EFFECTS OF MONENSIN AND RYEGRASS STRAW ON THE PERFORMANCE OF FEEDLOT LAMBS
Sixty single-reared lambs sired by Hampshire rams were used to study the effects of monensin and ryegrass straw on feedlot performance. Two levels of monensin (0 and 27.5 ppm) and two levels of ryegrass straw (0 and 25%) were arranged in a 2 × 2 factorial treatment arrangement. Each of the four pelleted diets was fed ad libitum to 15 lambs. Monensin-fed lambs had lower (P < 0.10) feed intakes, smaller (P < 0.05) loin eye areas and higher (P < 0.10) carcass quality grades than lambs not fed monensin. Although the difference was not significant (P > 0.10), monensin-fed lambs also had improved feed efficiencies over lambs not fed monensin. Crude protein, ether extract and crude fiber digestibilities of the diets were increased by the addition of monensin. Lambs fed ryegrass straw had lower (P < 0.01) average daily carcass gains, lower (P < 0.10) carcass quality grades, lower (P < 0.05) dressing percentages and smaller (P < 0.10) loin eye areas than lambs not fed ryegrass straw. Diet total digestible nutrients and dry matter, organic matter, crude protein, crude fiber and nitrogen-free extract digestibilities were reduced (P < 0.05) when ryegrass straw was present. Key words: Sheep, feedlot lambs, ryegrass straw, monensin