Effect of grass regrowth interval on intake, rumen digestion and nutrient flow to the omasum in beef cattle

The effect of increasing grass regrowth interval (RI) (28 days vs. 38 days (d)) on zero-grazed intake, rumen fermentation, in situ degradability, rumen digesta kinetics and nutrient flow to the omasum, was determined in six ruminally cannulated Holstein-Friesian steers in a 2 × 2 cross-over design experiment. Digesta kinetics was determined using the rumen evacuation technique and omasal flow was estimated using Co-EDTA, Yb-acetate and indigestible neutral detergent fibre (INDF) as digesta flow markers and purine bases as microbial markers. Increasing RI had no effect (P>0.05) on dry matter intake (DMI), total volatile fatty acid (VFA) concentration or molar proportions of VFA in the rumen but reduced (P<0.05) rumen ammonia nitrogen (N) concentrations. Rumen digestion and flow to the omasum of organic matter (OM), neutral detergent fibre (NDFom) and N did not differ (P>0.05) between the RI treatments. A higher proportion (P<0.05) of NDFom digestion occurred in the rumen with the 28 d than the 38 d RI treatment. There was no effect (P>0.05) of RI on microbial N flow or on efficiency of microbial nitrogen synthesis but the flow of non-ammonia non-microbial N was reduced (P<0.05) with the 38 d compared to the 28 d RI treatment. The digestion rate (kd), derived from the rumen evacuation technique, of DM and OM (P<0.05), and NDFom and DNDFom (P<0.01) decreased with increased RI. These results indicate that increasing the RI of a perennial ryegrass-based sward by 10 d from 28 to 38 d in spring/early summer had relatively little or no adverse effects on DMI, rumen fermentation and rumen or total tract digestion of OM and NDFom of growing beef cattle but reduced ammonia N levels in the rumen, potentially reducing nitrogen excretion to the environment.