With the diet pressures on cows this spring and the lack of physical buffer fibre available, it is vital to consider the options to minimise the risks to milk fat %. Supplementation with appropriate concentrate and specific fat products (‘high-C16’) can help alleviate these risks through the spring flush.


Turning cows out to pasture represents a period of considerable dietary change as animals adapt 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 %) is often affected. The seasonal variation in milk fat % and the major effect of turnout is clear from Figure 1.
Milk Fat Graph

Figure 1  Average monthly milk fat % recorded during the 2014/15, 2015/16 and 2016/17 seasons across UK dairy herds (AHDB dairy)


Early season grass is characterised by low fibre and high digestibility, along with high concentrations of rapidly-fermentable water-soluble carbohydrates. This results in low physically-effective fibre to stimulate the rumen, increasing acidity and driving down rumen pH. Coupled with this, lush spring grass generally contains around 3 % fat, much of which is poly-unsaturated and potentially highly rumen-active; α-linolenic acid (C18:3) usually comprises between 60 and 75 % of the total fatty acids in grass, with palmitic (C16:0) and linoleic (C18:2) acids the next most abundant (between 6 and 20 % of total fatty acids) (Table 1).


Table 1   Major fatty acids in perennial ryegrass (Dewhurst et al., 2002)


Fatty acid

% DM

% Total fatty acids










Total fatty acids




Grazing dairy cows can consume 400-500 g of highly unsaturated ‘free’ (non-rumen-protected) oil per day, leading to reduced fibre digestion and animal performance (such as lower milk fat %). These effects reflect the well-established negative influence of high levels of ‘free’ oil on rumen microbial fermentation and creation of the oil ‘slick’ effect which reduces digestion of fibre. Turning cows out to graze this lush grass without adequate supplementation to counter the negative effects, risks a major fall in milk fat %, particularly with forage stocks on many farms running low following the long indoor feeding period.


The analytical characteristics of grass suggest that higher fibre supplements may be more appropriate in early season, and higher starch supplements more appropriate later in the season. The issue of concentrate supplementation of high yielding cows at grass was addressed in work undertaken at the Agri Food and Biosciences Institute, Hillsborough, Northern Ireland.


As part of a series of studies, high genetic merit Holstein-Friesian cows were allocated to either high starch or high fibre concentrate supplements offered at 10 kg dry matter (DM)/cow/d. The study was divided into an early-mid season period (1 May to 10 July) and a mid-late season period (17 July to 25 September), and a high daily herbage allowance of 23 kg DM/cow/d was offered to all cows. Results from the study are presented in Table 2.


Table 2     Effect of supplement type (10 kg DM/cow/d) on animal performance



Early-Mid season

Mid-Late season





Herbage intake (kg DM/d)





Total DM intake (kg DM/d)





Milk yield (kg)





Milk fat (%)





Milk protein (%)





Liveweight change (kg/d)





Sayers et al. (2003)


The Hillsborough results show that concentrate type had little effect on either DM intake or milk yield in either the early or late season periods of the study. However, concentrate type had a major impact on milk composition, with strong trends for starchy concentrates to reduce milk fat % and increase milk protein % relative to cows offered fibrous concentrates.


Feeding additional fibre sources can help alleviate reductions in milk fat %, but may lead to reduced performance if energy intake is restricted as a consequence. High levels of starchy concentrates may potentially increase energy supply, this may also increase risk of acidosis and further exacerbate problems with milk fat. However, inclusion of appropriate ‘rumen-protected’ fat sources in the ration offers the potential to increase energy density of the ration and alter milk composition without increasing risk of rumen upset.


Specific increases in milk fat % can be achieved by inclusion of the so-called ‘high-C16’ fat supplements, containing high levels of the milk fat-stimulating palmitic acid. This fatty acid is proven to increase milk fat production low levels of supplement (300-500 g/cow/day) can typically increase milk fat by 0.2-0.3%. Mega-Fat 88 contains 88% of the active C16 fatty acid and is an excellent supplement for cows at turnout to help maintain milk fat % in challenging diets.


With improved weather conditions leading to strong grass growth and a lack of winter forage in the pit to help buffer the spring grass flush, the incidence and extent of milk fat depression is likely to be high following turnout. However, consideration of the type of concentrate supplement offered and appropriate rumen-protected fat supplement can reduce the issues and improve milk production and composition.