Feeding alfalfa hay increased (P < 0.01) C18:3n-3 andtotal C18:3 FA but decreased (P = 0.02) the percentageof medium-chain FA (MCFA, including C12:0,C14:0, C16:0, and total) in milk (Table 9). Alfalfa hay greater concentrations of total C18:3 FA and MCFA
(Table 3), which explains the increase in C18:3 FA but not the decrease in MCFA and C16:0 in the milk of cows fed alfalfa. Perhaps the MCFA were used preferentially for energy or adipose deposition or were just a larger percentage, because alfalfa hay had a numerically lower milk fat yield. Corn silage increased (P = 0.02) trans-11 C18:1 and total trans C18:1 compared with cows fed alfalfa hay, probably because corn silage has greater concentrations of unsaturated FA than alfalfa hay (Table 3). Alfalfa hay also had a greater percentage of coarse particles, which should stimulate rumination and potentially influence metabolism by the fibrolytic bacteria that are primarily responsible for biohydrogenation
(Maia et al., 2007).