RESUMO
Feed additives such as monensin (MON) and virginiamycin (VM) are commonly utilized in feedlot diets to enhance rumen fermentation. Nevertheless, the precise effects of combining MON and VM during specific feedlot periods and the advantages of this combination remain unclear. This study was designed to investigate the effects of withdrawal of MON when associated with VM during the adaptation and finishing periods on ruminal metabolism, feeding behavior, and nutrient digestibility in Nellore cattle. The experimental design was a 5 × 5 Latin square, where each period lasted 28 days. Five rumen-cannulated Nellore yearling bulls were used (414,86 ± 21,71 kg of body weight), which were assigned to five treatments: (1) MON during the entire feeding period; (2) VM during the entire feeding period; (3) MON + VM during the adaptation period and only VM during the finishing period 1 and 2; (4) MON + VM during the entire feeding period; (5) MON + VM during the adaptation and finishing period 1 and only VM during the finishing period 2. For the finishing period 1, animals fed T3 had improved potential degradability of dry matter (p = 0.02). Cattle fed T3 and T5 had the highest crude protein degradability when compared to animals receiving T2 (p = 0.01). Animals fed T2 and T3 had reduced the time (p < 0.01) and area under pH 6.2 (p = 0.02). Moreover, animals fed T4 had greater population of protozoa from the genus Diplodinium (p = 0.04) when compared to those from animals fed T2, T3 and T5. For the finishing period 2, animals fed T3 had greater starch degradability when compared to animals receiving T4 and T5 (p = 0.04). Animals fed T3, T4 and T5 had increased the duration of time in which pH was below 5.6 (p = 0.03). The area under the curve for ruminal pH 5.2 and pH 5.6 was higher for the animals fed T3 (p = 0.01), and the area under pH 6.2 was higher for the animals fed T3 and T5 (p < 0.01) when compared to animals receiving T2. There is no substantial improvement on the rumen fermentation parameters by the concurrent utilization of MON and VM molecules, where the higher starch and protein degradability did not improve the rumen fermentation.
RESUMO
Feed additives such as monensin (MON) and virginiamycin (VM) are widely used in feedlots diets to maximize rumen fermentation. However, the knowledge about the effects of MON and VM combinations in specifics feedlot periods and the benefits of this association are still unclear. This study aimed to evaluate the effects of withdrawal of MON when associated with VM during the adaptation and finishing periods on feedlot performance of Nellore cattle. The experiment was designed as a completely randomized block replicated six times (four animals/pen) in which 120 Nellore bulls (378.4 ± 24.4 kg) were allocated in 30 pens and fed for 112 days according to the following treatments: (T1) MON during the entire feeding period; (T2) VM during the entire feeding period; (T3) MON+VM during the adaptation period and only VM during the finishing period 1 and 2; (T4) MON+VM during the entire feeding period; (T5) MON+VM during the adaptation and finishing period 1 and only VM during the finishing period 2. After 112 days on feed, no treatment effect was observed for DMI (P ≥ 0.12). However, bulls fed T5 had greater (P = 0.05) final BW and ADG when compared to T1, T2, and T4. Cattle from T3 and T5 groups presented heavier HCW (P = 0.05) than that fed T1, T2, and T4. Nellore bulls fed T1 and T5 had lower (P < 0.01) DMI variation than those receiving T2. The withdrawal of MON when associated with VM during the final third of the feedlot period improved overall final BW, ADG, and HCW when compared to bulls fed either MON or VM, but did not positively impact feedlot performance when compared to cattle that had MON withdrawn at the end of the adaptation period.
