Form of calf diet and the rumen. II: Impact on volatile fatty acid absorption

摘要

Diet is known to affect rumen growth and development.Calves fed an all-liquid diet have smaller and lessdeveloped rumens and a decreased ability to absorbvolatile fatty acids (VFA) compared to calves fed bothliquid and dry feed. However, it is unknown how rumensrespond when challenged with a defined concentrationof VFA. The objective of this study was to assess theeffects of 2 different feeding programs on VFA absorptionin preweaned calves. Neonatal Holstein bull calveswere individually housed and randomly assigned to 1of 2 diets. The diets were milk replacer only (MRO; n= 5) or milk replacer with starter (MRS; n = 6). Dietswere isoenergetic (3.87 ± 0.06 Mcal of metabolizableenergy per day) and isonitrogenous (0.17 ± 0.003 kg/dof apparent digestible protein). Milk replacer was 22%crude protein, 21.5% fat (dry matter basis). The texturedcalf starter was 21.5% crude protein (dry matterbasis). Feed and ad libitum water intakes were recordeddaily. Calves were exposed to a defined concentrationof VFA buffer (acetate 143 mM, propionate 100 mM,butyrate 40.5 mM) 6 h before euthanasia on d 43 ± 1.Rumen fluid samples were obtained every 15 to 30 minfor 6 h to measure the rate of VFA absorption. Rumentissues were obtained from the ventral sac region andprocessed for morphological and immunohistochemicalanalyses of the VFA transporters monocarboxylatetransporter 1 (MCT1) and 4 (MCT4). Body growthdid not differ between diets, but empty reticulorumenswere heavier in MRS than MRO calves (0.67 vs. 0.39 ±0.04 kg) and MRS calves had larger papillae areas (0.76vs. 15 ± 0.08 mm2). We observed no differences betweendiets in terms of the abundance of MCT1 and MCT4per unit area. These results indicate that the extrapolatedincrease in total abundance of MCT1 or MCT4in MRS calves was not due to increased transporterdensity per unit area. Modeled VFA absorption metrics(flux, mmol/h, or 6 h absorbed VFA in mmol) were notdifferent across diets. These results demonstrate thatthe form of calfhood diet, whether solely MR or MRand starter, does not alter VFA absorption capacitywhen the rumen is exposed to a defined concentrationof VFA at 6 wk of age.