Relationship of corticoid, thyroid and metabolic profiles with fertility of beef females on sourveld and mixed sweetveld

English: During the 1981/82 season, first-calf Bonsmara cows on the sourveld (lower plane of natural pasture) supplemented with an NPN lick in winter (sourveld +N), and sourveld not supplemented with an NPN lick in winter (sourveld - N), had significantly lower mean blood (2.13; 2.12 and 2.30 mmol/l; P < 0.01), plasma (3.20; 3.10 and 3.38 mmol/l; P < 0.01) and red blood cell glucose (1.06; 0.98 and 1.09 mmol/l, P <0.05) concentrations, compared to the cows on mixed sweetveld (higher plane of natural pasture) supplemented with an NPN lick in the winter (sweetveld +N). Similar results were obtained with regard to total plasma protein, 66.0; 68.0 and 72.0 gil (P < 0.01), plasma albumin, 32.0; 32.0 and 34.0 gil (P < 0.01), and plasma globulin, 35.0; 36.0 and 38.0 gil (P <0.05) for the 3 treatments respectively. Plasma urea concentrations appeared to be a good indicator of protein status of first-calf cows on the 3 treatments, concentrations being 1.75; 1.35 and 3.75 mmol/l (P < 0.01) respectively.During the following year (1982/83), differences in energy and protein constituents were much smaller, and only plasma albumin and urea concentrations were indicative of the higher nutritive value of the mixed sweetveld. First-calf cows on the sourveld + N had significantly (P < 0.01) lower plasma cortisol concentrations than their counterparts on mixed sweetveld + N during the 1980 period (11.94 vs. 13.02 ng/ml respectively), and the 1981/82 period (8.78; 7.32 and 9.76 ng/ml for females in the 3 treatment groups respectively. During the subsequent year, virtually no differences in plasma cortisol levels were observed. No significant difference in plasma thyroxine concentrations was noted between experimental animals on the sourveld and mixed sweetveld during the 1980 period. During the following 2 seasons, plasma T4levels were significantly (P <0.001) elevated in the experimental animals on the mixed sweetveld (41.80; 40.80 and 48.1 0 ng/ml during 1981/82, and 39.10; 40.80 and 46.60 ng/ml during 1982/83 for females in the 3 treatment groups respectively. Pregnancy influenced the mentioned metabolites and hormones Bonsmara cows to varying degrees. Lactation suppressed blood and plasma glucose concentrations for 8 to 10weeks after parturition. The influence oflactation on total plasma protein, albumin and globulin concentrations was not significant, and variable. Plasma urea levels were considerably suppressed in lactating first-calf cows, compared to dry cows in the 3 treatment groups, viz. 1.28 vs. 1.96 (P < 0.05); 1.02 vs. 1.53 (P < 0.01) and 3.05 vs. 3.98 (P < 0.05) respectively. Little differences were noted during the subsequent year. The adrenal cortex and thyroid activity were suppressed by the effect of lactation in first-calf cows in all 3 treatment groups during the 1981/82 season. Cows having higher mean blood glucose concentrations after first parturition (8 weeks prior to mating) conceived at the next mating, compared to lower concentrations of infertile cows, viz. 2.49 vs. 1.93 mmolIl, and 2.20 vs. 2.07 mmol/l during the 1981/82 and 1982/83 seasons respectively. Similar results were obtained for plasma glucose (3.57 vs. 2.75, and 3.36 vs. 3.27 mmol/l respectively), plasma cortisol (8.2 vs. 4.0, and 8.3 vs. 5.3 ng/ml respectively) and plasma thyroxine (45.2 vs. 34.5, and 41.0 vs. 37.2 ng/ml respectively). Fertile cows were in a weight gaining phase from parturition to mating. Season of the year generally did not have a significant and consistent influence on blood and plasma metabolites. Energy, and some of the plasma protein metabolites, were significantly higher during the summer of 1981/82, than during the winter. During the following year (1982/83), results were generally reversed, with plasma albumin and urea levels being higher during the winter than summer seasons. Plasma cortisol concentrations were generally elevated during the summer season, compared to the winter season.