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Grazing Behavior Affects Daily Ruminal pH and NH₃ Oscillations of Dairy Cows on Pasture

Department of Dairy and Animal Science, The Pennsylvania State University, University Park 16802

Corresponding author: Lawrence D. Muller; e-mail: lmuller{at}psu.edu.

Grazing behavior of Holstein cows in late lactation at 2 pasture allowances without or with supplementation was studied in a single reversal design. Twenty multiparous cows (4 ruminally cannulated) grazed a bromegrass/orchardgrass pasture offered at 2 pasture allowances: 1) low, and 2) high, with 25 and 40 kg/d of DM per cow, respectively. Half of the cows were supplemented with a mineral/vitamin mixture (1 kg/ d of the mix in a corn/molasses carrier) and the other half supplemented with a corn-based concentrate (1 kg of concentrate per 4 kg of milk). Automatic behavior recorders were used to measure grazing time and number of bites. For the mineral/vitamin mixture-supplemented cows, grazing time and number of bites after the p.m. milking was greater and ruminal pH was numerically lower at the high pasture allowance. For the concentrate-supplemented cows, grazing behavior and ruminal pH did not differ between the 2 pasture allowances. Pattern of grazing time of mineral/vitamin mixture-supplemented and concentrate-supplemented cows influenced daily oscillations of ruminal pH and NH₃-N concentration. Pasture allowance affected grazing behavior of mineral/vitamin mixture-supplemented cows; however grazing behavior of concentrate-supplemented cows was not affected by pasture allowance.

Key Words: grazing behavior • pasture allowance • supplementation • ruminal fermentation

Abbreviation key: CS = concentrate-supplemented, MVMS = mineral/vitamin mixture-supplemented, PAL = pasture allowance

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