Sushil Kumar1 and Narender Singh2
Role of polyunsaturated fatty acids (Ω-3 &Ω-6 PUFA) in various biological process regulations is well studied and documented by many nutritionist in recent past. Different types of PUFA has been reported to perform essential role in regulating biological metabolism, endocrine functioning and disease control in various tissues. In the dairy animals, adequate nutrition is paramount requirement for growth, successful productive as well as reproductive functions such as early onset of puberty and ovarian cycle. Prolonged depletion of body reserves in the lactating animals, especially during early lactation and fast growing heifers can have significant detrimental effects on growth, production and recommencement of estrous as well as conception rate. Traditionally, supplemental lipid in the ration of dairy animals is included to increase dietary energy density, but the PUFA in the supplemental fat have encouraging impact on the metabolism of carbohydrate, protein and fat, as well as growth and differentiation of cells by regulating gene expression of reproductive tissues resulting in a noticeable improvement in production and fertility. In recent years, animal nutritionists are more focused on decreasing the ratio of dietary Ω-6 to Ω-3 fatty acids in the diet of bovines because of higher health benefits and improved reproduction efficiency of lower ratios. There are different oil-feed ingredients (Table 1)that are rich in polyunsaturated fatty acids and can be used in the dietary regimen of dairy animals.
Table 1. Major sources of unsaturated fatty acids for dairy animals
|C18:2 (Ω-6, %)
|C18:3 (Ω-3, %)
In ruminants, a major hurdle for delivery of PUFA is ruminal bio-hydrogenation. In fact, greater than 80% of PUFA from the diet are modified/ hydrogenated by the ruminal microflora. However, notwithstanding extensive bio-hydrogenation of unsaturated FA, feeding increased amounts of PUFA has been reported to alter tissue composition thereby influencing production and reproduction.Therefore, despite limitations in delivery of specific amounts of PUFA for absorption, altering the fatty acid composition of the diet is capable of influencing animal performance.
Changing the content of individual dietary fatty acids in ruminants are reflected as the change in the concentration of various plasma metabolites and hormones. Blood concentrations of NEFA are an indicator of fat mobilization from the adipose tissues and they are related to the mechanism of energy balance of cows. Increased plasma cholesterol and NEFA concentrations in the heifers fed whole raw soybean diet as source of Ω-6 fatty acids was reported by Gonthieret al. (2005). Further, the change in the plasma metabolites may also be affected by the form of PUFA presented in the rumen i.e. either in rumen protected or unprotected form. Heifers’ diet supplemented with Ω-3 PUFA in a partially protected form did not affect NEFA and glucose. There was a linear increase in blood cholesterolwith no effect on the concentrations of triglycerides and glucose level in cows supplemented with graded levels of PUFA. NEFA can be used as an energy source by different tissues like skeletal muscle and hepatocytes, re-packaged into triglycerides and exported as very LDL-cholesterol or stored within the liver, or can also be converted to ketones. Grummer and Carroll (1991) proposed that increased plasma NEFA resulted from one or more of the following: 1) incomplete removal of fatty acids by tissues after lipoprotein lipase hydrolysis of triglyceride contained within circulating lipoproteins; 2) increased net fatty acid release from adipose; 3) decreased NEFA clearance by tissues. Improvement in the reproductive efficiency of the animals fed PUFA rich feeds has been testified by various studies. Supplemental fatty acids have often been used to increase the dietary energy density to minimize NEB in animals, which is related to low fertility in cows. However, Lucy et al. (1991) stated that improved reproductive performance of animals fed PUFA is independent of the increased energy as the different diets were kept iso-energetic in nature and it was supported by the unchanged plasma NEFA concentrations between the groups fed different diets. Additionally, PUFAs have benefits that are independent of changes on the energy status of animals. For example, Ω-3 and Ω-6 FA are reported to be associated with prostaglandins’ metabolism in dairy cows.
Gulliver et al. (2012) postulated that Ω-3 and Ω-6 PUFA might influence the synthesis and metabolism of important reproductive hormones. Thomas et al. (1997) reported that dietary supplemental soybean oil increased the serum concentrations of insulin and GH as well as follicular IGF-I concentrations, which might positively affect reproduction by stimulating ovarian granulosa cell proliferation.Feeding cow supplemental dietary fat with varying saturation increase serum progesterone and cholesterol concentrations. The circulating cholesterol acts as precursor for luteal progesterone synthesis.Various studies have confirmed increased diameter of the dominant ovarian follicle of lactating dairy cows fed with unsaturated fat supplements. In addition to these benefits, PUFAs, or their bio-hydrogenated metabolites, are also supposed to be absorbed by the uterus and inhibit the production and release of PGF2α in the endometrium at the beginning of pregnancy. This prevents the regression of the corpus luteum in the ovary and thus, continuous production of progesterone encouraging favorable condition for survival of embryo.Inhibition of PGF2α by Ω-3 preventing regression of the CL resulting in sustained progesterone release has been reported by other researchers also. Watheset al. (2007) found that diets rich in Ω-6 are coupled with elevated cholesterol, steroidogenic acute regulatory (StAR) protein and PGE2, which might stimulated progesterone production.Mattoset al. (2000) also proposed that Ω-3 PUFA may reduce uterine PGF2α secretion and decrease the sensitivity of the CL to PGF2α thereby improving the fertility by reducing the degree of embryonic loss in early pregnancy. Post-breeding PUFA supplementation modulates PGF2α synthesis, luteolysis, and enhances maternal recognition of pregnancy.
On contrary to this, Dirandehet al. (2014) reported that the overall plasma progesterone concentrations did not differ between groups fed soybean and linseed oil but were greater than control treatments fed saturated fat. Thomas et al. (1997) showed that dietary polyunsaturated fat stimulated a significant increase in serum insulin with increased growth rate of ovarian follicles in cattle compared to animals fed saturated fats. Gandraet al. (2017) supplemented lactating cows’ diet with Ω-3 and Ω-6 rich oil and found greater blood cholesterol concentration and increased number of small and total ovarian follicles in early lactating cows. Westwood et al. (2002) observed a positive correlation between blood cholesterol concentration and reproductive performance including the expression of estrus and conception rate in dairy cows. Similar to this, Ghasemzadeh-Nava et al. (2011) reported that the size of dominantovarian follicle was significantly greater in cows that consumed fish oil or soybean oil in diet. The size of dominant follicle was significantlygreater in soybean oil supplemented group compared with the control cows, and intermediate values were noted in diet supplemented with fish oil.While, mean diameter of ovulatory follicle and CLwas found significantly higher in cows were fed higher dietary Ω-3.Garcia et al. (2003) reported moderate increase in circulating concentrations of IGF-1 during pubertal development in cattle by feeding PUFA.Increased number of medium-size follicles in cycling heifers fed soybean oil was also seen by Thomas and Williams (1996). Whitney et al. (2000) concluded that inclusion of soybean oil at 3% of a forage-based diet in heifers increased plasma cholesterol and growth hormone and;decreased the time to conception.Increasing the Ω-6/ Ω-3 PUFA ratio also increases plasma insulin and progesterone. Regarding the economic analysis of reproductive efficiency, the higher AI/conception rate observed in PUFA supplemented heifers may account for a reduction of one insemination, compared to the SFA supplemented group.
Despite the fact that bio-hydrogenation is a major impediment for delivery of unsaturated fatsin ruminants, feeding Ω-6 and Ω-3 rich oils alters hormone metabolism that further influences cellular functions, production and reproduction.Based on the available literature, it can be concluded that altering the fatty acid composition of the diet with unsaturated fats is capable of influencing animal performance.