Evaluación de eficiencia alimenticia en vaquillonas de carne en sistemas silvopastoriles

In the current context of climate change and increasing food demand, the development of sustainable and efficient livestock systems becomes essential. Silvopastoral systems (SPS) have emerged as an alternative due to their environmental, productive, and economic benefits. To evaluate the effect of the absence or presence of trees at different densities on the partitioning of consumed energy between maintenance and production, we quantified productivity variables (body weight, BW; average daily gain, ADG; and subcutaneous fat thickness), intake variables (dry matter intake, DMI; digestible organic matter, DOM; and metabolizable energy intake, MEI), and the use of consumed energy (energy retained in tissues, RE-tissue; heat production, HP; RE-tissue/MEI; RE-tissue/HP; residual heat production, RHP) in 36 Brangus and crossbred heifers (18 months, 349 ± 12 kg BW). Animals were randomly assigned to a randomized complete block design with three treatments: control without trees (CON), two tree rows (2R), and four tree rows (4R), with three replicates (4 heifers/treatment/block). The spatial arrangement in both tree treatments (2R, 4R) consisted of 5 x 5 + 20 meters. The experimental period lasted 15 days and took place over the summer. Heat production was determined using the heart rate (HR)–O₂ pulse technique, and forage OM digestibility was estimated from fecal N in samples collected during the trial. The MEI was estimated as HP + RE-tissue, and DMI was derived from MEI and forage ME concentration. Data were analyzed using mixed models, and means were considered different when P < 0.05. No differences were observed among treatments in ADG (156 ± 5 g/d) or RE-tissue (243 ± 3 kJ/kg BW0.75/d). Heart rate and O₂ pulse did not differ among treatments (75 ± 1.8 beats/min and 0.279 ± 0.013 µL O2/BW0.75/beat), but average HP was higher in CON than in 2R and 4R (668, 592, and 573 ± 34 kJ/kg BW0.75, respectively). Moreover, HR and HP varied over the day and there was a significant treatment x time interaction, showing minimum values at night (21:00–5:00 h) and peaks at the beginning and end of the day (6:00–9:00 h and 17:00–19:00 h). These patterns were more pronounced in CON heifers, which also showed higher midday values (11:00–13:00 h). The DMI, DOM intake, and MEI were higher in CON heifers compared with 2R and 4R (9.2, 7.9, and 7.9 ± 0.4 kg/d; 4.0, 4.5, and 4.5 ± 0.42 kg/d; and 72,6, 58,1, and 60,6 ± 2.8 MJ/d, respectively). However, when expressed as %BW or BW0.75, no differences in DMI were detected, although CON heifers showed 16% greater DOM intake and MEI, reflecting the higher forage OM digestibility in CON compared with 2R and 4R. Despite no differences in feed conversion (20.4 g/kg DM), the efficiency of ME use, expressed as RE-tissue/MEI and RE-tissue/HP, was lower, while RHP was higher in CON than in 2R and 4R (RHP of 4.77, –2.69, and –2.71 ± 2.57 MJ/d, respectively). These results suggest lower maintenance energy requirements in SPS, with no major differences between tree-density treatments. The greater partitioning of ME toward production in SPS was likely associated with reduced activity and thermoregulation costs resulting from a more favorable microclimate.