The Efficient Use of Grass.

The efficient use of grass" By W. F. RAYMOND, The Grassland Research Institute, Hurley, Berks. The output of animal products from grassland depends on the quantity and quality of herbage produced, and on the efficiency of utilization of this herbage by the animal; these factors interact, herbage production being affected by method and intensity of use, and efficiency of utilization by the overall level and seasonal distribution of herbage production. However, our ability to grow grass appears in general to be ahead of our ability to use it efficiently, and it was to consider this problem of utilization that this Symposium was arranged. Efficiency of utilization itself depends both on the proportion of the herbage grown that is eaten by livestock, and on the efficiency of conversion into animal products of that herbage. These two requirements are often contradictory, and an analysis of the causes and nature of this contradiction may help us to develop systems of grassland use that will more efficiently exploit the herbage we grow. It is useful first to consider the efficiency with which the ruminant animal uses the food it eats. The 500 lb bullock at maintenance (i.e. neither gaining nor losing weight) requires some 4 lb starch equivalent (SE)/day, whereas the same animal gaining I , z or 3 lb live weight/day requires about 54, 7 or 9 lb respectively. These figures, based on Rations for Livestock (Evans, 1960) may not be precise, but they are used here to illustrate how efficiency of food conversion increases with rate of animal production. Thus the higher the rate of gain the less is the total SE required per lb gain (58, 33 and 3 lb SEjlb gain at production levels of I , 2 or 3 lb/day). Such an increase in efficiency of food conversion is a key factor in the intensive pig and poultry industries, but less attention has been given to it in ruminant systems based largely on forages. Much of the current success of feeding beef cattle on diets containing a high proportion of barley, however, seems due to the high food conversion efficiencies achieved, which has directed attention to the low efficiency of conversion of the forage eaten in many grassland systems. This inefficiency is because many herbage-fed animals have a low daily intake of nutrients, of which the majority is used for maintenance, and relatively little for production; as a result the nutrient requirement per unit of production is high. It seems clear that to increase the efficiency of conversion of the herbage eaten we must increase the daily intake of nutrients by the individual animal. This requires a study of the factors that determine