Supplemental levels of protein and energy influence ingestion of Romerillo (Chiliotrichum diffusum) by sheep in southern Patagonia

Romerillo (Chiliotrichum diffusum) is an unpalatable and invasive shrub that reduces biodiversity and constraints livestock operations in the Magellan rangelands of southern Chile. Nutrient and energy inputs may enhance romerillo use by sheep, reducing its competitive capacity and restore biodiversity loss. In Exp. 1, 32 ewes were penned individually and randomly assigned to 4 groups (8 ewes/group), where they were supplemented with 1 (Group 1), 3 (Group 2), 5 (Group 3), or 7 g/Kg BW/d (Group 4) of canola meal, a high-crude protein (CP) supplement. Subsequently, all ewes had ad libitum amounts of freshly cut romerillo followed by 1% BW of grass hay. Intake of romerillo increased with the amount of canola meal offered up to 5 g/Kg BW (P < 0.0001), but ewes supplemented at 7 g/Kg BW ate amounts of romerillo comparable to ewes supplemented at 3 or 5 g/Kg BW (P > 0.05), reflected in a negative quadratic relationship between romerillo and canola meal intake (P < 0.001). In Exp. 2, 24 ewes were randomly assigned to three treatment groups (8 ewes/group), where they received a constant amount (2.5 g /Kg BW/day) of supplements with declining ratios of CP to metabolizable energy (ME): Canola meal (Group 1), a commercial pellet (Group 2), and a mix of corn:canola meal with a proportion (67:33) ewes typically select when consuming romerillo (Group 3). Romerillo and hay were fed as described for Exp 1. Ewes supplemented with canola meal ingested more romerillo than ewes supplemented with the mix (P < 0.05), and tended to consume more shrub than ewes supplemented with the commercial pellet (P = 0.06579). The relationship between romerillo intake and supplemental level of CP showed a positive linear effect (P < 0.05), whereas the relationship between shrub intake and supplemental level of metabolizable energy (ME) showed a quadratic effect (P < 0.001) with a minimum at the greatest values of ME intake. Collectively, these results suggest a negative effect of incremental levels of supplemental ME on romerillo intake, provided that the concentration of CP in the supplement declines. Thus, the level of supplementation and CP:ME ratio in the supplements both influenced romerillo intake by ewes, where the ratio observed in canola meal and the level of 5 g/kg BW/d represented the best options - out of those assayed - for maximizing romerillo intake. These quantities and ratios give insights into how nutrients interact with defended and unpalatable plants in the community during the foraging process, contributing to the optimization of targeted browsing programs, aimed at improving the efficiency of the vegetation management plan, as well as the nutrition and welfare of the herbivore.