Abstract
Feedlot cattle are usually adapted to high-concentrate diets containing sodium monensin (MON) in more than 14 days. However, for finishing diets with lower energy content, the use of MON during adaptation may hold dry matter intake (DMI), and virginiamycin (VM) may be an alternative. This study was designed to determine the potential of shortening the adaptation of Nellore cattle to high-concentrate diets using only VM as a sole feed additive relative to feedlot performance, feeding behavior, and ruminal and cecum morphometrics. The experiment was designed as a completely randomized block replicated six times (four animals/pen) in which 120 Nellore bulls (390.4 ± 19.0 kg) were fed in 30 pens for 111 days according to the following treatments: (1) MON and adaptation for 14 days (MON14), (2) MON + VM and adaptation for 14 days (MONVM14), (3) VM and adaptation for 14 days (VM14), (4) VM and adaptation for 9 days (VM9), and (5) VM and adaptation for 6 days (VM6). At the end of the adaptation, 30 animals (n = 1 per pen) were randomly slaughtered for rumen and cecum evaluations. The remaining 90 bulls were harvested at the end of the study. No effects of treatments were observed (P < 0.10) for final body weight, average daily gain (ADG), and hot carcass weight (HCW). Cattle fed VM14 presented a greater (P ≤ 0.03) DMI, expressed as percent of body weight (BW), than animals fed either MON14 or MONVM14; however, cattle fed either MON14 or MONVM14 improved (P ≤ 0.02) the gain-to-feed ratio (G/F) by 10.4 or 8.1%, respectively, when compared to bulls fed VM14. Bulls fed VM14 had smaller (P < 0.05) papillae area (0.34 vs. 0.42 cm2) and rumen absorptive surface area (28.9 vs. 33.8 cm2) than those fed MON14. The shortening of the adaptation period linearly decreased the 12th rib fat (P = 0.02) and biceps femoris fat daily gain (P = 0.02) of Nellore bulls fed only VM, which linearly decreased the final biceps femoris fat thickness (P < 0.01). Feedlot cattle fed VM as a sole feed additive should not be adapted to high-concentrate diets in less than 14 days. Regardless of either adaptation length or feed additive, feedlot cattle need at least 14 days to adapt to finishing diets.
Highlights
Adaptation studies have been conducted previously in Brazil to determine the most appropriate adaptation length to highconcentrate diets for Nellore cattle [1,2,3,4,5]
The cattle fed VM and adaptation for 14 days (VM14) presented a greater (P ≤ 0.03) dry matter intake (DMI), expressed as % of body weight (BW), throughout the study when compared to the cattle fed either MON and adaptation for 14 days (MON14) or MONVM14
The cattle fed VM14 showed a greater DMI per meal during feeding behavior evaluations on both adaptation and finishing periods, which explains the greater DMI throughout the study, expressed either in kilograms or as percent of BW, when compared to the cattle fed MON14 or MONVM14 since no differences in meals per day or meal length were detected
Summary
Adaptation studies have been conducted previously in Brazil to determine the most appropriate adaptation length to highconcentrate diets for Nellore cattle [1,2,3,4,5]. All adaptation studies conducted previously in Brazil with Nellore cattle used feedlot diets containing sodium monensin (MON) as the sole feed additive. Monensin is known for its negative effect on dry matter intake (DMI), which typically results in a positive effect on the feed efficiency of feedlot cattle [7]. Since finishing diets in Brazil contain less energy than a typical feedlot diet in North America [9, 10], the use of a feed additive that does not impact the DMI, especially during adaptation, may be recommended
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