Effect of Supplementing Grass Silage-Based Diets with Concentrate Carbohydrate Sources with Different Fermentation Profiles on N Metabolism of Beef Heifers Fed to Maintenance

The synchronous supply of energy and nitrogen (N) substrates to the rumen microbes on grass silage (GS)-based diets can potentially lead to reduced levels of N excreted in the urine. The objective of this study was to evaluate the effect of supplementing GS-based diet with carbohydrate sources differing in rumen fermentation profile on N metabolism of beef heifers. Six Belgian Blue × Holstein Friesian cross beef heifers (487 ± 29 kg BW) were used in a 3 × 3 Latin Square design (n = 6). Dietary treatments were: (RB) GS supplemented with rolled barley; (MM) GS supplemented with maize meal and; (SH) GS supplemented with soya hulls offered at 40:60 forage to concentrate ratio on a dry matter (DM) basis, at maintenance feeding (40 g DM/kg BW^0.75). Carbohydrate source had no effect on DM, organic matter, or N intake or total N excretion and the amount of N excreted in the urine (p > 0.05). Animals offered MM excreted a higher percentage of N in the faeces and a lower percentage of N in the urine compared to animals offered RB (p < 0.05). There was a time by interaction for ruminal ammonia (NH₃) concentrations (p < 0.01). Ruminal NH₃ concentrations peaked at 2 h post-feeding for all treatments. At 3 h post-feeding, ruminal NH₃ concentrations for the RB treatment remained higher compared to MM and SH treatments. Molar proportions and total ruminal volatile fatty acids were similar among dietary treatments (p > 0.05). Supplementing GS-based diets with different carbohydrate sources had no impact on the total level of N excreted or the amount of N excreted in the urine. However, there was a higher percentage of N excreted in the faeces and a lower percentage of N excreted in the urine when animals were offered MM compared to those offered RB (p < 0.05).