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

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 strategies (Lax, Moderate and Intense) starting from two herbage masses (1200 or 1500 kg DM/ha) during early spring were used to provide data of potential ewe feed intake and feed quality from lambing to weaning for a breeding ewe flock. Ewe and lamb feed intakes and animal performance were then simulated using this data to represent the potential lamb and ewe liveweight changes using three different stocking rates (4.5, 8 and 10.5 twin-bearing ewes/ha) that closely represented the three defoliation strategies originally studied in the mowing trial. The impacts of these early-spring outcomes 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 again using simulation modelling. A high pasture cover (1500 kg DM/ha) at the start of lambing enabled higher per ha lamb weaning weights to be produced when an Intense grazing (10.5 ewes/ha) strategy was chosen, compared to all other grazing strategies and starting covers (506 vs < 471 kg lamb liveweight/ha, respectively). However, little extra benefit in terms of ewe and lamb performance was seen from the high starting pasture cover if Lax (4.5 ewes/ha) or Moderate (8 ewes/ha) grazing strategies were used. Lax grazing maximised both individual lamb weaning weight (35.9 kg) and ewe liveweight (73.4 kg), but produced the lowest per hectare lamb performance by weaning (282 kg lamb liveweight/ha). An Intense grazing strategy, irrespective of starting cover, produced the lightest individual ewes and lambs, but the greatest total lamb liveweight per hectare at weaning (471-506 kg lamb liveweight/ha), while the Moderate grazing strategy was intermediate (441-452 kg lamb liveweight/ha). For all four geo-climatic regions studied, the Intense grazing strategy generally produced the greatest gross revenue per ha from lamb sales out of the three grazing strategies investigated. 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 improved the gross revenue received from lamb sales even further especially in the Canterbury and Waikato environments but less so in the Hawkes Bay and Southland environments. These variations were the result of interactions between the weaning weight of lambs and the post-weaning pasture growth and/or feed quality depending on the geo-climatic region.