Simulating change in cattle liveweight during the transition period in a dry monsoonal climate

The change in liveweight of tropical crossbred steers grazing Townsville stylo or mixed Townsville stylo–sabi grass pastures was recorded for 3 years during the annual transition period between the dry and wet seasons, a period commonly associated with a loss of liveweight. The experiment was conducted at Katherine, Northern Territory (14.3°S., 132.3°E.). A non-linear regression model was developed relating weekly changes in liveweight to the quantity of living fodder on offer at the commencement of the week and to the rainfall for the week. The model was used to estimate the mean start and duration of the period during which the rate of gain in liveweight is below the dry season rate, on the basis of local meteorological data covering 25 years. Predictions from the model were compatible with previous findings. The root mean square error of the model increased when the yield of either nitrogen, phosphorus or total digestible nutrient was substituted for the yield of living dry matter. Therefore the difference in yield of living dry matter, rather than its botanical composition, accounted for the different rates of gain in liveweight recorded on the two pasture treatments. Consequently the inclusion of the perennial sabi grass allowed higher gains in liveweight (or less loss) because of the higher growth rate (and therefore greater living dry matter yield) of the perennial grass compared with the annual Townsville stylo. Furthermore it is suggested that predictions from the model could apply to other pasture species provided that the quality of juvenile growth during the transition period is relatively constant.