Effects of Increasing the Proportion of High-Quality Grass Silage in the Diet of Dairy Cows

The Swedish dairy cow has almost doubled its annual milk yield over a few decades. This has simultaneously led to an increase in the dietary proportion of concentrate in order to meet the increased nutritional requirement of dairy cows. The proportion of forage is often below 50% of dry matter (DM) intake in conventional dairy production on an annual basis. However, the regulations for organic dairy production stipulate a minimum of 60% DM of forage in the diet, with a permissible decrease to 50% in early lactation. The price of concentrate is currently high, so feed is a large expense for dairy farmers and has a large impact on total farm profits. Since the techniques for forage preservation have improved, enabling earlier harvesting, the nutritional quality of grass silage has increased. This may allow partial replacement of concentrate by high-quality grass silage without any adverse effects on milk production. This thesis examined the effects of replacing concentrate with grass silage of high nutritional quality over the entire lactation. The effects were examined for milk production, enteric methane emissions, milk fatty acid composition and profitability. Over two consecutive years, a total of 92 dairy cows were randomly assigned to one of three diets that differed in the proportion of forage. After peak lactation, from lactation month four, the proportion of forage was gradually increased over the entire lactation up to a maximum in late lactation of 50%, 70% or 90% on a DM basis. Overall, increasing the dietary proportion of silage up to 70% in late lactation did not significantly affect milk yield compared with feeding 50% silage. Positive effects were shown in terms of milk fatty acid composition and profitability. Increasing in the dietary proportion up to 90% silage in late lactation significantly decreased milk production, but the positive effects on milk fatty acids persisted. Enteric methane emissions showed a tendency to increase with increasing proportion of silage in the diet. The overall conclusion was that there is great potential for increasing the proportion of high-quality grass silage in the diet of high-yielding dairy cows after peak lactation, up to 70% silage in late lactation.