Evaluación del comportamiento de vaquillonas de carne en sistemas silvopastoriles

Silvopastoral systems (SSP) have gained increasing interest in recent years due to the expansion of forestry in Uruguay and the potential benefits for animal production. In order to explore the differences between traditional rearing systems and a silvopastoral scheme, the impact of incorporating low-density trees on behavior (grazing vs. resting; lying down/standing) and its relationship with individual feed efficiency, this study with beef heifers was conducted on a commercial farm (Cerro Largo, Uruguay) during February 2025. Thirty-six Brangus heifers (18 months of age, 336±66 kg BW at the start of the measurement period) were used, randomly assigned to a complete block design and three treatments: a Control without trees, two rows of trees (2F), and four rows of trees (4F) with three repetitions (4 cows/treatment/block). The spatial arrangement consisted of distances of 5 meters between trees within the row, and 5 meters between adjacent rows, (5x5 layout). These rows (double or quadruple) were separated by 20-meter alleys, which determined a density of 160 trees/hectare for 2F, and 240 trees/ha for 4F. For 8 days, posture (standing/lying down) and activity (grazing/non-grazing) were evaluated using three-axis accelerometers (HOBO pendant G data logger, Onset Corp., MA, USA), placed on the medial face of the left hind leg and on muzzles, respectively. The recordings from both accelerometers, taken every 1 minute, were unified to define three distinct activities for the heifers: grazing, resting + rumination standing, and resting + rumination lying down. The data were analyzed using mixed models with repeated measures, and the effect of the temperature - humidity index (THI) on behavior was evaluated by multiple linear regression, with THI considered a continuous variable. Patterns dictated by the circadian rhythm regarding animal behavior were evidenced, with more important grazing events at dawn and dusk, and minimal nocturnal grazing activity with a preference for the lying down posture during the night. In turn, increases in ITH were associated with reductions in grazing time, which were explained by an increase in the number of sessions and decrease in their duration in the 4F treatment. Heifers in SSP (2F and 4F) had a greater daily resting + rumination lying down time, while no differences were found in the time dedicated to grazing. However, the forage intake rate (g DM/min) was higher in the Control heifers, which also exhibited greater grazing time during midday and nighttime hours. In turn, there was a preference for standing in the Control heifers, and the proportion of lying down time out of the total non-grazing time increased with ITH during daylight hours only in the SSP. These results would suggest greater animal welfare in the SSP, which could be attributed to the microclimate generated by the presence of trees. Individual feed efficiency measured as residual heat production (RHP) of the heifers was positively correlated with the forage intake rate, but not with other grazing or postural behavior variables. In conclusion, the inclusion of trees in a low density caused modifications in the heifers' behavior, which reflected greater thermal comfort, favoring animal welfare, without large differences between the distinct tree densities.