Eficiência alimentar e características produtivas de bezerras Gir em aleitamento

Abstract

The objectives of this study were: 1) to classify animals into groups of high and low feed efficiency using two feed efficiency indexes (residual feed intake (RFI) and residual feed intake and body weight gain (RIG)), and 2) to evaluate if pre-weaning heifer calves divergent for feed efficiency indexes exhibit differences in performance, body measurements, blood and ruminal parameters, digestibility, energy partitioning, nitrogen partitioning and thermography. 32 Gyr heifer calves were enrolled in a 63-d trial and classified into 2 feed efficiency (FE) groups based on RFI and RIG (mean ± 0.5 SD). The groups were classified as high efficiency (HE) RFI (HE RFI, n = 9; HE RIG, n = 10), and low efficiency (LE) RFI (LE RFI, n = 10; LE RIG, n = 11). HE and LE calves had RFI values of - 0.052 and 0.049 kg/d (P < 0.05), respectively). At birth, the amount of whole milk provided for each animal was calculated based on their metabolic weight at birth (42% x BW0.75). The liquid diet was divided into two meals at 0700 and 1400. The total solid diet (TSD) was composed of 92% concentrate and 8% of tifton 85 hay chopped in 5 cm length as-fed. Intake was measured daily. Body weight and measurements were performed at birth, 3rd and 7th day and weekly onward. Blood concentrations of insulin, BHB, urea and glucose, and pH, N-NH3 and volatile fatty acids (VFA) in ruminal fluid were evaluated at 14, 28, 42, 56 and 70 days of age, respectively. From the 50th to the 55th day of age the animals were submitted to a digestibility assay. At 55th (±6) days of age, respiratory exchanges were measured by one open-circuit respirometry chamber. Thermal images of the calves were taken with an infrared camera on d 56 (±3) at 0600 h, before the morning feeding. The HE RFI group consumed 8.9% less solid diet than the LE RFI group. HE RFI animals exhibited an increased digestibility of crude protein and ether extract and tended to have higher total dry and organic matter digestibility. LE RFI animals had higher gross energy and nitrogen intake, though higher fecal losses resulted in a tendency to reduce energy and nitrogen use efficiency. Total VFA concentration (µmol/mL), proportion of acetate and propionate (%VFA) in ruminal fluid were greater for LE than HE RFI heifers. Blood urea concentration tended to be higher in LE than HE RFI heifers. HE and LE calves had RIG values of 0.080 and -0.077kg/d (P ≤ 0.01), respectively. HE RIG animals exhibited higher average daily gain (9.4%), body weight (BW), and heart girth, though HE RIG group exhibited narrower hip width. HE RIG animals tended to have higher ether extract digestibility but higher methane losses (% of gross energy). HE RIG animals tended to have greater ruminal proportion of acetate and lower propionate (%VFA). Blood insulin concentration was greater and blood glucose tended to be greater for LE than HE RIG animals. LE RIG had higher left rib, left flank and anus surface temperature measured by infrared thermography in comparison to HE RIG group. HE RFI in pre-weaning heifers seems to be related to differences in digestibility and not related to differences in ruminal fermentation. Differences in protein metabolism seems to affect RFI during this phase. Divergent animals for RIG during the assessed phase appear to differ in body measurements, and glucose and insulin metabolism, which may be related to differences in the composition of the gain. Infrared thermography seems to be correlated to RIG but not to heat production (HP) in pre-weaning calves.