Intake, digestibility and rumen characteristics in cattle offered whole-crop wheat or barley silages of contrasting grain to straw ratios

The effects of varying the grain (G) to straw (S) ratio (G:S) of whole-crop wheat and barley silages on intake and digestibility and whole-crop barley silage on rumen fermentation characteristics were examined in two parallel studies. For the intake and digestibility study, eight Aberdeen Angus cross-bred steers (mean bodyweight 407 kg (S.D. 24.2)) were used in two (barley and wheat) 4 × 4 Latin Square designed experiments. The dietary treatments were four G:S ratios: 0:100, 30:70, 60:40 and 90:10. Intake of grain linearly increased (P<0.001) while that of straw decreased (P<0.001) as the ratio of G:S increased for both cereals. No effect (P>0.05) was observed in total dry matter (DM) intake (DMI) or in DMI per kg liveweight. There was a positive linear (P<0.001) effect on the digestibility of the DM and organic matter (OM) and a negative linear effect on neutral detergent fibre (aNDFom) digestibility (P<0.01) as the G:S ratio increased for both cereals. Both a positive linear (P<0.05) and quadratic (P<0.01) effect were observed for the G:S ratio on nitrogen (N) digestibility of barley and a corresponding positive linear increase (P<0.01) for wheat. A negative linear effect was found for digestibility of starch (P<0.01) and a positive linear effect for faecal grain content (P<0.01) with increasing G:S ratio. Four Holstein–Friesian steers (mean bodyweight 659 kg (S.D. 56.9)) fitted with rumen cannulae were used in the rumen study. A negative linear effect of G:S ratio was found on rumen pH (P<0.001) while a positive linear effect was found on rumen ammonia (P<0.001) and total volatile fatty acid (VFA) concentration (P<0.01) with increasing G:S ratio. A negative linear effect (P<0.01) was found on the molar proportion of acetic acid. However, this decrease was offset by linear increases in the molar proportions of iso- and n-butyric acid, iso- (P<0.01) and n- (P<0.05) valeric acid, and to a lesser extent in propionic acid (P<0.01). No effect of treatment was found on rumen pool sizes of DM or its constituents. A positive linear effect (P<0.01) was found on the effective degradability (ED) of the DM, OM, N and starch while it was found to be negative in aNDFom (P<0.05). No effect (P>0.05) was found on the fractional clearance rates of DM, OM, aNDFom or starch or on liquid passage rate. It is concluded that increasing the G:S ratio in whole-crop wheat or barley silage linearly increased the intake of digestible nutrients for both wheat and barley and increasing the G:S ratio for whole-crop barley increased the concentration of fermentation products (total VFA, ammonia and the molar proportions of the VFAs, except acetic acid) in the rumen.