Trade-offs between productivity and ground cover in mixed farming systems in the Murrumbidgee catchment of New South Wales. (Special Issue: Grain & Graze: an innovative triple bottom line approach to collaborative and multidisciplinary mixed-farming systems research, development and extension.)

摘要

Inadequate ground cover because of over-grazing of pastures, fallowing and stubble burning or excessive cultivation exposes land to degradation, yet maintaining cover can constrain productivity. We used long-term simulation analysis to investigate the impact of modifying stock and crop management practices on the trade-off between farm productivity (grain, meat and wool) and ground cover levels. Management of mixed farms at five locations (426-657 mm mean annual rainfall) in the Murrumbidgee catchment of New South Wales was represented for simulated farms that included permanent pastures, crop-pasture rotations, and a meat/wool sheep enterprise. Locations varied in their stocking rate, soil characteristics and proportion of land area allocated to various crops and pastures. At each location we investigated the impact on ground cover and farm productivity of (1) retention of wheat stubble, (2) altering stocking rate (up to +or-25%) and (3) moving stock elsewhere on the farm when the mean value of ground cover fell below a given threshold (50, 60, 70, 80 or 90%) for either, all pasture on the farm or an individual paddock. The analysis showed that seasonal conditions were the dominant effect on the mean farm cover (mean range 64-98%); cover fell as low as 43-57% in severe droughts, depending on location. Retention of wheat stubble increased long-term mean cover by 1-4%, with little impact on grain yield. Altering stocking rate had the largest impact on cover at all locations (up to 4%), although there was a trade-off with productivity. Maintaining a constant stocking rate, although moving stock within the farm, had a small, positive effect on the whole-farm cover, although it reduced gross margin because of the cost of supplementary feeding. Confining stock to a 3 ha feeding lot was the most effective strategy for maintaining ground cover and minimising financial loss. Our analysis did not account for the benefits of maintaining high levels of ground cover on the long-term productive capacity of the farm through on-farm or off-farm environmental benefits. We conclude that ground cover is dominated by climatic effects, and that within the range of stock management practices examined in the present study, practices that increase minimum cover levels also reduce farm productivity. Reducing total stocking rate, or removing stock from the farm during periods of low cover, will have the greatest impact on maintaining cover, although it will reduce whole-farm profit.