The effects of monensin and monensin-containing supplements on performance of steers grazing winter wheat pasture

Scope and method of study. The purpose of this study was to examine the effects of monensin on growth performance of steers grazing winter wheat pasture. Additionally, the effect of different delivery methods for monensin and other supplemental nutrients was investigated. Finally, a study was conducted to determine the effects of monensin on intake, site and extent of nutrient digestion, and rumen fermentation of steers grazing winter wheat forage. Two years (2004-2005 and 2005-2006 wheat pasture grazing seasons) were dedicated to evaluating the growth performance of steers. This included observations on 369 steers across the two grazing seasons. The site and extent of nutrient digestion study utilized 15 ruminally and duodenally cannulated steers grazing wheat pasture during one grazing season.; Findings and conclusions. Providing steers grazing winter wheat pasture with a free-choice, non-medicated mineral mixture increased daily live weight gain (ADG) by 0.14 kg/steer (average across both years). Inclusion of monensin in a free-choice mineral mixture reduced intake of the mineral mixture by 60%. However, while intake was reduced, ADG of steers receiving the monensin-containing mineral mixture was increased by 0.12 kg as compared with those receiving the non-medicated mineral mixture. No advantage in ADG was observed by combining supplemental energy, minerals, and monensin into a single package compared with feeding a commodity feedstuff (soybean hulls) and the free-choice, monensin-containing mineral separately. The site and extent of nutrient digestion study showed that wheat forage intake was not altered by monensin; however, the extent of ruminal digestion of the fiber fractions of wheat forage was increased. Additionally, monensin decreased the acetate:propionate ratio of ruminal fluid and increased the molar proportion of propionate. In total, these factors which improve the energetic status of the cattle lead to the increased growth performance observed in the performance study.