations post partum