The effects of phosphorus supplementation on body mass and reproduction of grazing beef cows supplemented with different levels of phosphorus at Armoedsvlakte
In an attempt to quantify the requirement for supplementary phosphorus (P) of grazing beef cattle at Armoedsvlakte (which is notorious for its P deficient pastures), 66 cows received supplementary P per fistulam from 1984 to 1989. Supplementation was provided according to a 3 × 2 factorial of three levels of P (Low, Medium and High), in periods of either all year round (LP 12, MP 12 and HP .12) or six months only (LP 6, MP 6 and HP 6), i.e. September to February. The body mass of cows, averaged over the 59 months of the experiment, displayed no Period x Level interaction and, of the two main effects, only Period was significant 0.0162). Cows re-ceiving P all year round averaged 33 ± 13.0 kg heavier than those receiving P for only six months of the year. Regarding reproductive performance, the birth mass of calves varied significantly from year to year, but there was no year by treatment interaction, indicating that the treatment effects were consistent over years. Amongst treatment factors, birth mass varied only according to Level of P supplementation (P = 0.0463). Weaning percentage varied almost significantly according to Period of P supplementation (P = 0.0563) and displayed an almost significant linear response to Level of P supplementation (P = 0.0560). The joint significance level for this model was P = 0.0261. The weaning mass of calves and their average daily growth (ADG) showed significant differences only according to year of birth. The significant effect of treatment (level of P supplementation) on calf birth mass but not on weaning mass suggests that if there is a P deficiency, cows will draw on their reserves to shield the calf from such a deficiency. A financial analysis suggests that the greatest financial advantage would be accrued by supplementing reproducing beef cows according to the HP 12 treatment, i.e. 16 g P day-1 during late gestation to late lactation (September to February) and 9 g P day-1 during the remainder of the year.