Respostas adaptativas da pele de novilhos Nelore suplementados em pastagem tropical

Abstract

Tropical areas have high levels of solar radiation and high temperatures, which seem to be the main factors that affect the adaptation of livestock in this environment. The following hypotheses were tested, i) that supplementation with different protein sources (true protein × non protein nitrogen - NNP) does not cause changes in the skin adaptability of Nellore steers to the sun in different seasons of the year; and, ii) that the different seasons of the year affect the skin adaptation of Nellore steers. The objective of this work was to evaluate the effects of replacing true protein (soybean meal) by NNP (extruded urea) in the supplement on the characteristics of the skin of steers in the growing phase maintained on pastures. Thirty-six Nellore steers with an initial weight of 256.6 kg± and 15 months old were used. The animals were identified and distributed in paddocks of Brachiaria brizantha cv. Marandu. The experimental area consisted of four paddocks (four hectares each), two paddocks per treatment with nine animals each paddock, according to a completely randomized design. Two supplements were formulated, containing two sources of nitrogen (extruded urea and soybean meal) with 41% and 26.7% of crude protein based on dry matter, for the rainy season (wet) and dry period (wet-drought transition), respectively. Supplements were provided two months before the start of skin collection at 0.45% of body weight (BW) during the rainy season (wet) and 0.70% BW in the dry period (wet-drought transition). Skin biopsies were performed in three seasons: summer (December), autumn (March) and winter (July). Skin slides were used to measure epithelial thickness (linear µm) and dermal thickness (linear µm); in the sweat glands were measured: area (µm²), depth (linear µm) and count of sweat glands and hair follicles. The protein source of the supplement has effects on the length of the glandular portion, number of follicles and gland area in Nellore steers during the rearing phase at different times of the year. The skin of steers supplemented with soybean meal showed a greater number of follicles in the summer and a smaller area and length of sweat glands, which leads to greater adaptation to the environment. There were changes in the skin at different times of the year as a function of sunlight and ambient temperature for all other variables evaluated. Keywords: extruded urea; protein; sweat gland; skin; supplementation