Ruminant Nutrition: Dairy: Rumen Metabolism

The objective of this experiment was to investigate the effects of lauric and myristic acids on ruminal fermentation, production, and milk fatty acid (FA) profile in lactating dairy cows. The experiment was conducted as a replicated 3 × 3 Latin square. Six ruminally-cannulated cows (95 ± 26.4 DIM), were subjected to the following treatments: 240 g/cow/d of each, stearic (SA, control), lauric (LA), or myristic (MA) acids. The basal diet contained (DM basis) 15.2% CP and 33.9% NDF. Experimental periods were 28 d and cows were refaunated between periods. LA reduced (P < 0.001) protozoal counts in the rumen by 96% (no effect of MA), acetate, total VFA, and ammonia concentrations, and microbial N outflow from the rumen (by 32%; P = 0.002), compared with SA. Ruminal methane production was not affected by treatment. DMI was severely depressed (P < 0.001) by LA (18.6) compared with SA (26.7) and MA (24.7 kg/d), which decreased (P = 0.017) milk yield (35.8 and 43.6 kg/d, LA and SA, respectively). Feed efficiency, however, was the highest (P = 0.008) for LA (1.91), followed by MA (1.81), and SA (1.63 kg/kg). Milk fat content was also severely depressed (P = 0.021) by LA (2.59) compared with SA (3.42%). Treatment had no effect on milk protein content. Milk N efficiency was greater (P = 0.022) for LA (34.9) than SA (30.6%). Concentration of milk FA < C16 was 20% lower (P = 0.011) for LA than MA. Concentration of C12:0 was more than doubled (P < 0.001) by LA and C14:0 was increased by MA (by 45%; P < 0.001), compared with SA. LA resulted in lower (P = 0.027) C16, but greater (P = 0.014) long-chain FA, compared with SA and MA. Concentrations of trans C18:1 and cis-9, trans-11 CLA were doubled (P < 0.01) by LA compared with SA and MA. In conclusion, LA had profound effects on ruminal fermentation, mediated through inhibited microbial populations, and decreased DMI, milk yield, and milk fat content. Both LA and MA modified significantly milk FA profile.