‘Fats and fertility’: a much-discussed and long-researched topic with some contradictory data and a variety of claims for different feed fat products. What does the evidence really say? Dr John Newbold, Volac Wilmar Feed Ingredients discusses.

A recent meta-analysis, conducted by scientists in Australia, gives us a solid foundation. Meta-analysis? This is a commonly-used and very robust statistical method to integrate results from different experiments and pull-out key messages. Across all studies, use of fat supplements increased risk of pregnancy by 27%¹.

So there’s something real here, but of course it does not mean that this response will be seen in every cow on every farm. Different fats affect different biological systems at different time windows between late gestation and the establishment of a secure next pregnancy. Use of a dietary fat to aid reproduction – and choice of fat – depends on correct diagnosis of the reproductive problem.

Is the uterus failing to recover (to ‘involute’) after calving? Does negative energy balance delay the restart of activity in the ovaries? Is there enough support for the young embryo both before and after ‘maternal recognition’, when it implants into the uterus and the placenta begins to form? Directly or indirectly, fat nutrition can affect all of these vital processes, but for the rest of this article let’s focus just on the survival and growth of the young embryo.

For several days after conception, the embryo is nourished by histotroph, a ‘soup’ of nutrients and growth factors secreted from the uterus in response to the hormone progesterone. The availability of progesterone in blood delivered to the uterus is the net result of its synthesis from cholesterol in the corpus luteum in the ovaries and its breakdown in the liver. It has been argued that high-yielding cows, with high rates of dry matter intake, show chronically low progesterone (and chronically poor reproduction) because of high flow of blood – and therefore rapid progesterone breakdown – in the liver.

It’s been shown repeatedly that enrichment of dairy cow diets with a good rumen-protected fat will increase circulating levels of progesterone. Work from the University of Nottingham provides one of the clearest demonstrations, with a linear increase in blood progesterone in the crucial few days after ovulation as Megalac increased from 0 to 3% of total diet dry matter².

There are probably multiple mechanisms underlying this effect. Firstly, improved energy nutrition in early lactation – before ovulation – supports follicle growth, resulting in a higher number of cells capable of producing progesterone after ovulation. Secondly, after absorption, fatty acids are packaged into lipoproteins for distribution around the body. This increases supply of cholesterol – the precursor of progesterone – to the ovaries.

Back in the 1990’s, research conducted with beef cows in the US suggested that dietary rumen-protected fat (and specifically Megalac) could reduce the rate of progesterone breakdown in the liver³. More recent research from Ireland with dairy cows lends support to this idea4.

There’s an apparent contradiction between these positive effects of dietary fat on reproduction (via progesterone) and the negative effects of body fat mobilised in early lactation. However, these two routes of fat supply are not the same: fatty acids from body fat are carried around the body as free fatty acids bound to protein (‘non-esterified fatty acids’, or NEFA), whereas most dietary fat is carried as lipoproteins – along with the cholesterol needed for progesterone synthesis. Also, higher NEFA in early lactation is associated with other factors that negatively affect reproduction (such as low levels of the hormones insulin and IGF-1), independently of fatty acid supply. Finally, there’s evidence that some of the negative effects reported for individual NEFA are not apparent when a mix of fatty acids is supplied.

With multiple touch-points between dietary fat and fertility, it’s tempting to think of a programmed approach, using different types of dietary fat to optimising fat nutrition at every stage from late gestation to stable mid-pregnancy. However, targeting individual cows with different diets depending on their reproductive stage is challenging in practice, depending on how cows are grouped and fed.

One approach was implemented in the final stages of the Nottingham project described above. The rate at which cows became stably pregnant was greatest when dietary fat (as Megalac) was supplemented once ovarian activity had re-started after calving5.

‘Fat and reproduction’ are undoubtedly closely-linked. A deeper understanding of the underlying biology can lead to correct matching of the level and source of fat to the target cow, resulting in improvements in this key driver of dairy farm profitability.

1. Rodney et al. (2015) J. Dairy Sci. 98: 5601-5620
2. Garnsworthy et al. (2008) J. Dairy Sci. 91:3824–3833
3. Hawkins et al. (1995) J. Anim. Sci. 73:541-545
4. Hutchinson et al. (2012) Theriogenology 78: 878-886
5. Garnsworthy et al. (2009) Reproduction 137: 759-768