More milk from more forage

Farming Monthly National|June 2020

More milk from more forage
Dr Cate Williams: IBERS, Aberystwyth University

As pressure is placed on farmers across the UK to improve the sustainability and environmental impact of their farming practices, many dairy farmers are considering a move to pasture and forage-based systems. One of the key difficulties of a pasture-based system is the reduction in milk yield, particularly in high-yielding cows, and sometimes a loss of body condition (Table 1). This stems from inconsistencies in the quality of grass and problems maintaining dry matter intake (DMI); even with ideal pasture conditions, grass naturally has a lower nutrient density than concentrates. However, pasture-based production systems are not without their advantages, when properly utilised, grazing can improve farm profitability, mostly through reducing bought-in feed costs (Table 1). In a pasture-based system, time and labour costs may also be reduced that would otherwise be needed to feed animals or harvest forage making the business more profitable and sustainable. Through this mechanism, greenhouse gas emissions stemming from the import and transport of bought-in feed are significantly reduced, as well as associated issues with land-use change and deforestation in the exporting country. There is also evidence that the fatty acid profile of milk produced by grazed cows is healthier compared to those fed a total mixed ration (TMR) .

Changes in milk composition

The link between the ruminant diet and the resulting products is well established and can be used to produce milk and meat with specific qualities. One of the key components of milk that is easily manipulated by diet is the fat profile which is also of interest in terms of human health. In grass the human health beneficial conjugated linoleic acid (CLA) and polyunsaturated fats (PUFA) are of most interest. Grass is an excellent source of PUFA, however, because of its toxicity to the rumen microbes, they are saturated with hydrogen upon entry to the rumen. It is this action that determines the saturated fat (SFA) content of ruminant products like meat and milk. Nevertheless, if the cow is fed a good supply of PUFA a certain amount will escape the rumen and pass through to the intestine where it may be absorbed and incorporated into meat and milk. Research comparing milk from cows on a mixed diet versus pasture-fed has found significant differences in the fat profile – after transitioning to pasture, milk produced contained double the amount of CLA when compared to those fed a mixed diet. Overall, the lipid profile of milk from grazing cows is more beneficial to human health than those fed silage or a total mixed ration (TMR), containing lower levels of SFA and higher levels of PUFA. Whilst a trend towards low-fat milk and milk products has emerged, there has also been a shift in consumer interest to ‘healthier’ foods. As such, milk with a healthier fatty acid profile may have the potential to be marketed at a premium.

There is contradictory evidence for differences in milk protein content from cows fed a TMR vs. pasture-fed. Some studies find that pasture-fed milk contains higher protein and casein levels, whilst others report that a grazing diet reduces protein yield. Whilst grass contains a high level of crude protein and amino acids, it is low in rapidly fermentable carbohydrates which limits the supply of energy, possibly leading to lower milk yields and protein content.

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June 2020