Growth, nutrient digestion and thyroid hormone concentrations in weanling horses fed added dietary fat

Abstract

Thirty, 19-wk old Quarter Horses were utilized in a 112-d study to compare growth, nutrient utilization and thyroid hormone concentrations in weanling horses fed a 10% added fat diet (FD) or a conventional diet (CD). Diets contained the same nutrient:calorie ratios and were fed with bermudagrass hay in a 70:30 concentrate:hay ratio. Initially and at 28-d intervals horses were weighed, measured for height, circumference of heart girth and cannon bone, length of body and cannon bone and subcutaneous thickness of fat over the rump and ribs. Radiographs were taken of the right carpal and metacarpal-phalangeal joints at the beginning and end of the experimental period for evaluation of physeal maturation. Digestion trials were conducted with each horse 10 d prior to completion of the study. In addition, blood samples were collected from 6 females in each treatment over a 6 hr intensive collection period for evaluation of hormone concentrations. Concentrate intake was lower (P<.05) for weanlings fed FD (3.84 kg/d) than those fed CD (4.17 kg/d). Weanlings fed FD tended to have higher average daily gains (.80 kg/d) than their control counterparts (.74 kg/d), and the feed-to-gain ratio was lower (P<.05) for the weanlings fed added fat (6.3) than the weanlings fed the control concentrate (7.3). Gain in heartgirth circumference was greater (P<.05) for the fat fed weanlings (20.2 cm) than for the control weanlings (17.9 cm). All other linear measurements of growth were similar between treatments. There were no radiographic indications of abnormalities in the physes of horses on either treatment. Closure of the physes occurred at normal rates in foals on both treatments. Digestibility of protein and energy tended to be higher and digestibility of ether extract and neutral detergent fiber were higher for weanlings fed FD than for those fed CD (69.57% vs. 67.54%; 59.22% vs. 57.54%; 65.39% vs. 57.67%, P<.05; 40.55% vs. 35.62%, P<.06; respectively). Weanlings consuming FD tended to have lower circulating concentrations of glucose and insulin, however these differences were not significant due to wide individual variation. Concentrations of thyroxine and triiodothyronine were similar between treatments.