Corn silage or high moisture corn supplements for beef heifers grazing temperate pastures: effects on performance, ruminal fermentation and in situ pasture digestion

Effects of replacing fresh pasture with corn silage (CS) or high moisture corn (HMC) on average daily gain (ADG) and rate of subcutaneous fat deposition (RFD) were evaluated in 48 Angus × Hereford heifers (196 ± 6 kg) under grazing conditions. Six additional heifers (284 ± 25 kg), fitted with ruminal cannulaes, were assigned to a replicated 3 × 3 Latin square in 15-days periods, to study the effect of supplementation on ruminal fermentation and in situ pasture digestion characteristics. All heifers were assigned to one of three treatments as follows: in T0 fresh pasture (800 g kg−1 grass and 20 g kg−1 legumes) was the sole component of the diet; whereas CS and HMC were included at 400 and 310 g kg−1 of the diet on a DM basis in T1 and T2, respectively. No differences in ADG (910 g/day, P = 0.34) or RFD (0.022 mm/day, P = 0.62) were detected among treatments. Total DM intake was higher in T2 (8.54 kg/day) when compared to T0 (7.24 kg/day) and T1 (6.68 kg/day). Average ruminal pH values were higher (P < 0.01) in heifers that received CS (pH = 6.3) when compared to T0 (pH = 5.9) and T2 (pH = 5.8). Treatment by hour of sampling interactions were detected for rumen ammonia nitrogen concentration (P < 0.02); being higher in T2 at hours 19 and 23, but similar among treatments for hours 3, 7, 11 and 15 of sampling. Total volatile fatty acid (VFA) concentrations and molar proportions of acetate and n-butyrate tended to differ among treatments, but molar proportion of propionate was similar among treatments. Potentially digestible fractions of pasture DM and NDF were higher in T1 (P < 0.05). The rate of digestion of pasture NDF, degradable NDF fraction and effective degradability were decreased by HMC feeding (P < 0.05). Supplementation of temperate pastures with CS or HMC permitted an increase in stocking rates from 4.2 in T0 to 7 (T1) and 6.2 heifers/ha (T2). Animal live weight gain increased from 320 in T0 to 499 (T1) and 467 kg ha−1 (T2), without affecting ADG or FDR of the animals. These results suggest that CS supplementation had a positive associative effect on high quality pasture digestion, being a more appropriate supplement than HMC to be used in grazing beef heifers.