The effect of ewe wastage in New Zealand sheep and beef farms on flock productivity and farm profitability

Changes in the relative value of meat and coarse wool have led to the majority of income for New Zealand sheep farms now coming from the sale of animals for slaughter rather than wool, shifting the production focus for many farmers. Ewe wastage, on-farm mortality and premature culling before the potential end of the ewe's productive life, is a limiting factor for sheep meat production. Current estimates of wastage rates in New Zealand commercial ewe flocks range from approximately 3 to 20%. A bio-economic model of a representative New Zealand North Island Hill Country sheep farm was constructed to investigate the effects of wastage rates on farm profitability. The model used system dynamics with annual time steps over 30 years. The proportion of ewes that were culled up to 6 years of age and all mature ewe deaths combined were considered as wastage in this study. Modelled scenarios covered ewe wastage rates ranging from 5 to 21% for a self-replacing flock of 1879 ewes producing coarse wool and lambs for sale on 423 ha where 63% of total feed was consumed by sheep. Greater wastage of ewes resulted in higher numbers of young, less productive replacement ewes in the flock, lowering average flock age from 4.18 years to 3.54 years over the wastage rate range tested. The reproductive performance of the flock decreased with flock age from 1.33 lambs weaned per ewe presented for breeding to 1.22 lambs weaned per ewe presented for breeding with wastage rates of 5% and 21%, respectively. The lower numbers of lambs weaned and lower proportion available for sale reduced income from lamb sales. Subsequently, total cash operating surplus for the sheep enterprise of the representative farm was reduced by NZ$ 1069 per year per 1% increase in ewe wastage. For a New Zealand North Island Hill Country sheep farm with 15% of the flock lost due to wastage, a 10% reduction in wastage could increase cash profit by 17%. Sheep energy requirements were reduced across the production year with greater ewe wastage leading to a greater positive end of year cumulative feed surplus.