Effects of corn processing and supplemental hay on rumen environment and lactation performance of dairy cows grazing grass-legume pasture

The effect of corn processing (9 kg of dry matter/d of ground dry shelled o r 9 kg of dry matter/d of steam rolled) and supplemental hay (9 or 3.2 kg o f dry matter/d of alfalfa hay) on milk yield and composition, rumen environ ment, and starch utilization by lactating cows grazing grass-legume pasture was studied. Twelve rumen cannulated, multiparous Holstein cows in early l actation (95 d in milk), were assigned to a 4 x 4 Latin square design repli cated three times. Treatments were ground shelled corn-based concentrate, g round shelled corn-based concentrate plus alfalfa hay, steam-rolled, corn-b ased concentrate, or steam-rolled, corn-based concentrate plus alfalfa hay. Supplements were fed in equal proportions twice daily. Cows fed steam-roll ed corn tended to have higher percentage of milk protein and lower milk ure a nitrogen concentrations than cows fed shelled corn. Milk yield was not af fected by corn processing or hay supplementation. Intake of pasture forage but not total dry matter intake was reduced by hay supplementation. Starch plus free glucose digestibility in the total tract was not affected by grai n processing; however, starch plus free glucose digestibility tended to inc rease with hay supplementation. Supplemental hay increased starch plus free glucose digestibility through changes in rumen digestion kinetics. Hay sup plementation reduced the liquid rate of passage, and tended to reduce parti culate turnover. Rumen degradability of pasture forage organic matter tende d to be higher for cows fed supplemental hay. Supplemental hay in these die ts had a greater impact on starch utilization than corn processing.