Milk fat content was changed by ruminal infusion of mixed VFAs solutions with different acetate/propionate ratios in lactating goats

Although it is well documented that conjugated linoleic acids (CLA), such as trans-10, cis-12CLA are potent inhibitors of milk fat synthesis, there might be other factors that could depress milk fat synthesis. Previous studies revealed that milk fat synthesis of dairy goats was not depressed by supplementing with either vegetable oil or fish oil [Morand-Fehr, P., Sauvant, D., Bas, P., 1984. Effet de la fraction grasse des graines de soja sur les performances des che'vres laitie' res. In: 35th Annual Meeting of the EAAP, The Hague, The Netherlands; Martin, L., Rodriguez, P., Rota, A., Rojas, A., Pascual, M.R., Patón, D., Tovar, J., 1999. Effect of protected fat supplementation to lactating goats on growth and fatty acid composition of perirenal fat in goat kids. J. Anim. Sci. 68, 195-200; Morand-Fehr, P., Sanz Sampelayo, M.R., Fedele, Y.V., Le Frileux, Y., Eknaes, M., Schmidely, P.H., Giger Reverdin, S., Bas, P., Rubino, R., Havrevoll, O., Sauvant, D., 2000. Effect of Feeding on the Quality of Goat Milk and Cheeses in Seventh International Conference on Goats, Tours, France, Tome 1, May 15-21, pp. 53-58], which indicated that milk fat synthesis of dairy goats might not be influenced by CLA. Therefore, dairy goats were used in the present study to examine the influence of ruminal acetate: propionate ratio on milk fat content. Four volatile fatty acid mixtures with respective acetate/propionate ratios (APR) of 75:15 (VFAs1), 65:25 (VFAs2), 55:35 (VFAs3), and 45:45 (VFAs4) were infused ruminally in different experimental periods. Buffer solutions were infused to maintain a stable ruminal pH. Milk fat content, plasma insulin concentration, and absorption rate of major volatile fatty acids by the mammary gland were monitored. There was a trend that plasma insulin concentration increased with increased propionate ratio of VFA mixtures infused, and the plasma insulin concentration when VFAs4 was infused was significantly higher than those when the other three VFA mixtures were infused (p < 0.0001). The milk fat content was lowest when VFAs4 was infused, but only significantly different from that when VFAs2 was infused (p < 0.05). Unexpectedly, compared to the VFAs2, milk fat content was significantly depressed by the infusion of VFAs1 (p < 0.05), in which the acetate ratio is extremely high. One of the possibilities was that the shortage of gluconeogenesis precursors inhibited milk fat synthesis. Correlation analysis revealed that milk fat content was positively related to mammary extraction efficiency (MEE) of acetate and propionates linearly, but only the relationship with that of acetate was significant (p < 0.05).