Reducing rumen starch fermentation of wheat with 3% NaOH does not reduce whole tract starch digestibility and increases energy utilization in wethers during heat stress

Abstract

Previous studies have suggested that feeding slowly fermentable grains might improve heat-tolerance in ruminants, due to increased post-rumen starch digestion and a reduction in the heat increment of feeding. During heat stress gastrointestinal function may be compromised which could impact on the site and extent of digestion. To characterize the in vitro rumen starch and DM disappearance of corn, wheat, and 3% NaOH-treated wheat grains, grain samples were incubated during 0, 5, 8, and 24 h at 39 °C in buffered rumen fluid. Then faecal pH and whole tract apparent starch, DM, and organic matter (OM) digestibility were determined in wethers fed, with either corn (CD), wheat (WD) or 3% NaOH- treated wheat (TWD) based diets during heat stress in two experiments. In experiment 1, 22 wethers were fed either CD or WD (n = 11 per diet) during three different stages in climate controlled rooms: stage 1 (TNFR), 7 days of thermoneutral conditions 18–21 °C and 40–50% relative humidity) and 1.3 times maintenance feed intake; stage 2 (HSFR), 7 days of heat stress (28–38 °C and 30–50% relative humidity) and feed intake as TNFR; and stage 3 (HSFU), 7 days of heat stress as in HSFR stage and 1.5 times maintenance feed intake. In experiment 2, 31 wethers were fed either CD (n = 10), WD (n = 10) or TWD (n = 11) during three different stages with the same temperature and relative humidity regimes as experiment 1, however during TNFR and HSFR stages wethers received 1.7 times maintenance feed intake while in HSFU stage wethers received 2 times maintenance feed intake. After 24 h of incubation, untreated wheat had the fastest rate of starch and DM disappearance followed by 3% NaOH-treated wheat and corn grain (P < 0.001). Wethers fed CD had lower apparent starch (P < 0.001) and higher DM (P < 0.001) and OM (P < 0.001) digestibility than those fed WD, which was associated with lower faecal pH (P < 0.001) and higher faecal starch content (P < 0.001) than wheat diets. 3% NaOH treatment of wheat did not affect the whole tract starch digestibility of wheat being faecal pH and starch content of TWD like WD. At low feed intakes (1.3 times maintenance) there was no effect of heat stress on digestibility whereas at higher feed intakes heat stress reduced starch, OM, and DM digestibility. However, starch digestibility of TWD was similar to WD and metabolizable energy intake, DM and OM digestibility were higher. It is concluded that 3% NaOH treatment of wheat could reduce the rumen starch disappearance (in vitro condition) without reducing the whole tract starch digestibility (tested in vivo) of wheat in heat-stressed wethers. The study demonstrated the treated wheat with NaOH can be a strategy to increase OM digestibility and energy retention of wethers under heat stress.