Options for manipulating nutrition if feed supply is immutable

Abstract

The Australian pastoral industries depend almost entirely on the nutrients derived from the available pasture and browse. This is particularly true for the northern cattle industry, where the production of pasture, and hence animal productivity, depend on the length of time that sufficient soil moisture is available during and after the wet season to sustain pasture growth. Animal nutrition is further affected by the relatively low digestibility of tropical grasses even at early stages of maturity. Hence animal production is marked by relatively low annual weight gain and poor reproductive performance. Research to date has demonstrated that lack of nitrogen both as ammonia for rumen microbes and as amino acids for the animal's tissues is the major nutritional deficit, which causes a depression in feed intake and hence exacerbates an already existing energy deficit. In many areas, too, deficiencies of P, Na, S and trace elements have been identified. Broadly speaking, the technology has been developed to overcome these nutritional deficiencies and to investigate remaining problems of undernutrition and low productivity. It is probable that new tropical legumes suited for areas with less than 750 mm annual rainfall will also be developed. However, despite these efforts, animal production will tend to be low, because for much of the year the high fibre low protein forage is eaten in only restricted amounts, and because only 40% or so of the energy in the forage becomes available to the animal. There is, therefore, need to develop rumen microbes capable of more rapid and more extensive breakdown of plant fibre and to achieve new methods to improve the conversion of amino acids into animal protein.