Effects of replacing maize silage with lucerne silage and lucerne silage chop length on rumen function and milk fatty acid composition

The objective of this study was to investigate whether higher lucerne (medicago sativa; alfalfa) silage inclusion rate and longer lucerne chop length improves rumen function through increased provision of physically effective fiber, when included in a maize and lucerne silage-based total mixed ration. Diets were formulated to contain a 50:50 forage:concentrate ratio (dry matter [DM] basis) and be isonitrogenous and contain equal levels of neutral detergent fiber (320 g/kg). The forage portion of the offered diets was comprised of maize and lucerne silage DM in proportions (w/w) of either 25:75 (high lucerne; HL) or 75:25 (low lucerne; LL). Second cut lucerne was harvested and conserved as silage at either a long (L) or a short (S) chop length (geometric mean particle lengths of 9.0 and 14.3 mm, respectively). These variables were combined in a 2 x 2 factorial arrangement to give four treatments (HLL, HLS, LLL, LLS) which were fed in a 4 x 4 Latin square design study to four rumen-cannulated, multiparous, Holstein dairy cows in mid-lactation. Effects on dry matter intake (DMI), chewing behaviour, rumen volatile fatty acid (VFA) concentration, rumen pH, rumen and fecal particle size, milk production and milk fatty acid (FA) profile were measured. Longer chop length increased rumination times/kg DMI (+2.8 min/kg) relative to the S chop length, with HLL diets resulting in the most rumination chews. Rumen concentrations of total VFA, acetate, and n-valerate were higher for the HLS diet than the other three diets, while rumen propionate concentration was lowest for the HLL diet. Physically effective fiber (particles >4 mm) percentage in the rumen mat was increased when L chop length was fed regardless of lucerne inclusion rate. No effect of treatment was observed for milk yield although milk protein concentration was increased by L for the LL diet (+1.6 g/kg) and decreased by L for the HLL diet (-1.4 g/kg). Milk fat concentrations of total cis-18:1 (+3.7 g/100g FA) and 18:3 n-3 (+0.2 g/100g FA) were greater with HL. In conclusion, longer lucerne silage chop length increased time spent ruminating per kg DMI, but had no effect on rumen pH in the present study. Increasing dietary lucerne inclusion rate had no effects on rumination activity or rumen pH, but decreased the ratio of n-6:n-3 polyunsaturated fatty acid concentrations in milk fat