Importance of protein and amino acids in nutrition
Although less well explored than metabolisable energy (ME) balance, metabolisable protein (MP) balance is also important for successful reproduction. In this review study, improved early-lactation MP balance tended to increase the proportion pregnant. Increasing crude protein (CP) intake may increase nutrient loss via increased milk production and have negative effects on fertility, in association with higher urea nitrogen concentrations in blood. Increasing CP content of the diet does not necessarily increase MP availability, but decreasing the degradability of protein or increasing the fermentability of the diet may be more effective in increasing MP availability. The review also highlights that alterations in dietary protein may not simply affect MP balance, but also specific amino acids (AA) composition and supply of metabolisable AA. Specific roles for AA in reproductive performance are not well defined. Lysine and methionine have been suggested to be the most co-limiting AA for production, and supplementation of these may increase milk yield, but results are inconsistent. Supplementation of lactating cows with rumen-protected methionine or lysine has had positive or negligible effects on reproductive outcomes.
Positive effects of fatty acids
Microbial lipolysis and biohydrogenation in the rumen ensure that intake of fatty acids and those available for absorption in the duodenum differ; hence, these issues were explored separately. Fats not only provide an energy source but also are essential precursors for steroid hormones, and the beneficial effects of fat have been observed independently of the provision of energy. The researchers of this review noted that intakes of many of the fats (C14:0, C16:1, C18:0, C18:1 trans, C18:1 cis, and other, g/d) were uni-variably associated with an increased proportion of cows pregnant. No association was found between C18:3 identified with the proportion of cows pregnant, but fatty acids present in fish oil, docosahexaenoic acid, docosapentaenoic acid, and eicosapentaenoic acid are included in the other fatty acids associated with the proportion of cows pregnant.
Role of carbohydrates
Carbohydrates are important sources of energy for cows, as well as for rumen microorganisms and generally increase the efficiency of protein utilisation and microbial protein production. However, increased concentrations of rapidly fermentable carbohydrates can increase the risk of acidosis. The type of carbohydrate also influences the risk of acidosis, with sugars posing a greater risk than starches. Consequently, carbohydrates have positive and negative effects on fertility. The positive associations of starch percentage and intake (kg/d), possibly because of a slower fermentation rate than sugars, and negative associations of soluble fibre and sugar percentage and intake (kg/d) with proportion pregnant were identified in uni-variable analyses. The review also mentioned that a negative association was found between increased CPM-estimated physically effective NDF intake (kg/d) and proportion pregnant in the uni-variable analysis. The only variable that remained in the multivariable models was the effect of sugar intake (kg/d), which was negatively associated with the proportion of cows pregnant. Ruminal acidosis may decrease fertility by reducing feed intake, producing a metabolic acidosis leading to detrimental alteration of uterine environment, stimulating inflammation that induces prostaglandin release, and resulting in luteolysis in a process analogous to that of mastitis.
This study highlights several important findings for future research on the effects of transition nutrition on fertility. It confirms that nutritional management of cows during the transition period can have substantial effects on reproductive success, and this finding is consistent with previous meta-analytical studies in this area. Overall, this study confirmed earlier findings that excessive protein intake can impair fertility, but that a positive MP balance is consistent with better fertility. However, it may be necessary to increase protein intake when feeding fats, and other work suggests a need to control the MP balance before calving. The role of specific metabolisable AA needs further study. This study also, critically, identified potential effects of specific carbohydrate fractions, especially sugar (kg/d), starch (kg/d), and physically effective NDF (kg/d) on reproductive outcomes.
This article is a short summary of the original paper: Effects of nutrition on the fertility of lactating dairy cattle, by R.M. Rodney, P. Celi, K. Breinhild, J.E.P. Santos and I.J. Lean, published in the Journal of Dairy Science, Volume 101, Issue 6.