Effects of Grazing Hour in a Part-Time Grazing System for Dairy Cows on Herbage Intake Assessed by Acoustic Detection of Prehension Events

A study was carried out to evaluate the within-session grazing dynamics on bite mass, number of bites and herbage intake in dairy cows, combining acoustic detection of prehension events with hand-plucking estimates of bite mass. Nine lactating cows were managed under a part-time grazing system, with animals allowed access to pasture for 5 h per day. Measurements were conducted on four sampling days at 15-day intervals over a 2-month period. Measurements were taken at pasture entry and after 1, 2, 3, and 4 h of grazing. The number of prehensions during grazing was automatically detected from sound recordings using a custom-developed software tool. Hourly herbage intake was estimated as the product of hourly prehension number and estimated bite mass. Daily intake was also independently calculated from the difference between pre- and post-grazing herbage biomass. All measurements were aggregated at the hourly level to ensure temporal alignment among pasture characteristics, bite mass, and acoustic data. Intake was also independently estimated from biomass disappearance. A significant decrease in biomass (p = 0.025) and herbage height (p = 0.008) was observed during the grazing session. Bite mass was significantly affected by grazing hour (p = 0.008), showing a progressive decrease over time. Grazing hour also affected (p < 0.001) the number of prehensions and herbage intake both of which decreased progressively from the beginning to the end of the grazing session. The temporal decline in intake components may reflect both the progressive reduction in available herbage and a decreasing motivation to graze as satiety developed. Mean daily intake estimated from biomass disappearance and from the acoustic approach combined with hand-plucking was 10.71 ± 1.84 and 10.50 ± 2.25 kg DM/cow, respectively. These results suggest that the combined use of acoustic detection of prehensions and measured bite mass may provide a reliable and relatively easy-to-apply method for estimating individual herbage intake in short-duration grazing systems.