106 Effects of 28-d rotation of ionophores on commercial cattle performance during the growing phase.

Abstract Rotating ionophores in feedlot cattle diets has been suggested to enhance performance, but short-term rotations have not shown consistent benefits and are not compliant with FDA regulations for continuous feeding. Trial objectives were to investigate the influence of multiple 28 d rotations of ionophores in the growing phase on gain and efficiency of gain (F:G) in a commercial feedlot. Data were collected from 584 crossbred heifers (BW = 290 ± 26 kg). Upon arrival to the feedyard, heifers were blocked by arrival date and allocated to pens within block based on processing order, resulting in 59 hd ± 15 per pen with 5 pens/treatment. Within placement time block, pens were randomly assigned to either MON or ROT treatment. Cattle fed MON treatment were presented with a basal diet that included 200 mg/h/d monensin. Cattle fed ROT treatment were presented with the same basal diet but alternated between 200 mg/h/d monensin and 75 mg/h/d laidlomycin propionate every 28 d for 2 rotations (112 d total). 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). Weights were collected every 28 days. Dry matter intake was calculated from daily feed delivery weights. Data were analyzed using lmer package in R. Effects of treatment were also evaluated both within period and across periods (56 d, 84 d, 112 d increments) for ADG, DMI, and F:G with random effects of placement time. From d 0 to d 28 when all cattle were fed monensin, there were no effects of treatment on ADG, DMI, or F:G (P>0.10). From d 29 to d 56 when ROT cattle were transitioned to laidlomycin propionate, ADG and DMI did not vary by treatment (P>0.10), but a tendency for improved F:G was observed for ROT heifers (P=0.08). From d 0 to d 56, F:G was improved for ROT compared to MON cattle (P < 0.01). From d 57 to d 84 when all diets contained monensin, there were no differences in ADG, DMI, or F:G by treatment (P>0.10). From d 85 to d 112 when ROT cattle were presented with laidlomycin propionate for the second time, ROT heifers had higher ADG (P=0.01) and improved F:G (P< 0.01) compared to MON heifers with no difference in DMI (P=0.70). From d 0 to d 112, ADG and F:G were improved for ROT compared to MON cattle (P>0.01). The results suggest that multiple rotations of monensin and laidlomycin propionate in growing cattle diets improved performance characteristics when cattle are limit fed compared to continuous feeding of only monensin.