The milk form BO cows in North-Benin seems to be healthier than cow milk from South-Benin for it contained higher MUFA and higher unsaturation indices compared to the BO milk from South-Benin.
The more favorable combination of bovine milk FA composition to human health was suggested to be around 30% SFA, 60% MUFA, and 10% PUFA (Hayes and Khosla, 1992). In the present study, the milk FA compositions were 58.48% SFA, 21.98 % MUFA, 19.56 in BO and 68.19 % SFA, 19.41% MUFA and 12.40% PUFA in WF. It’s clear that the current milk FA composition of BO and WF is far from optimal and there is therefore a need of modifying the FA composition towards the ideal composition. The linoleic acid (PUFA, C18:2 cis-9, cis-12) and oleic acid (MUFA, C18:1 cis-9) have been reported to reduce the risk of coronary heart diseases by decreasing serum total cholesterol and low-density lipoprotein cholesterol levels in humans (Mensink et al., 2003). Oleic acid (C18:1 cis-9) belonging to the w-3 family has anticancer and antiatherogenic properties (Haug et al., 2007 in soyeur et al 2009). The linoleic acid (C18:2 cis-9, cis-12), the most important in the w-6 family reduces the incidence of types 2 diabetes by improving the sensibility to insulin (Hu et al 2001 in soyeur et al 2009). On the other hand, Palmitic acid (C16:0) is considered as hypercholesterolemic, responsible for the increase in quantity of Low Density Lipoproteins (LDL) that are responsible for heart coronary diseases in humans (Rotta et al., 2009). In the present study BO presented milk with higher linoleic acid (P<0.001), higher oleic acid (P<0.05) and lower total SFA while WF presented the highest total SFA (P<0.001) and palmitic acid (C16:0, P<0.001). Hence, the BO milk would be preferred to WF milk with regard to heart diseases and the type 2 diabetes prevention in humans. In paradox, WF presented higher milk, protein and fat yields compared to BO breed (Table 1). A crossbreeding between BO and WF may be suggested to optimize the milk production traits as well as the milk FA composition. The w-6:w-3 ratios were 1.14 and 1.3 in BO and WF, respectively. These ratios are in the range of w-6:w-3 lower than 5 recommended to reduce the risk of cardiovascular diseases, obesity, cancer, autoimmune disorders, allergies and some mental disorders (Sabikhi 2004 in soyeur et al 2009). Effects of SCD1 A293V polymorphism In the present study, the frequencies of SCD1 293V were 0.84 and 0.94 in BO and WF respectively. A higher frequency of the V allele (0.82) is also reported in Italian Brown cows (Conte et al., 2010). However, our results did not agree with the higher frequency of the A allele reported for Dutch Holstein-Friesian heifers (0.73), Italian Holsteins (0.57), and Canadian Jersey cows (0.80; Kgwatalala et al., 2009; Schennink et al., 2008; Mele et al., 2007). The difference in SCD1 A293V allele frequencies between the indigenous breeds in this study and the western breeds may be due to the selection pressure for milk traits in the western dairy cattle breeds. The SCD1 AV genotype was associated with higher C14 index and higher total index compared to the VV genotype in BO breed. This result did not agree with Conte et al. (2010) who found that the SCD1 VV genotype was associated with higher C14 index in Italian Brown cows. Moreover, Kgwatalala et al., (2009) showed that it's the AA genotype of SCD1 which is associated with higher C14 index. The effect of SCD1 A293V genotypes on C14 index seems to vary from one breed to another. In the current study, SCD1 genotypes did not significantly affect protein or fat percentage nor fat or protein yields in BO (data not shown) which is similar to the results of Schennink et al. (2008) in Dutch Holstein-Friesian heifers. However, in the present study, the SCD1 V allele showed a significant negative association (-5.68%, P<0.05) with C14 index compared to the A allele in BO.The allele A of SCD1 is therefore significantly associated with 5.68% more C14 index in BO. The positive significant association among the allele A of SCD1 and C14:1 cis-9 and C14 index was reported in several previous studies (Mele et al., 2007; Schennink et al., 