Effects of 3-nitrooxypropanol on feeding behavior of dairy cows fed grass-clover silage-based diets

The objective of this study was to determine the effects of 3-nitrooxypropanediol (3-NOP) supplementation and silage source on feed intake and eating behavior in cows fed grass-clover silage-based diets and to further investigate if any observed effects were consistent across exposure period, parity, and lactation stage. Forty-eight Holstein cows (balanced for multiparous vs. primiparous and early vs. mid lactation) were enrolled in a balanced 4 × 4 Latin square design. The dietary treatments consisted of inclusion or not of 3-NOP (3-NOP: 60 mg/kg DM; or control: 0 mg/kg DM), combined with 2 different sources of grass-clover silage (spring growth or first regrowth), which were arranged in a 2 × 2 factorial arrangement. The diets were delivered as a partial mixed ration in addition to a maximum of 0.83 kg DM/d of concentrate feed in a methane-measuring unit. The entire experiment had 4 periods, each lasting 3 wk. When cows were fed 3-NOP, they had a lesser total DMI (20.7 vs. 21.6 kg DM/d). When mid-lactation cows were fed first regrowth grass-clover silage with 3-NOP, they made more attempts to access unassigned feed bins, particularly during period 1, whereas this effect was not observed in early-lactation cows. There was no main effect of 3-NOP supplementation on meal frequency or meal size. In conclusion, the reduction in feed intake due to 3-NOP supplementation was not more acute during initial exposure, whereas attempts to access unassigned feed bins by mid-lactation cows on first regrowth silage were elevated primarily during period 1 and declined in subsequent periods. In grass-clover silage-based diets, reduced feed intake in cows supplemented with 3-NOP may indicate altered rumen dynamics and feeding motivation. However, the causes of these changes and potential welfare implications associated with them require further study.