Effects of dry season supplements of protein and phosphorus to pregnant cows on the incidence of first post-partum oestrus

Pregnant multiparous Brahman cross cows grazing native pasture at Samford in south-east Queensland were allotted to three treatment groups in late winter; one group received a daily supplement of 180 g crude protein (as peanut meal) plus 10 g phosphorus (P), the second group 10 g P and the third (control) group neither protein nor P. The P and control groups received approximately 320 g DOM per day, mainly composed of sugar and grain, to balance the DOM provided to the protein + P group as peanut meal. Supplementation continued for a period approximately spanning the third to eighth months of gestation. The protein plus phosphorus supplement significantly reduced the interval from calving to first postpartum oestrus by 46 per cent; all animals in this group had exhibited oestrus within two months of calving. By three months after calving, only 50 per cent of controls and 70 per cent of those given phosphorus had exhibited oestrus. Calf growth was unaffected by the supplements. Samples of grazed pasture were collected using non-pregnant cows with oesophageal fistulae. The provision of P or P plus protein tended to render the animals less selective in their grazing. The evidence suggested that the higher feed requirements of the pregnant cows removed any possible effect of the supplements on their selectivity. Native pasture provided a slightly sub maintenance ration during the period of supplementation, and a major limiting nutrient appeared to be protein.