The effect of abomasal infusion of corn starch and β-hydroxybutyrate on hindgut microbial fermentation kinetics in early lactating dairy cows measured by the <i>in vitro</i> gas production technique

Abstract The effects of abomasal infusion of corn starch and β-hydroxybutyrate (BHB) on hindgut microbial fermentation characteristics and end-products in early lactation dairy cows were determined via in vitro gas production (GP). Four substrates, either fibre or starch sources differing in expected rate of degradability (slow – cellulose and corn grain; rapid – beet pulp and pregelatinized corn flour), were incubated with faecal inoculum from cows abomasally infused with water only, 1.5 kg corn starch/d + 0.0 mol BHB/d, 3.0 kg corn starch/d + 0.0 mol BHB/d, 0.0 kg corn starch/d + 8.0 mol BHB/d, 1.5 kg corn starch/d + 8.0 mol BHB/d, or 3.0 kg corn starch/d + 8.0 mol BHB/d in a 6 × 6 Latin square design. In vitro GP was measured using an automated GP system with methane (CH 4 ) measured at specific times. After 72 h, volatile fatty acids (VFA), pH, ammonia, and in vitro organic matter digestibility (IVOMD) of incubation fluid were determined. Infusion of BHB had limited effect on hindgut microbial fermentation. Infusion of 3.0 kg corn starch/d increased GP at 3 h of incubation for all substrates but resulted in lower total GP, CH 4 production, pH, ammonia concentration, and IVOMD after 72 h, while increasing total VFA concentration and molar proportions of propionate and butyrate vs. 0.0 and 1.5 kg corn starch/d infusions. Among substrates, IVOMD of cellulose was most affected by 3.0 kg corn starch/d infusion. Results suggest that in vitro fermentative activity of hindgut microbes decreases when higher levels of starch enter the hindgut.