Interrelationships Between Forage Nutritive Value and Quantity in Affecting Animal Performance.

Forage quantity and nutritive value are two key factors impacting performance of grazing ruminants, but they often interact. The objective of this paper is to assess the responses of grazing animals to forage quantity and nutritive value individually and then explore how they interact to influence animal response. Stocking rate is a key grazing strategy influencing performance per animal and per hectare. The greatest impact of stocking rate is mediated through its effect on forage mass (forage quantity). Forage mass decreases as stocking rate increases, causing performance per animal to decrease. While increased stocking rate is associated with decreasing forage mass (quantity), forage nutritive value generally increases in response to increasing stocking rate. To achieve a given level of animal performance, less forage is needed when nutritive value is high, but generally the effect of stocking rate on forage mass has a more profound effect on animal performance than its effect on nutritive value. Forage allowance (kg of forage per kg of animal liveweight) is a useful way of relating animal performance to forage quantity because it integrates both forage mass (numerator) and stocking rate (denominator). This provides a potentially powerful predictive tool across forage species, environments, and grazing methods. The relationship of performance per animal with herbage allowance is generally linear when allowance increases from a low level, but at some point it plateaus. For grazing studies with allowance at high levels for all treatments, there is no relationship of performance per animal and allowance, and in this case forage nutritive value becomes a more important determinant of performance than forage quantity. Thus, pasture conditions dictate whether forage nutritive value or mass will have the greatest impact on animal performance.