by Dr Richard Kirkland, Global Technical Manager, Volac Wilmar Feed Ingredients
YOUR cows being turned out to grass this spring entails a period of considerable dietary change as animals move from fermented silages and high concentrate levels to fresh grass with lower levels of supplementation.
High intakes of fresh grass can lift milk yield, but milk composition, particularly milk fat percentage, is often affected. This primarily reflects the intake of large quantities of rapidly fermentable sugars and ‘free’ oil, as well as low fibre concentrations in pasture-based diets at turnout.
Feeding at grass should aim to support milk production while maintaining or improving milk composition. Feeding additional fibre sources can help alleviate reductions in milk fat percentage, but may lead to reduced performance if energy intake is restricted as a consequence. High levels of starchy concentrates may potentially increase energy supply, but may also increase risk of acidosis and further exacerbate problems with milk fat.
However, inclusion of appropriate fat sources in the ration offers the potential to increase energy density of the ration without increasing risk of rumen upset. Lush spring grass generally contains around three per cent fat, much of which is poly-unsaturated and potentially highly rumen-active; linolenic acid (C18:3) usually comprises between 60 and 75 per cent of the total fatty acids in grass, with palmitic (C16:0) and linoleic (C18:2) acids the next most abundant (between six and 20 per cent of total fatty acids).
Grazing dairy cows can consume 400-500g of this highly unsaturated ‘free’ oil per day, leading to reduced fibre digestion and animal performance (such as lower milk fat percentage). These effects reflect the well-established negative influence of high levels of ‘free’ oil on rumen microbial fermentation and metabolism in the animal, as well as the formation of specific fatty acid intermediates in the rumen which act to switch off milk fat production in the udder. Hence, fat supplements used must be rumen-protected and contain an appropriate blend of individual fatty acids to best balance requirements.
Higher-C16 fatty acid supplements will help drive milk fat production, but care needs to be taken on body condition loss, while formulations with higher C18:1 fatty acids are more-appropriate for fertility targets. Volac Wilmar’s Mega-Max formulation provides a suitable balance of both key fatty acids to target requirements at grass.