Effects of 3-NOP on feeding behavior of dairy cows fed maize silage-based diets.

The primary objective of this study was to determine the effects of 3-nitrooxypropanol (3-NOP) supplementation and silage source on feed intake and eating behavior in dairy cows fed maize silage-based diets. A secondary objective was to investigate if any observed effects were consistent across silage source, exposure period, parity, and lactation stage. Forty-eight Holstein cows (balanced for multiparous vs primiparous and earlier vs later lactation) were enrolled in a balanced 4 × 4 Latin square design. The dietary treatments consisted of no inclusion or inclusion of 3-NOP (Control: 0 mg; or 3-NOP: 60 mg of 3-NOP/kg of DM), combined with 2 different sources of maize silage (high stubble or low stubble), which were arranged in a 2 × 2 factorial design. The diets were delivered as a partial mixed ration (PMR), in addition to a maximum of 0.81 kg DM/d of concentrate feed in a methane measuring unit (MMU). The entire experiment had 4 periods, each lasting 3 wk. Differences in PMR and total DMI between Control and 3-NOP were dependent on experimental period or week. Cows on 3-NOP spent less time eating PMR (158 vs 187 min/d; SEM = 5.59), made more attempts to access unassigned feed bins (32.5 vs 10.6 visits/d; CI = 19.3 - 54.7 and 6.11 - 18.5 respectively), and more unrewarded visits to the MMU concentrate feeder (2.16 vs 1.54 visits/d; CI = 1.73 - 2.68 and 1.21 - 1.95 respectively). Meal size and meal length were also lesser overall when cows were fed 3-NOP compared with Control (2.44 vs 3.06 kg DM/meal; SEM = 0.08 and 24.7 vs 30.3 min/meal; SEM = 1.03, respectively), with the differences being dependent on experimental week. When cows were fed the high-stubble silage, they had a longer meal criterion if they were also fed 3-NOP compared with Control (32.9 vs 30.8 min; SEM = 1.47). In conclusion, 3-NOP supplementation reduced feed intake, meal length, and meal size, and increased attempts to access alternative sources of feed. However, these effects were not more acute during initial exposure than in the remaining period of exposure. In maize silage-based diets, reduced feed intake in cows fed 3-NOP may indicate alterations in feeding motivation and rumen dynamics, but the mechanisms causing these changes and their potential effects on welfare require further study.