AVALIAÇÃO DE GRUPOS GENÉTICOS DE NOVILHAS TERMINADAS EM CONFINAMENTO COM DIETAS CONTENDO DIFERENTES FONTES DE NITROGÊNIO NÃO-PROTEICO

Abstract

Finishing beef heifers in feedlots is a strategy for producing high-quality meat. The aim was to assess the production performance, economic evaluation and carcass and meat quality of feedlot-finished cull beef heifers fed diets containing different sources of non-protein nitrogen. 552 beef heifers were used, 292 ½ Angus + ½ Nellore (AN) and 260 ½ Charolais + ½ Nellore (CN), with an initial weight of 374.23±55 kg and 24 months of age. The heifers were fed isoprotein and isoenergy diets, but with different sources of non-protein nitrogen (NPN). The LPU diet contained a combination of livestock urea and protected urea, and the EU diet only extruded urea. Three studies were carried out, divided as follows: Study 1 (adaptation) evaluated two protocols for adapting to confinement, a traditional protocol and an experimental protocol, divided into two periods, 0-14 and 15-42 days. In the traditional protocol, the heifers were given a specific diet for adaptation (41% roughage: 59% concentrate) for the first 14 days, with no restriction on consumption. In the experimental protocol, the heifers were given the final finishing diet (34% roughage: 66% concentrate) with a 50% restriction on the first day and a 5% daily increase. Dry matter intake (DMI), productive and economic performance were measured in the periods evaluated. DMI was higher in the traditional protocol in all periods. However, performance was better with the experimental protocol in the initial period (0-14 days). After adaptation (15-42 days), the traditional protocol showed better performance. The cost of gain was lower in the experimental protocol in the initial period and in the first 42 days. The nutritional protocol of gradual adaptation to total ration is recommended for heifers adapting to confinement. Study 2 assessed the finishing period after adaptation. The same heifers and diets described in study 1 were used, distributed in a 2×2 factorial arrangement, two genetic groups (AN and CN) and two diets (LPU and EU). The AN heifers had greater dry matter intake, better feed conversion, greater average daily gain and final weight (P < 0.01). The AN heifers had greater revenue and, despite the greater total cost, had a greater gross margin (P < 0.05). Regarding diets, heifers fed the LPU diet had greater final weight, average daily gain and feed efficiency (P < 0.01). The combination of LPU sources in the diet provided better performance. Study 3 assessed the carcass and meat quality of the heifers described in study 2. AN heifers had greater hot carcass weight, loin eye area, subcutaneous fat thickness and lower shear force than CN heifers (P < 0.05). The meat from the AN heifers had a greater proportion of saturated fatty acids, with no effect on the proportion of monounsaturated and polyunsaturated fatty acids. A×N heifers have better carcass and meat characteristics, while CN heifers have meat and fat with a better fatty acid profile.