Ruminal pH and epithelial function as affected by increasing compound feed supply in growing Holstein heifers
Adaptation to high-concentrate diets involve changes in ruminal milieu and epithelium that remain largely undescribed. Changes in ruminal pH and epithelial function were studied by gradually introducing compound feed (CF)(from 0 to 8kg/d as-fed; Δ+0.5kg/d) in the diet of four fistulated Holstein heifers (8±0.32 mo of age) with ad libitum access to chopped barley straw. Loggers placed in the ventral rumen continuously monitored pH over 16 d. Cumulative time (min/d) spent below pH cut-off points was calculated for each animal and fitted to a logistic curve (AlZahal et al., 2007), where the slope (β0; indication of daily pH range) and inflection point (β1; median pH value) were calculated. Linear correlation between these parameters and CF supply was calculated. Rumen papillae samples were biopsied for RNA extraction and subsequent gene expression analyses when cattle consumed 0, 4 and 8kg of CF/d. Total DM intake was 5.8±0.76, 6.9±1.08 and 8.9±0.71kg/d and straw intake 4.0±0.76, 3.3±1.08 and 1.7±0.71kg/d for CF levels of 0, 4 and 8kg/d, respectively. The daily minimum pH was 6.4±0.19, 6.0±0.12 and 5.8±0.17 at 0, 4 and 8kg/d CF, respectively. In contrast, no changes in daily maximum pH (from 7.3±0.17 to 7.3±0.31) and subtle decreases in daily mean pH (from 6.9±0.1 to 6.7±0.24) were observed with increasing CF intake. The slope of the logistic curve decreased (P<0.05) as the intake of CF increased resulting in 8.7±0.69, 6.6±0.69 and 2.1±0.69 for 0, 4 and 8kg CF/d, respectively. The inflection point decreased (P<0.05) with CF supply but was not affected by CF dose (6.8±0.13, 6.6±0.11 and 6.3±0.21 for 0, 4 and 8 kg CF/d, respectively). Slope correlated better to CF intake [β0=9.5(±0.48)-0.6(±0.10)×CF(kg); R2=39] than inflection point [β1=6.8(±0.06)-0.008(±0.010)×CF(kg); R2=0.01]. The relative mRNA expression of tight junction genes claudin1 (CLDN1) and claudin4 (CLDN4) were down-regulated (P<0.05) by 0.77±0.03 and 0.85±0.06 fold between 0 and 4kg CF inclusion, respectively. Moreover, the relative mRNA expression from 0 to 4 and 0 to 8kg CF supply was up-regulated (P<0.05) for putative anion transporter 1 (PAT1) (1.25±0.07; 1.39±0.15) and carbonic anhydrase 1 (CA1) (1.20±0.09; 1.30±0.20). Based on the results of this study, adaptation to high-concentrate diets was associated more extreme fluctuations in rumen pH rather than obvious declines in average ruminal daily pH. In addition, adaption to high-concentrate diets involved changes in gene expression of key transport (PAT1), metabolic (CA1) and tight junction genes (CLDN1 and CLDN4) in the epithelium.
Keywords: rumen-pH, adaptation, gene expression