Abstract
Rotating ionophores in feedlot cattle diets may influence rumen fermentation and greenhouse gas emissions, but limited data are available in beef × dairy crossbred steers. This trial evaluated the effects of rotating monensin and laidlomycin propionate in a backgrounding diet on enteric methane (CH₄), carbon dioxide (CO₂), predicted dry matter intake (DMI), and animal performance over a 112-d growing period. Steers (n = 59; BW = 268 ± 22 kg) were limit-fed a basal growing diet and assigned to either a monensin (Rumensin® 200 mg/hd/d) control (MON; n = 26) or a rotational ionophore treatment (ROT; n =33). The rotation of ionophores consisted of monensin (200 mg/hd/d) on d-0 through d-56 and d-85 through d-112 and laidlomycin propionate (Cattylst ®75 mg/hd/d) on d-57 to d-84. The basal growing diet was comprised of steam-flaked corn (43.5% of diet DM), mixed alfalfa hay (32.4% of diet DM), wet distillers grains (18.4% of diet DM), supplement (4.9% of diet DM), and corn oil (0.8% of diet DM). Greenhouse gas flux was measured using automated GreenFeed emissions monitoring systems (C-Lock Inc., Rapid City, SD). Emissions data were grouped into an early baseline phase (d-0 to d-56) and a treatment phase during which ionophore inclusion was rotated (d-57 to d-112). A least square means approach was used to generate g/d CO(2) and CH(4) values for each steer, which allowed for consideration of individual animal effects, average airflow, and hour of day. Results were analyzed using a general linear model in R with fixed effects of treatment and initial BW. In the baseline phase (d-0 to d-56), CH₄ emissions did not differ between treatments (P = 0.56), nor did CO₂ emissions differ between treatments (P = 0.14). During the treatment phase (d-57 to d-112), CH₄ emissions were significantly higher in ROT steers compared to MON (P = 0.02), while CO₂ remained similar (P = 0.46). ROT steers exhibited greater average daily gain (1.62 vs. 1.57 kg/d, P = 0.03) and improved gain:feed (0.323 vs. 0.315, P < 0.01) relative to MON steers. Predicted DMI, estimated from CO₂ output and performance parameters, did not differ between treatments in either phase (P ≥ 0.10). These results indicate that rotational feeding of monensin and laidlomycin propionate influences rumen fermentation and improves growth performance in limit-fed, beef x dairy steers when compared to feeding monensin for the entire feeding period.