OPTIONS FOR MANIPULATING NUTRITION IF FEED SUPPLY IS IMMUTABLE

The Australian pastoral industries depend almost entirely on the nutri ents derived from the available pasture and browse. This is particular ly true for the northern cattle industry, where the production of past ure, 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 annu al 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 exacerbate s 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 defi ciencies and to investigate remaining problems of undernutrition and l ow productivity. It is probable that new tropical legumes suited for a reas with less than 750 mm annual rainfall will also be developed. How ever, despite these efforts, animal production will tend to be low, be cause 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 i n the forage becomes available to the animal. There is, therefore, nee d to develop rumen microbes capable of more rapid and more extensive b reakdown of plant fibre and to achieve new methods to improve the conv ersion of amino acids into animal protein.