2008; Kgwatalala et al., 2009). However, in our study the allele A of SCD1 did not show significant effect on C14:1 cis-9 FA. This may be explained by the large sample size used in the other studies, namely 1,725 Dutch Holstein-Friesian heifers (Schennink et al., 2008), 297 Italian Holstein Friesian cows (Mele et al., 2007) and 525 Canadian Jersey cows (Kgwatalala et al., 2009). The literature also reported significant associations of the SCD1 A293V polymorphism with the C10 index, C12 index, C16 index and C18 index (Kgwatalala et al., 2009; Schennink et al., 2008). However, in our study, no significant effect of SCD1 polymorphism was observed for C18 index, and the C10:1 and C12:1 FAs were not present in our samples. The contribution of the SCD1 A293V polymorphism to the genetic variance of C14 index (4.6%) in BO breed was much lower than the 52 % reported in Dutch Holstein-Friesian heifers (Schennink et al., 2008) and 8.94% in Italian Brown cattle (Conte et al., 2010). The fact that the SCD1 A293V polymorphism has significant effect on C14 index and not on the total index suggests that the activity of the stearoyl CoA desaturase enzyme is not affected (Schennink et al., 2008). Because, the C14 index is the best indicator for the stearoyl CoA desaturase 1 enzyme activity since the C14:0 in milk fat is almost exclusively from de novo synthesis in the mammary gland and almost all the C14:1 cis-9 is likely to be synthesized by the stearoyl CoA desaturase enzyme (Bernard et al., 2006 in Mele et al 2007). Effects of DGAT1 K232A polymorphism The frequencies of DGAT1 232K were 0.77 and 0.92 in BO and WF breeds respectively. A higher frequency of the K allele was reported in a previous study on BO and WF breeds in Benin (Houaga et al., 2017). However, a lower frequency of DGAT1 K allele (0.40) was reported in the Dutch Holstein-Friesian heifers (Schennink et al., 2008). In the present study, the DGAT1 K allele was associated with lower C18 index (P<0.05), total unsaturation index (P<0.01), and MUFA (P<0.01), and with higher saturated fatty acid (P<0.05) in WF breed. These results are similar to the study of Schennink et al. (2007), who studied 1762 Dutch Holstein Friesian cows and found that the DGAT1 232K allele was associated with more saturated fat, but no significant effect on C18 and total unsaturation indices in contrast to the present study. Moreover, the authors, in the same study, showed that the DGAT1 K allele was associated with a larger fraction of C16:0 and lower fractions of C14:0, unsaturated C18 and conjugated linoleic acid (CLA; Schennink et al., 2007). Another study, showed also that the DGAT1 K allele was associated with lower C18 and total indices, similar to our results (Schennink et al., 2008), but with higher C10, C12, C14 and C16 indices, on which we found no significant effect. The majority of milk fatty acids are present in the form of triacylglycerols and the DGAT1 enzyme plays an important role during the last step in triglycerides synthesis. The DGAT1 K232A polymorphism was reported to have significant association with milk FA composition and unsaturation (Conte et al., 2010). However, in our study, DGAT1 K232A polymorphism didn't show any significant association with individual FA but with SFA and MUFA. This is conceivable because the effect of DGAT1 on FA composition and saturation may be due to a higher activity and alteration of specificity of Diacylglycerol Acyltransferase 1 enzyme (Schennink et al., 2007) which may vary between breeds. The discussion of our results on DGAT1 K232A polymorphism on fatty acid traits was limited to western dairy breeds because of the scarcity of data on African indigenous cattle breeds. The genetic variance explained by the DGAT1 K232A polymorphism on C18 and total indices in WF breed were 4.4% and 7.1% respectively which is lower than 15% and 29%, respectively reported by Schennink et al. (2008) in Dutch Holstein Heifers. Although the genetic variance explained by DGAT1 K232A and SCD1 A293V polymorphisms is lower, DGAT1 and SCD1 could serve as genetic markers to improve the milk FA unsaturation indices in indigenous WF and BO cattle breeds respectively.