Potential impacts of different spring grazing strategies on lamb finishing in hill country

Abstract

How does the pasture cover at the beginning of lambing and the grazing strategy used in early spring influence the ability of a hill country sheep and beef farm to finish lambs? The results from an intensive pasture-mowing trial investigating three defoliation intensities (Lax, Moderate and Intense) starting from two herbage masses (1200 or 1500 kg DM/ha) during early spring were used to provide data on potential pasture consumption and feed quality from lambing to weaning for a breeding ewe flock. Using simulation modelling, this pasture data was used to estimate the potential lamb and ewe liveweight changes from birth to weaning at each of three different stocking rates (4.5, 8 and 10.5 twin-bearing ewes/ha) that closely represented the three defoliation intensities originally studied in a mowing trial. The impacts of these early-spring herbage masses and grazing intensities/ stocking rates on subsequent feed requirements and potential lamb finishing during the later summer and autumn period in four contrasting geo-climatic regions of New Zealand were then further investigated using simulation modelling. Variations in ewe and lamb weaning weights were evident and reflected the amount of pasture available through the different grazing management approaches. For all four geoclimatic regions studied, the Intense grazing, high stocking rate strategy generally produced the greatest gross revenue per ha from lamb sales out of the three grazing strategies investigated, though not significantly greater than the moderate stocking rate in summer dry environments. However, under Intense grazing, a high pasture cover (1500 kg DM/ha), in comparison to a low pasture cover (1200 kg DM/ha) at the start of ewe lactation further improved the gross revenue received from lamb sales, especially in the Canterbury and Waikato environments but less so in the Hawke's Bay and Southland environments. These variations demonstrate how iterations based on the same starting properties impact on weaning weight of lambs and post-weaning pasture growth and/or feed quality. Keywords: emergence, feed supply, geo-climatic regions, iteration, lactation, lamb weaning weight, pasture quality.