Fertility and seasonal changes in liveweight of Droughtmaster cows grazing a Townsville stylo-spear grass pasture

Abstract

An experiment was conducted to examine the fertility and seasonal changes in liveweight of cows grazing a Townsville stylo–spear grass pasture for over 4 years. There were two replicates of 12 treatments, which were factorial combinations of two stocking rates and three levels of superphosphate with and without conservation of hay.

All experimental factors influenced cow liveweight. Cows on the pastures receiving superphosphate were heavier throughout the experiment than cows on unfertilized (F₀) pasture; in most years there was little difference between pastures receiving superphosphate at 377 kg/ha (F₃) and 126 kg/ha per year (F₁). Stocking rate and conservation of hay had smaller but significant effects throughout the experiment. There were also marked seasonal fluctuations in cow liveweight associated with the amount and distribution of rainfall.

Superphosphate was the only treatment that significantly affected conception and calving rate (F₃ > F₁, F₀ at P < 0.01). Numerical analyses published elsewhere of average cow liveweight before and after mating and of conception rate strongly indicated a direct effect of superphosphate on conception rate that could not be explained by liveweight differences. This effect was primarily associated with the highest rate of superphosphate (F₃), the F₁ effect being more equivocal.

There were significant differences between the three rates of superphosphate in the rate of change of cow liveweight during the first lactation (F₃ > F₁ > F₀ at P < 0.05) but the differences appeared inadequate to influence conception rate markedly.

Without fodder conservation, 1 cow unit to 2.4 ha was a safe stocking rate for fertilized pastures. This is three to four times the carrying capacity of unimproved spear grass pastures in the region.