By Barry Bradford, Michigan State University for Parnell
Nutritionist vs. reproductive physiologist sparring matches are among the greatest pleasures of working in the Animal Science Department at Michigan State University. "Our work accounts for 60% of the input costs on a dairy," we nutritionists like to point out, "so obviously what we do is the most important." The predictable retort from the repro side: "Good luck having cows to milk if they never get pregnant!"
One of my favorite aspects of working in animal science is that we have the opportunity to work toward an integrated understanding of the animal – and not just the inner workings of the cow, but also how management and physiology intersect. These integrative perspectives are critical in dairy cattle fertility. Genetics, health, breeding programs, environment - and yes, nutrition - all play key roles in setting cows up to have a fair shot at becoming pregnant.
Nutritional influence on fertility remains an area needing more research, but we nonetheless have a growing understanding of critical nutritional variables that contribute to fertility in dairy herds. For the sake of brevity, I will bypass the discussion of the importance of fat-soluble vitamins (especially vitamin A) in reproduction, which has been established for nearly 100 years.
Essential fatty acids
Although fats typically comprise only about 5% of a dairy cow's diet, this component has received a lot of attention lately. We now recognize that fats do more than provide a concentrated source of energy – they also serve as "bioactive" nutrients. Simply put, a bioactive nutrient has the capacity to change the function of the animal. Some fatty acids have been recognized as playing a uniquely important role for many years because they serve as the starting material for producing key signals in the body, including prostaglandins. The essential fatty acids, typically grouped into omega-3s and omega-6s, must be in the diet to support the cow's normal physiological function.
Omega-3 fatty acids include alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Omega-6 fatty acids are more abundant in the cow's diet, in the primary form of linoleic acid. In diets typically fed to dairy cows, plasma omega-6:omega-3 ratios generally exceed 10:1. Despite the ability of the microbes in the rumen to alter (biohydrogenate) unsaturated fatty acids, it is possible to impact this ratio. A review of dozens of studies shows that feeding unprotected fish or flaxseed oil (common sources of omega-3 fatty acids) has little impact on plasma omega-3 fatty acids. But, feeding products designed to limit ruminal availability of fatty acids can decrease the plasma omega-6:omega-3 ratio up to 40%. Therefore, although all of these polyunsaturated fatty acids are absorbed in small quantities in ruminants, some feed products can shift the profile of absorbed lipids.
One of the main reasons that essential fatty acids are needed is because they allow the animal to create prostaglandins and other hormonal signals that drive processes like reproduction. One study demonstrated that supplementing either omega-3 or omega-6 fatty acids during early lactation increased peak progesterone concentrations during the estrus cycle, indicating that essential fatty acid deficiency may be a concern. Additionally, these fatty acids influence the cow's immune system differently; omega-6 fatty acids nudge the system toward greater inflammation, while omega-3 fatty acids tamp down on inflammation. During implantation of the embryo in the uterus, it must not be treated like an invader, and controlling inflammation can help prevent early embryonic loss. In at least two studies – feeding either calcium salts of fish oil or rolled flaxseed (both sources of omega-3's) through the breeding period decreased pregnancy loss, pointing to the importance of considering dietary effects on overall fertility and not just pregnancies per AI.
Feeding fat during early lactation has been linked to improved fertility, probably due to the functional impacts described above and improved energy balance (less weight loss) in the month after calving. Therefore, fat supplementation is warranted for optimal reproduction, and the ratio of omega-3:omega-6 fatty acids in the diet is worth evaluating.
A quantitative analysis of many studies suggested that metabolizable protein balance in early lactation was positively associated with fertility. Those cows with more than enough absorbed amino acids for maintenance and milk production became pregnant more quickly after calving than those cows, which showed a deficiency in amino acid supply versus needs. It isn't too surprising – amino acids are the building blocks for all cells in the body, and if the cow is scrounging to survive and make milk, the cow may choose not to start growing a fetus at the same time.
Supplying adequate metabolizable protein involves the appropriate formulation of the entire diet, which does not lend itself to large, well-controlled studies to test this link. However, a recent study specifically tested whether adding rumen-protected methionine to enhance the supply of just this one amino acid would be sufficient to improve reproductive outcomes. In this study, the supplemental methionine had no detectable effect on first-lactation cows, nor did it influence conception rates in multiparous cows. However, methionine did decrease pregnancy loss between 28 and 61 d pregnancy checks – from 20% in control groups to just 6% in treated cows. Consistent with this observation, methionine-supplemented cows had significantly larger embryos on day 33 of gestation. Although the impacts of rumen-protected methionine could be due to several factors, another study demonstrated clear implications of a similar treatment on the programming of day 7 embryos, suggesting the methionine supply potentially impacts oocyte development.
Managing body condition
The most important impact of nutrition programs on reproductive success comes at late gestation. Managing body condition is among the essential factors in determining a cows' health in the transition period. Transition health has vast impacts on fertility. Cows fed with excessive energy as milk production declines in late lactation, and over-fed during the dry period, can gain significant body condition. The excess body fat contributes to metabolic disease, and for reasons not completely understood, it also promotes infectious disease in early lactation. Cows suffering from these disorders have trouble rebreeding, resulting in prolonged lactation and an even greater risk of over-conditioning before the next calving.
In contrast, we want to set cows up for the high fertility cycle. When cows are fed to calve with a body condition score of 2.75 to 3, they are more likely to have a healthy transition period, more likely to become pregnant within 100 days, and therefore at less risk for excessive body condition at the next calving. Optimal body condition sets cows up perfectly for another round of success. The nutritional keys to achieving these goals are low-energy dry cow diets and maintenance diets for late-lactation cows.
Ongoing research is highlighting the impacts of nutritional management for a successful reproduction program. Transition management and body condition score are critical for cows to return to normal cyclicity quickly after calving. Supplementing nutrients such as omega-3 fatty acids and methionine can limit early pregnancy losses.Ultimately, what we've established is nutritionists play a pivotal role (alongside reproductive physiologists) when it comes to getting cows pregnant.
Learn more about how nutrition and herd health impacts reproduction, with the Parnell Webinar featuring Dr. Jose Santos from University of Florida, on March 25, 2021 at 3:00pm CT.
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