Fat in ruminant diets

Dr Richard Kirkland, Volac Wilmar ruminant nutritionist

Fat is an essential component of balanced diets and is often added to increase energy density – crucially without increasing the acid load in the rumen. However, fats are not simply energy sources and individual fatty acids have different metabolic functions which help determine the effect they may have on animal production.

Traditional forage/concentrate-based diets typically have a low fat concentration (around 3% of diet DM) and increasing diet fat above this level with vegetable or fish oils, or other high fat ingredients, can lead to problems in the rumen, including:

1. Formation of an oil ‘slick’ preventing rumen bacteria from digesting fibre.
2. Fatty acid toxicity to strains of rumen bacteria.
3. Formation of milk fat-depressing trans fatty acids.
4. Binding of minerals, reducing availability for bacterial growth and rumen function.

These issues can be reduced by use of rumen-protected (rumen-inert) fats which pass through the rumen intact with the fat released for digestion when it passes to the small intestine.

Fat supplements

There is a plethora of rumen-protected fat supplements on the market targeted to ruminant animals and in particular, dairy cows. A summary of the primary product types is presented in Table 1.

Table 1      Common fat supplement types

Calcium salts are most-commonly based on palm fatty acid distillate (PFAD). Digestibility is high (reflecting their content of unsaturated fatty acids), which contributes to the high net energy (NE) value, (27.3 MJ/kg DM) measured in vivo (Andrew et al., 1991). Furthermore, the post-ruminal delivery of unsaturated oleic acid (C18:1), a known amphiphile, to the small intestine can also act to improve digestion of total fat in the ration, leading to improved feed efficiency and increased energy available from the diet.

Rumen stability is an important aspect of calcium salts and can vary by fatty acid composition and physical structure (grist size). Research undertaken with calcium salts of PFAD has reported rumen stability of 90% even at pH as low as 5.5 (Sukhija and Palmquist, 1990).

In addition to increased milk production, improved fertility is a key target with calcium salt supplementation, with potential mechanisms including improved energy status, increased progesterone production and improved viability of ovulated eggs. As highlighted in Table 2, cows offered a higher fat diet containing calcium salts had a significantly higher rate of blastocyst production from both in vitro fertilised (IVF) oocytes and cleaved embryos. Similarly, higher total and trophectoderm cell numbers indicate better quality blastocysts that may improve their subsequent development.

Table 2      Increased blastocyst production through provision of higher-fat diet

Fouladi-Nashta et al. (2007)

A further common group of fat supplements are those categorised as ‘high-C16’ fats, containing close to 100% fat and characterised by palmitic acid (C16:0) concentrations typically 80% or above and near-neutral odour. Several studies have demonstrated the milk fat-stimulating effect of C16:0, with potential mechanisms including an increased absorption efficiency from the blood, preferential usage for milk fat triglyceride synthesis and stimulation of triglyceride synthesis through preferential attachment of C16:0 at specific sites on the triglyceride.

As demonstrated in Tables 3 and 4, responses to C16:0 supplementation (2% of dietary DM) are variable and may reflect differences in stage of lactation, basal diet fat concentration and genetics amongst others. These data indicate differential use of the C16:0 fatty acid supplements in support of milk and milk fat production and it is important to remember that the greater the proportion of the C16:0 supplement used for milk fat production, then the lower the proportion available for oxidation to supply energy for milk and other productive purposes.

Table 3      Milk production effects of dietary supplementation with C16:0 

Piantoni et al., 2013

Table 4      Effects of C16:0 on milk and milk fat production

De Souza et al., 2015

Many of the high-C16 supplements available have an unsaturated fatty acid concentration of 10% or more. These ‘softer’, low melting point, fatty acids are potentially released in the rumen where they may exert negative effects as noted previously.  

Hydrogenated fats available in Europe have primarily been based on PFAD, resulting in products with a more-balanced supply of C16:0 and C18:0 with the characteristic aroma of the raw material. At near 100% fat these supplements commonly have a C16:0 contribution to promote milk fat, while data indicates a potential tissue preference for C18:0 as a metabolic energy source to promote milk yield and productive functions compared to straight C16:0 sources. However, research indicates that digestibility of C18:0 declines with increasing supply, such that these products can have lower digestibility and NE than the calcium salt products. These formulations are useful supplements as general energy sources to increase milk and milk solids.

While those fat supplements discussed previously have been based on free fatty acids, a further category of product commonly included in animal feeds for ruminants, as well as monogastrics, is based on triglycerides. These products are often highly saturated, with the primary fatty acid as C16:0 and contain a proportion of glycerol which may contribute to glucose supply in the lower digestive tract.

Though triglycerides can provide an energy source and a supply of milk fat-boosting C16:0, they are generally considered the least digestible for ruminants, with rate of ruminal lipolysis indicated as the limiting factor. Hence, NE supply from these product types is typically lower than the near-100% fat content may indicate, though as triglycerides they are less likely to stimulate hypophagic sensors in the upper small intestine than are free fatty acids. These formulations are useful supplements as generic energy for ruminants (and monogastrics) to increase energy density.

With so many options available it is important to have clear unbiased advice on what type of feed fat to use for ruminants to meet specific objectives at farm level.