Whole corn grain utilisation in dairy goats is determined by rumen retention rather than intestinal digestive...

The small intestinal starch digestion of ruminants is limited, potentially due to inadequate pancreatic α-amylase secretion. Previous studies confirmed that leucine stimulates amylase release. But the core scientific problem remains whether increasing intestinal amylase availability can improve dry whole corn (DWC) utilisation, and which digestive site dominates DWC starch digestion in goats. This study aimed to verify the effects of rumen-protected leucine (RP-Leu) and amylase (RP-Amy) on DWC utilisation of goats, and identify the primary site of starch digestion. In Experiment 1, eighteen castrated Guanzhong dairy goats (47.70 ± 1.27 kg BW) were randomly assigned to three groups (n = 6) receiving a basal DWC‑containing diet without (control) or with RP‑Leu or RP‑Amy. In Experiment 2, eighteen goats were evenly divided into three groups, each receiving a diet of DWC, dry rolled corn (DRC), or dry ground corn (DGC). Digesta from the rumen, reticulum, and jejunum were collected to determine starch content and particle size distribution. The results showed that RP-Leu increased oxygen consumption and heat increment but decreased the respiratory quotient and the efficiency of converting metabolic energy (ME) to net energy (NE). RP-Amy reduced both respiratory quotient and heat increment and improved the conversion efficiency of ME to NE, whereas starch digestion, digestible energy and ME did not differ among groups. The DWC group had significantly lower starch content in the omasal and jejunal digesta than the DRC and DGC groups, but retained more starch in the rumen. The DRC group showed a higher quantity of corn particles entering the omasum, with increased particle size and starch content. The large particle size of DWC physically impedes its passage through the reticulo‑omasal orifice, reducing the amount of starch reaching the small intestine. This physical restriction overrides the potential benefits of RP‑Leu and RP‑Amy in enhancing amylase activity. Our findings demonstrate that the rumen, not the small intestine, is the primary site of DWC starch digestion in goats, and that efficient DWC utilisation relies on prolonged ruminal retention and gradual fermentation rather than on small‑intestinal amylase activity. Consequently, conventional feed processing (e.g., grinding) is unlikely to improve DWC utilisation. The study provides a theoretical basis for applying whole corn in goat production while highlighting the need to consider physical constraints rather than enzymatic limitations.