Effects of feeding sheep three different forage diets on nitrogen balance and methane emissions

Context In New Zealand, there is a need to examine whether alternative forage mixes could reduce greenhouse gases from grazing sheep and increase resilience to climate change. Pastures with a wider range of species, including plantain, may result in a lower urinary nitrogen excretion and/or lower methane production. Aims The aim was to determine the effect of feeding sheep three different forage diets on urinary nitrogen excretion and methane production. Methods Ram lambs were fed one of three forage diets (ryegrass/clover [RYE], herb/clover [HERB] or a diverse [DIV] mix) ad libitum in metabolism crates. Nitrogen balance and methane emissions were measured. The main components of the HERB diet offered were plantain (62%) and clover (14.5%). The main components in the DIV diet offered were plantain (22.7%), clover (13.7%) and ryegrass (12.3%). The main components of the RYE diet offered were clover (17.9%) and ryegrass (58.2%). Feed intake, urinary and faecal outputs were measured. Methane concentration at the nostrils was recorded over a 3 min period at the same time on 10 different days using a portable Laser Methane Detector. Key results There were significant differences in feed quality of the diets, with the HERB having the highest ME. Lambs fed the HERB diet had the highest intakes of dry matter, water and nitrogen. Lambs on the HERB diet had a lower urinary nitrogen concentration but a higher urine volume, meaning the total daily nitrogen excretion from lambs fed the HERB diet was less than lambs fed the RYE diet but not different from lambs fed the DIV diet. The methane concentration tended to be lower on the HERB diet (P = 0.07). It was significantly less on a per unit of dry matter basis compared to the RYE diet (P = 0.03), whereas the DIV diet resulted in intermediate methane concentrations. Conclusions The sheep fed the HERB diet produced more dilute urine and their urinary nitrogen excretion was lower than the RYE lambs. However, HERB lambs produced a greater urine volume, so these differences were not statistically different from lambs fed the DIV diet. Implications Diet can alter nitrogen balance and excretion and may lead to reduced emissions. In this study, the urinary nitrogen concentration was lower on the HERB diet; however, the higher urine volume meant the difference in urinary nitrogen excreted relative to the other diets was reduced. The lower nitrogen content of the HERB diet was counteracted by a greater dry matter intake compared to the other two diets.