The effect of undernutrition on some hormones and metabolites in the blood of intact and ovariectomized beef cows
The main objective of this work was to evaluate whether or not differences in ovarian function induced by severe under-nutrition of beef cows could be attributed to specific changes in one or more indicators of metabolic status in the blood. In the first experiment 12 Hereford x Friesian heifers were allocated at random to one of two nutritional levels post partum. The high plane (HP) animals (6) were offered a ration providing energy and protein requirements for maintenance plus a milk yield of 10 kg/day for 133 days while the low plane (LP) heifers received a ration providing approximately 70% of maintenance requirements for 63 days before being transferred to the HP diet. During the experiment the calves were only permitted to suckle for 30 minutes twice daily and the difference in calf weight before and after feeding was used as an estimate of milk yield. Throughout the trial the liveweight of the animals was measured weekly. Blood samples were taken at regular intervals during the trial and the following determinations were performed: glucose, non esterified fatty acids (NEFA), urea, total protein, albumin, insulin, prolactin, luteinizing hormone and progesterone. Onset of heat was estimated by visual observation and intra-vaginal electrical resistance measurement while ovulation was confirmed by rectal palpation. Between 103 and 126 days post partum the heifers were artificially inseminated (A.I.) having been synchronized by two prior injections of a prostaglandin analogues After 133 days the animals were turned out to grass with a bull. Nutrition had a significant effect on reproductive function with only 3 of the LP heifers recommencing normal cyclic activity after parturition. Subsequent analyses included a comparison of the low plane cyclic animals (LPR) with the low plane acyclic (LPI) heifers as well as a general comparison between the HP and LP groups. Significant differences between the HP and LP were found in terms of body weight, milk yield, glucose, urea, NEJTA, albumin and prolactir but the only parameter to show, any difference between the LPI and LPR groups was milk yield; yield was higher in the LPI group. Fertility was significantly impaired in the LP heifers with none conceiving to A.I. compared with 3 in HP group. Diurnal changes in several blood variables were recorded. A second experiment was performed in 3 cyclic INT and 3 ovariectomized (OVX) heifers. These animals were fed for 90 days on a diet providing for maintenance plus a growth rat of 0.5 kg/d (HPP). They were subsequently transferred to a low plane diet for 140 days (LPP). This diet provided approximately 50% of the energy and protein requirements for maintenance and during this period the heifers lost about 19.7% of their peak liveweight. During the experiment blood samples were obtained on both a daily and a diurnal basis. On several occasions during the trial adrenal function was tested with injections of ACTH and pituitary function with injections of TRH. In addition, pituitary function of the QVX heifers was assessed by injections of LHRH. Blood samples were assayed for the same constituents as in the first experiment with the additional measurement of total cortisol. Undenutrition was not found to have any influence on the cyclic ovarian activity of the XNT heifers but LH peak height tended to be higher at oestrus during the LPP, Levels of urea and NEFA were increased whereas concentrations of insulin, prolactin and cortisol were found to be lower during the LPP. Adrenal responsiveness was found to be increased during underfeeding only in the XNT animals. The response of prolactin to TRH injections altered with daylength but was found to be suppressed by low feed intake in the INT heifers. This did not appear to be the case with the OVX group. The OVX heifers exhibited a delay in the time to peak LH and an increase in total quantity of LH released as a result of underfeeding when LHRH was administered.