Effect of P-status on phosphorus absorption and excretion rates and reproductive function in pregnant and lactating beef heifers

Summary

The study was performed at Lansdown Pasture Research Station (Townsville) using 21 maiden Droughtmaster heifers. The heifers grazed three pasture types, selected to represent an improved high soil P pasture (Sabi grass + stylo), an improved low soil P pasture and an unfertilized native grasslstylo pasture. P kinetic studies were undertaken in June, October and December of 1990 and in May 1991 to correspond with early to mid-pregnancy, pre-calving, post-calving and late lactational physiological stages. Additional measurements of dietary P concentration, digestibility of forage, plasma inorganic phosphate and faecal P concentration, were made between the P kinetic studies, particularly during lactation when the demand for P was highest. Post-partum reproductive function was monitored by regular plasma progesterone assay. Rib bone biopsy and tail bone densitometry were used to determine the treatment effects on animal P status. The weather detracted from the success of this project as there were late rains in May-June 1990 which extendkd the growing season through much of the normal dry season and pregnancy period of the cows. Then shortly after calving very heavy rain, causing extensive flooding and waterlogging of the pastures, was followed by no significant further rain after February 1991.

The result of these abnormal weather conditions was better than normal pastures, in terms of yield, % green matter and nutrient composition during much of pregnancy (1990) but poorer pastures during lactation in 1991. There were substantial differences between treatments in the liveweights of the animals, the cows on the fertilized improved pasture lost less liveweight than those grazing the other pastures whilst the calves gained most weight. The differences between treatments in calf growth were associated with differences in milk production. The cows grazing the fertilized improved pasture had significantly higher intakes of dry matter and P during both pregnancy and lactation. Detailed measurements of the endogenous faecal P losses of the cows showed these values were low, much lower than those to be found in the literature and somewhat lower than those found previously in growing cattle on the same site in previous years. The coefficient of P absorption was high, higher than those reported elsewhere but similar to those recorded in growing cattle in previous years.

The treatments had no effect on reproductive parameters with cattle on all treatments returning to oestrous progressively between January and July. The P status of the cows as measured by plasma inorganic P was higher on the fertilized improved pasture than the unfertilized pastures but declined markedly during lactation. In June 1991, there was no difference between treatments, they were all below 20 mgA. At the start of lactation, densitometer measurements of the ninth coccygeal vertebra showed that the cattle grazing the fertilized improved pasture had better bone mineralisation, but this advantage had disappeared by the end of the lactation.

An alternative measurement using the rib biopsy technique also showed no differences between treatments at the end of lactation. Rib cortical bone thickness was marginal in all treatments. It was concluded that there are substantial production advantages in terms of liveweight gain and calf growth in grazing breeders on improved fertilized pastures. While there was no difference in post-partum reproductive performance in this study, it could be reasonably expected that the improved, fertilized pastures would confer an advantage in this trait under more normal seasonal conditions. It is also concluded that P utilization in breeding cows is very efficient as a result of high absorption coefficients of dietary P and low excretion rates of endogenous P. Current published recommended P requirements for breeding cows are much too high. The data from this study will allow provisional estimates of the desirable P intakes of breeding cattle in the same way as has been determined for growing cattle.