System impacts of introducing crop grazing into pasture-based systems: the McClymont Memorial Lecture

Abstract

The present paper briefly discusses the impacts of introducing dual-purpose (DP) crops as a grazing resource into what were previously pasture-only grazing systems. The emphasis is on the high-value crops wheat and canola, since these have the greatest potential to increase profits by providing both winter grazing and a high-value grain or seed crop at harvest. A major potential impact of grazing on such crops is that the reduction in grain yield caused by grazing offsets the value of the grazing obtained. The paper shows that this is not the case and that if crop grazing is performed correctly, there will be minimal impact on grain or seed yield; yield may even be increased. ‘Correctly’ in this context refers mainly to the timely removal of livestock from the crop. Increases in grain yield after grazing arise principally from the conservation of soil water from the winter period through to the grain-ripening stage. The paper emphasises that although the digestibility and crude protein content of DP crops are high, the use of long-season bread wheats as a grazing resource requires that system managers pay greater attention to the sodium and magnesium status of the grazing livestock. This is because of the often high potassium and very low sodium concentrations of wheat forage, which leads to high potassium : sodium ratios in the rumen and reduced absorption of dietary magnesium. Supplementation with sodium and/or magnesium has significantly increased the liveweight gain of stock grazing wheat, but is contra-indicated when grazing canola. The paper also considers the interaction between the amount of grazing provided directly by the crop(s), relative to the possibly increased grazing available from the pasture component of the system, due to ‘pasture spelling’ while livestock are grazing the crop. Data are presented to show that, overall, the introduction of crop grazing into a previously pasture-based system greatly increases the total amount of grazing obtained. In addition, of the total extra grazing obtained in crop–pasture versus pasture-only systems, up to 40% has been found to come from spelled pasture. This has major implications for the management of the total system. The paper concludes by suggesting that, in the future, research on crop–pasture systems should continue, but should pay greater attention to the grazing of DP crops by cattle and particularly by breeding livestock. In addition, there is a need for much better data on the contribution of crop residues to the total grazing system, plus data on the effects of the introduction of DP crops on the wool production of the total grazing system.