The effects of mixed v. mono-grazing by steers and sheep on individual animal growth rate, pasture carrying capacity and live-weight output per ha were measured in a 4-year (1978 to 1981) experiment, after a preliminary familiarization year, 1977. Annual stocking rate treatments consisted of three monosteer, three mono-sheep and seven mixtures of steers and sheep. Annual average stocking rates were 2·11 steers † 8·1 ewes per ha under mixed grazing and, under mono-grazing, 4·44 steers and 15·2 ewes per ha. The range from low to high in stocking rate in mono- or mixed grazing was close to 40%. Over the 4 years a total of about 280 steers, 900 ewes and 1100 lambs were used.
Overall, mixed grazing increased average lamb daily live-weight gain (ADG) to weaning and to drafting from 246 to 265 g (P < 0001) and from 211 to 223 g (P < 0·001) respectively. Steer ADG for these periods was increased from 1·419 to 1·520 kg (P < 0·01) and from 0·950 to 1·094 kg (P < 0·001). The choice of reference mono-grazing stocking rates for comparisons of mixed v. mono-grazing ADG can affect these results. Average live-weight outputs (kg/ha) from grazing for the mono-steers, mono-sheep, and mixed grazing were 663, 690 and 714, the range in the latter over the seven mixed grazing treatments being from 605 to 805. Stocking rate was the main factor affecting output per ha. Because of the management rules used in this experiment mixed grazing effects are more appropriately assessed through ADG and measures based on it than on output per unit of area.
Models were fitted relating steer and lamb ADG to lamb weaning and lamb and steer drafting to stocking rates of steers and ewes. Mixed grazing benefits to steer and lamb ADG to drafting were greater as their proportion in the mix declined and increased with stocking rate. At the 50% proportion, lamb and steer ADG were improved by 5·2 and 3·4% respectively at low stocking rate and 9·4 and 6·6 at high stocking rate. Predicted steer ADG to lamb weaning for a given steer stocking rate increased with increases in ewe proportion up to five ewes per ha and decreased rapidly with further increments in ewe proportion.
Mixed grazing efficiency was also evaluated through the Relative Resource Total. This showed that under mono-grazing 10 to 13% more area was required to produce the same grazing season output as under mixed grazing. The 10% improvement in carrying capacity was exceeded for ewe: steer frequencies ranging from 1·5: 1 to 10: 1. Explanations for this greater efficiency in resource capture/use under mixed grazing are discussed.
Substitution rates for lamb ADG to weaning (2·35) and to drafting (2·86) and for steer ADG to drafting (0·21) were fairly constant over the 4 years 1978 to 1981.
Selection of mixed stocking rates to suit growth rate targets for different animal types and to match food supply with demand under varying soil/climatic/topographical conditions is discussed.