Effects of feeding a calf starter on molecular adaptations in the ruminal epithelium and liver of Holstein dairy calves

The objective of this study was to elucidate the effect of feeding a calf starter on the volatile fatty acid (VFA) profile in the rumen and on expression of genes involved in epithelial intracellular pH regulation, butyrate metabolism, and hepatic urea cycle during the weaning transition. Twenty Holstein bull calves were fed either milk replacer and hay (MR) or milk replacer, hay, and a commercial texturized calf starter (MR+S) in a randomized complete block design. All calves were fed 750 g/d of milk replacer as the basal diet. Calves on the MR+S treatment were also fed starter ad libitum, and the energy intake of calves within blocks was maintained by supplementing the MR group with extra milk replacer that was equivalent to the energy intake from calf starter. Calves were killed 3 d after they consumed 680 g/d of calf starter for 3 consecutive days. Calves fed MR+S had higher VFA concentrations in the rumen (99.1 ± 8.1 vs. 64.6 ± 8.6 mM) and a higher molar proportion of butyrate (15.6 ± 1.7 vs. 7.9 ± 1.9%) than calves fed MR. Relative abundance of mRNA for monocarboxylate transporter isoform 1 was higher (1.45 vs. 0.53), and that of Na+/H+ exchanger isoform 3 (0.37 vs. 0.82) and 3-hydroxy-3-methylglutaryl synthase isoform 1 (0.40 vs. 0.94) lower for the MR+S treatment compared with the MR treatment. In the liver, relative mRNA abundances of argininosuccinate synthetase isoform 1 (2.67 vs. 1.56), argininosuccinate lyase (1.44 vs. 0.99), and arginase isoform 1 (3.21 vs. 1.74) were greater for MR+S than for MR calves. Calf starter consumption appeared to increase fermentation in the rumen and affected expression of genes involved in cholesterol synthesis and intracellular pH regulation in ruminal epithelium, and those involved in urea cycle in the liver.