PSXIV-15 Effects of rotation of ionophores on rumen fermentation of growing phase beef x dairy steers using ex vivo techniques in a commercial feedlot.

Abstract Rotating ionophores in feedlot cattle diets has been proposed to alter rumen microbial dynamics and enhance performance, but limited research has examined its effects in beef x dairy crossbred steers. The trial objective was to investigate the impact of ionophore rotation on rumen fermentation using ex vivo rumen fluid collection from ruminally cannulated beef x dairy steers cohabitated with commercial beef x dairy steers. Rumen fluid donors (n=5) were ruminally cannulated Holstein x Angus steers (443 ± 15 kg). Cannulated steers were randomly allocated to commercial feedlot pens and co-housed with 71 beef x dairy crossbred steers (269 ± 23 kg). Steers (both commercial and cannulated) were randomly assigned treatment. Cattle fed MON treatment (n=3 cannulated; n=37 commercial steers) were limit fed a basal growing diet that included 200 mg/h/d monensin for 112 d. Cattle fed ROT treatment (n=2 cannulated; n=39 commercial steers) were presented with the same basal diet, but received 200 mg/h/d monensin for d-0 to d-56, 75 mg/h/d laidlomycin propionate for d-57 to d-84, and 200 mg/h/d monensin for d-85 to d-112. Rumen fluid was collected from cannulated steers every 28 days for 48 h ex vivo batch culture runs, along with individual body weights on all steers. Ex vivo gas production runs were conducted (d-28, d-56, d-84, d-112) by isolating fluid from each cannulated donor steer in a vented batch culture system (ANKOM) provided with respective treatment diet by steer as substrate. Variables of interest were analyzed using lme4 package in R, with a fixed effect of treatment. No effect of ROT compared to MON was observed in ex vivo runs conducted at d-28, d-56, d-84, or d-112 for total VFA concentration (mM), acetate: propionate ratio, total gas produced (mL), or CO2 produced (mL) (P > 0.10). Rumen fluid collected on d-84 from ROT steers exposed to laidlomycin propionate for 28 d demonstrated CO2: CH4 was higher for ROT compared to MON (P < 0.01). No differences between treatments was observed for gas production kinetics variables (P > 0.10). To better understand fermentation results, ADG was analyzed for commercial beef x dairy steers cohabitated with rumen cannulates (n = 37, and 39 for MON and ROT, respectively). No differences were observed for ADG from d-0 to d-28, d-29 to d-56, and d-86 to d-112 when all cattle were fed monensin. ROT steers had higher ADG from d-56 to d-84 than MON steers (P = 0.03). Results suggest rotating monensin and laidlomycin propionate in growing beef x dairy steer diets has a role in altering rumen fermentation and ADG in limit-fed growing beef x dairy cattle compared to continuous feeding of only monensin.