Restricted versus step-up dietary adaptation in Nellore bulls: Effects over periods of 9 and 14 days on feedlot performance, feeding behavior and rumen morphometrics

Previous studies have shown that beef cattle can be adapted for 14 days to high-energy diets; however, the effects of adaptation periods less than 14 days on feedlot performance have not been documented in the literature. In the present study, the effects of restricted intake of the final finishing diet as a means of dietary adaptation compared with diets increasing in concentrate content (step-up) over periods of 9 days and 14 days on feedlot performance, feeding behavior, blood gas profile rumen morphometrics, nutrients digestibility and carcass characteristics of Nellore cattle were evaluated. One-hundred twenty 24 month-old Nellore bulls (361 +/- 30 kg) were randomly allocated in 24 pens and fed for 84 days in a completely randomized block design with a 2 x 2 factorial arrangement of treatments: adaptation using both 9-day and 14-day step-up protocol, and 9-day and 14-day restriction protocol. Each treatment was replicated 6 times. One bull per pen was slaughtered at the end of adaptation period for ruminal evaluations. Overall, no main effect of protocols or adaptation length was observed for any of feedlot performance or carcass traits variables evaluated, with the exception of gain to feed ratio (G:F). Bulls on restriction protocol had improved G:F (P = 0.04) when compared to cattle adapted by step-up protocol. Regarding rumenitis score, there was no main effect of protocols and adaptation length (P > 0.05). For rumen morphometrics, an interaction between the effect of adaptation length and period was observed (P = 0.02) for absorptive surface area (ASA), where at the end of the adaptation period, Nellore bulls adapted for 14 days presented larger ASA than the ones adapted for 9 days; however, at the end of the finishing period, ASA from animals adapted for 14 days was similar to the ones adapted for 9 days. Thus, Nellore yearling bulls should be adapted in 14 days regardless of the protocol adopted (step-up or restriction). (AU)