A1 Optimize the protein intake of cattle

Taking into account animal welfare and production concerns, a reduction of 1% in the absolute protein content of animal feed generally corresponds to a potential decrease in ammonia emissions of 5%–15% from animal housing, manure storage, and manure application to land (Sutton et al., 2022). This is influenced by urine and manure pH levels. In addition to curbing ammonia, adopting a low-protein animal feeding approach also effectively reduces nitrous oxide emissions and boosts nitrogen utilisation efficiency in animal production. 

The average crude protein content of diets for dairy cattle should not exceed 15–16% in the dry matter (DM) (Broderick, 2003; Swensson, 2003). Young animals and high-productive animals require more protein concentration than older, less-productive animals. Special emphasis is placed on the nutritional needs of lactating cattle, requiring higher levels of nutrients, due to the demands of milk production. Dairy cattle in early lactation may consume >30 kg feed/day with a crude protein concentration of 16-18%. For beef cattle over six months old, protein intake could be further reduced to 12% (Sutton et al., 2022). Phase feeding can gradually decrease dairy diet protein content from 16% of DM before parturition to below 14% in late lactation; Phase feeding in beef cattle can gradually lower diet protein from 16% to 12% over time. Bittman et al., (2014) provide further details on recommended ranges broken down into low, medium and high ambition levels. These values serve as indicative target ranges, subject to potential adjustments based on local conditions. 

Increasing energy/protein ratio via older grass or supplemented grass with high-energy feeds (e.g., maize silage) effectively cuts crude protein, yet this might be limited in grassland systems due to grass quality issues and purchase costs. Grass-based diets tend to have a protein surplus due to rich grass content, contributing to high nitrogen excretion, which is highest with intensively fertilised young grass or grass-legume mixtures. Extending grazing periods reduces ammonia emissions per animal offering a strategy to curb emissions, although careful grazing management should be followed to avoid potential elevation of nitrate leaching and nitrous oxide emissions.  

For grassland-based ruminant production systems, the feasibility of this strategy may be limited, as older grass may reduce feeding quality. However, certain low-protein diets might contain elevated levels of fibre, which can stimulate the production of methane in ruminants (Dalgaarda et al., 2015). Methane is a potent greenhouse gas, and ruminants are major contributors to global methane emissions. The objective here is to identify the optimal protein and fibre levels within ruminant diets to minimise both ammonia and methane emissions (Hristov et al., 2019; van Gastelen et al., 2019). Particularly for ruminants, achieving a balance between protein degradability (and potentially tannins) and energy levels, including high sugar concentrations, is crucial. These measures may enhance palatability, intake, and even contribute to the ensiling process, thereby reducing losses due to spoilage.