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Effect of time of maize silage supplementation on herbage intake, milk production, and nitrogen excretion of grazing dairy cows.

The objective of this study was to evaluate the effect of feeding maize silage at different times before a short grazing bout on dry matter (DM) intake, milk production, and N excretion of dairy cows. Thirty-six Friesian × Jersey crossbred lactating dairy cows were blocked in 9groups of 4 cows by milk solids (sum of protein and fat) production (1.26±0.25kg/d), body weight (466±65kg), body condition score (4±0.48), and days in milk (197±15). Groups were then randomly assigned to 1 of 3 replicates of 3 treatments: control; herbage only, supplemented with 3kg of DM/cow of maize silage after morning milking approximately 9h before pasture allocation (9BH); and supplemented with 3kg of DM/cow of maize silage before afternoon milking approximately 2h before pasture allocation (2BH). Herbage allowance (above the ground level) was 22kg of DM/cow per day for all groups of cows. Cows were allocated to pasture from 1530 to 2030 h. Maize silage DM intake did not differ between treatments, averaging 3kg of DM/cow per day. Herbage DM intake was greater for control than 2BH and 9BH, and greater for 9BH than 2BH (11.1, 10.1, and 10.9kg of DM/cow per day for control, 2BH, and 9BH, respectively). The substitution rate (kilograms of herbage DM per kilograms of maize silage DM) was greater for 2BH (0.47) than 9BH (0.19). Milk solids production was similar between treatments (overall mean 1.2kg/cow per day). Body weight loss tended to be less for supplemented than control cows (-0.95, -0.44, and -0.58kg/cow per day for control, 2BH, and 9BH, respectively). Nitrogen concentration in urine was not affected by supplementation or time of supplementation, but estimated urinary N excretion tended to be greater for control than supplemented cows when urinary N excretion estimated using plasma or milk urea N. At the time of herbage meal, nonesterified fatty acid concentration was greater for control than supplemented cows and greater for 9BH than 2BH (0.58, 0.14, and 0.26mmol/L for control, 2BH, and 9BH, respectively). Timing of maize silage supplementation relative to a short and intensive herbage meal can reduce the substitution rate and increase herbage DM intake of grazing dairy cows.