Fats and fertility

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Fertility remains among the major issues in dairy herds with conception rates to first service having fallen to around 40% in the UK.

There are no silver bullets to solve fertility issues on farms, but there are many nutritional and practical options that can be implemented to try to improve reproductive performance.

And this is why we are interested in fat. Diets based on forage and cereals will contain around 3 to 3.5% fat in the diet dry matter (DM), whereas research data indicate a requirement of over 6% for high yielding cows. So what are the links between fat and cow fertility?


The key factor influencing cow fertility is energy. High yielding dairy cows suffer a period of negative energy balance in early lactation when their energy requirement to support milk production is greater than their energy intake from the diet due to low feed intakes in the weeks after calving. Consequently, they mobilise body fat – ‘milk off their backs’ – to make up the shortfall in energy supply – clearly visible as cows losing body condition through the early lactation period.

Excessive body condition loss is directly correlated with reduced fertility, with cows taking longer to start cycling after calving and reductions in conception rate of 10% per 0.5 unit body condition score loss having been reported. Maximizing energy density of the diet – providing more energy per bite – is a key target for dairy cows and this is one area where fat supplements play a unique role.

Proven fat supplements contain around 2.5 times the metabolisable energy (ME) concentration of typical cereals, but crucially are not fermented in the rumen so do not add to the acid load in the rumen like fermentable starch sources of energy. So, at a very basic level, fat supplements can help improve energy status of cows with knock-on effects on fertility.

However, fats can also support fertility via mechanisms not related to their high energy contents.


The clearest example of this is often reflected in an increase in production of progesterone – the key hormone for pregnancy – when fat supplements are added to rations.

In simple terms, increasing diet fat leads to an increase in the building blocks required for production of this vital reproductive hormone. Work at the University of Nottingham reported an increase in blood plasma progesterone concentration of 62% when diet fat was increased from 3.5 to 5.9% of DM by addition of a fat supplement.

Egg quality

Some fats have been proven to improve quality of ovulated eggs, leading to an increased chance of successful pregnancy. Work at the University of Nottingham reported that cows offered diets supplemented with Megalac rumen-protected fat to increase diet fat from 4.1 to 5.9% of DM – produced more-viable eggs.

Developing embryos grow rapidly after fertilisation, increasing over ten-fold in size in the three days prior to implantation in the uterus. Fat can supply energy for this rapid growth, while the more unsaturated fatty acids play important roles in the structure and function of biological membranes.

Rumen protection

Fat supplements vary widely in parameters such as % fat in the product, the type or balance of the individual fatty acid building blocks from which the fat is composed, and the degree of rumen protection of the fat. It is important that the fat used is rumen-protected – in other words it passes through the rumen for digestion in the small intestine, avoiding the negative effects on fibre digestion and rumen function associated with non-protected fat sources which can reduce milk fat production and energy supply.


In some situations Omega-3 fatty acids can help improve fertility if supplemented at the appropriate time in the oestrous cycle. This is more likely where cows have limited Omega-3 in their basal rations such as cows with no access to grazing or those with a low proportion of grass silage in their ration through winter.

However, potential benefits to Omega-3 inclusion can only be achieved if these fatty acids are protected from the actions of rumen microbes which would otherwise change their structure and hence mode of action. This is very difficult to achieve with current methods.

With many low-energy silages on farms this winter, fat supplementation will be a common nutritional tactic to increase energy density of diets. Selection of a fat supplement proven to improve fertility will pay dividends in terms of returns from production and reproduction benefits.

For further information browse www.volac.com or Freephone 0800 919 808.