Interrelationships between metabolic status and the onset of ovulation a puberty and postpartum in beef cattle

Nutrition is known to have an important bearing on reproductive function in female cattle. Major reproductive events that are influenced by nutrition include the onset of ovulation at puberty and the resumption of ovulation postpartum. Although the impact of nutrition on reproduction in female cattle is generally recognised, information is lacking on details of the interrelationships between nutrition, metabolic homeostasis and ovarian function. This applies in particular to beef cattle maintained on subtropical pastures. The primary aim in the present thesis was to characterise the interrelationships between nutrition, metabolic homeostasis and ovarian function in Zebu (Brahman, Bos indicus) and Zebu crossbred (Droughtmaster, Brahman x Shorthorn (Bos taurs)) beef cattle in relation to the onset of ovulation at puberty (Brahman) and resumption of ovulation postpartum (Droughtmaster). In the first major study, pre-pubertal Brahman heifers were assigned to either moderate nutrition (MN, n = 11)) or improved nutrition (IN, n = 11). Supplementation with cottonseed meal and steamed flaked barley was used to ensure differences in live weight gain and body condition score (BCS) between heifers on MN and IN. Heifers were monitored at 2-week intervals for body weight, BCS and circulating concentrations of GH, IGF-1, leptin, insulin and glucose. Ovarian follicular status was recorded by ultrasonography at the same time. Liver biopsies were obtained at 8-10 weeks intervals. Faecal samples were collected at the time of liver biopsies to ascertain dietary crud protein (CP) and dry matter digestibility (DMD). Pastures were sampled on six occasions at approximately 8-10 weeks intervals to determine trace mineral content. The trace minerals of particular interest were copper (Cu), manganese (Mn) and zinc (Zn). Heifers on IN had greater (P 0.05) between the IN and MN for trace minerals. The analysis of liver concentrations of Cu, Mn and Zn indicated that there was no difference (P > 0.5) between heifers on MN and IN. Heifers on IN had greater (P 0.05) between the heifers on IN and heifers on MN. All heifers on IN had attained puberty by 23 months of age whilst only one heifer on MN had reached puberty at the same time. Puberty in heifers on IN appeared to be influenced, in part, by increasing day length as the majority of heifers attained puberty over a 3-month period between 21 and 23 months of age which coincided with the winter-to-spring transition. It was concluded from the findings that heifers on IN had a generally improved metabolic homeostasis which was linked with greater body weight and body condition, and this induced an earlier attainment of puberty. In the second major study, Droughtmaster cows were assigned to moderate pasture (MP, n = 7) or improved pasture (IP, n = 7) from 6 to 7 months of gestation until 6 months after calving. Starting at 1 week after calving, cows were monitored for body weight and body condition at 2-week intervals. Circulating concentrations of metabolic regulators (GH, IGF-1, leptin, insulin and glucose) were monitored weekly and ovarian follicular function was recorded at the same time by rectal ultrasonography. Faecal samples were collected at 3-week intervals to determined dietary CP and DMD. Pastures were sampled on four occasions at 5-weeks intervals to ascertain trace mineral status. Cows underwent liver biopsies at 3-week intervals during the postpartum period to determine the liver concentrations of trace elements (Cu, Mn and Zn) and to evaluate gene expression. Cows on IP had greater (P 0.05) for liver concentrations of Mn and Zn between cows on MP and IP, but the latter cows had greater (P 0.05) between the cows on IP compared with cows on MP. All cows on IP had resumed ovulation by Week 16 postpartum whereas only 1 of 7 cows on MP had shown ovulation by Week 19 (P < 0.01). It was concluded from the findings that cows on IP had a significantly improved metabolic homeostasis than cows on MP and that this induced an earlier resumption of ovulation postpartum. This is the first study to demonstrate long- term differences in metabolic homeostasis achieved by pasture-based nutritional management in postpartum beef cows. The most notable feature of liver gene expression was an apparent greater expression of genes linked with fatty acid and cholesterol metabolism and IGF-1 at Week 6 postpartum for cows on IP compared with cows on MP. It is not known how an apparent increase in expression of these genes at Week 6 postpartum might be temporally related to the resumption of ovulation between Weeks 11 and 16 for cows on IP. The above reseach has provided important new information on the interrelationships between nutrition, metabolic homeostasis and reproductive function in Zebu and Zebu crossbred pubertal heifers and postpartum cows. Lont-term nutritional management can be used to achieve differences in metabolic homeostasis that are related to the initiation of ovulation at puberty and resumption of ovulation postpartum. Differences in circulating concentrations of IGf-1, leptin, insulin and glucose that are linked to the initiation (puberty) and resumption (postpartum) of ovulation could be interpreted to suggest that these metabolic indicators act at both the brain and somatic tissues to influence reproduction in female cattle. Potential actions of metabolic indicators at the brain to influence reproduction are particularly intriguing given the presence of specific receptors for IGF-1 on GnRH neurons and leptin on KISS-1 neurons within the hypothalamus