Intensive beef production on cultivated pastures in a sub-tropical environment.
The comparative performance of spring-calving beef cowherds comprising either Dual Purpose (Simmentaler) or British-cross (Hereford x Africaner) cows and calves was investigated at stocking rates of 3,0, 4,12, 5,34 and 6,74 cows and calves/ha on Kikuyu pasture over three seasons. An increase in stocking rate was associated with a decrease in the length of the grazing period, an increase in the mass and condition loss of cows, a decrease in calf livemass gains, a decrease in milk production, an increase in the production of beef per hectare and a decrease in margins over feed costs per cow. Stocking rate did not influence reconception rates, which averaged 65% over the three seasons. Simmentaler cows produced calves approximately 2 kg heavier at birth and 24 kg heavier at weaning, approximately 1,4 litres of milk more per day, lost more condition and attained higher margins over feed costs per cow and per hectare than Hereford-cross cows. Season influenced the length of the grazing season, birth mass, livemass gains in calves and mass changes in cows. Milk production accounted for 40% of the variation in calf gains on pasture. The conversion of milk to calf gain was influenced by milk yield, stocking rate, breed and calf sex. Ovarian and endocrinological responses associated with normal and restricted suckling for a 15-day period at two postpartum stages (Days 35 to 50 and 60 to 75) were examined in Bos taurus and Bos indicus cows. All cows received progestagen therapy, followed by GnRh administration during the periods of variable suckling intensity. Restricted suckling exerted little influence in Bos taurus cows, but tended to improve the ovulatory response, to decrease the incidence of "short" cycles and to increase reconception rates in Bos indicus cows. Treatment exerted no influence on calf growth and on tonic LH and oestrogen secretion. Bos indicus cows suckled normally released more LH in response to GnRh than Bos taurus cows. Bos indicus cows suckled once daily secreted less LH in response to GnRh than those suckled normally. Seasonal effects on tonic LH, and progesterone secretion were studied in cycling Friesland cows. Mean tonic LH levels during autumn were significantly higher than those during summer and spring. Season. exerted no influence on progesterone secretion.