Value for animal production (VAP): a new criterion for tropical forage evaluation

Abstract

Most feed evaluation systems were developed in the context of intensive animal production systems in temperate areas. Such systems concentrate on obtaining accurate energy values for ration formulation. For tropical forages, the simpler value of digestibility is often combined with voluntary intake to determine intake of digestible dry matter (IDDM). Due to the heterogeneity of tropical forages, these values vary widely depending on the amount of offered DM (ODM), which determines the amount of feed refusals and the degree of selective consumption. The effect of varying the amounts of excess feed on IDDM has been grossly underestimated in the past due to inappropriate methods of experimentation and data analysis. To avoid arbitrary degrees of selection, IDDM must be estimated for a wide range of ODM. On this basis, response curves can be developed and analysed to estimate the optimum degree of selection that maximises the ratio (IDDM−MR)/ODM, in which MR equals the level of IDDM needed to meet the maintenance energy requirements of animals. This ratio, defined as value for animal production (VAP) integrates feed intake and digestibility, as well as the amount of feed offered. For many forages, obtaining maximum VAP values (VAP m) requires high levels of ODM and feed refusals. Response curves for unchopped forage closely represent the effect of stocking rate on performance of grazing animals. With most feeds, chopping suppresses selection and may be counter-productive to achieving maximum potential animal production per unit of feed offered.