Effect of maize or grass silage on dry-matter intake, milk solids yield, substitution rate and nitrogen excretion of dairy cows grazing lucerne (Medicago sativa) in Canterbury, New Zealand

Lucerne (Medicago sativa L.) is a major forage legume grown in approximately 45 million hectares worldwide. It is an excellent candidate for irrigation and can produce large quantities of quality herbage due to its higher water use efficiency and deep root system. However, lucerne lacks growth during the cold season. Supplements are often offered during these periods to sustain higher intakes. This study investigated the effect of feeding maize or grass silage supplements on dry matter intake (DMI), substitution rate (subR), milk-solids (MS) and nitrogen excretion of dairy cows grazing lucerne in autumn. A field experiment was conducted at Lincoln University Research Dairy Farm, New Zealand, between March and May, 2013. Three diets consisting of: lucerne only and lucerne plus maize or grass silage were allocated to 30 dairy cows in an incomplete randomised crossover design. The silage supplementation, regardless of silage type, increased total DMI and metabolisable energy (ME) intake, and reduced consumption of lucerne by up to 1.36 kg DM/cow/d resulting in a subR of 0.44 kg DMI of lucerne/kg DMI silage. The type of silage had no effect on MS and feed conversion efficiency. Both silage supplements improved the nitrogen use efficiency by +1.73%, reduced excretion of nitrogen via urine by 19 % and increased excretion of N in dung by 8.7%. The results indicated the use of either maize or grass silage as a mitigation strategy to reduce nitrogen excretion through urine in dairy cows grazing autumn lucerne, as well as sustaining high DM and ME intakes when lucerne lacks growth.