Nutrient and energy digestibility, mineral status and antioxidant defense in goat wethers and periparturient does supplemented with poplar and willow leaves

Background Fast-growing poplar ( Populus spp. ) and willow ( Salix spp. ) trees are frequently cultivated in agroforestry systems in temperate latitudes, and the foliage from these trees is widely used as feed for sheep and cattle. However, little is known about their feed value for goats and their impact on the oxidative and metabolic stress situation during the lambing period. The first aim was to determine the macronutrient and energy digestibility and the usability of minerals of poplar and willow foliage in goat wethers. The second aim was to examine whether supplementing the diet with poplar foliage would improve antioxidant defense status, calcium and metabolic homeostasis during the periparturient period in dairy goats. Methods In Experiment 1, four Saanen goat wethers (1.5 years, 51.8 ± 1.3 kg) were fed hay (HAY) for ad libitum intake or hay plus 650 g black poplar (PNIG), balsam poplar (PMAX) or willow (SVIM) for 3 weeks followed by a 1-week washout in a 4×4 Latin square design. In the third week, animals were subjected to a balance trial in respiration chambers and blood sampled. Experiment 2 was conducted with 16 1-year-old pregnant Saanen goats (51.8 ± 4.4 kg) fed hay ad libitum and 200 to 300 g/day wheat. After parturition, half of the does were fed hay ad libitum and increasing amounts of wheat (100 -250 g/d) (CON), whereas the other half was additionally supplemented with 700 g/d P. nigra × deltoides (POPLAR) during the first 5 weeks of lactation. Animals were blood sampled before and after parturition. Results In Experiment 1, leaves had up to 10 times higher concentrations of crude protein, minerals, phenols, and antioxidant capacity than hay. Wethers fed PNIG had higher plasma Se concentrations than animals fed PMAX or HAY (p< 0.05). Foliage supplementation did not affect plasma heavy metal concentrations, metabolizable energy intake, nitrogen excretion or microbial protein production but reduced methane yield. The PMAX diet had higher organic matter digestibility than HAY or SVIM (p< 0.05). The SVIM diet revealed the lowest aNDF OM and crude protein digestibility compared to PNIG, PMAX or HAY (p< 0.01). In Experiment 2, dry matter intake of Saanen goats increased over time in the CON and POPLAR group to the same extent. The POPLAR group tended to have higher plasma Se concentrations (p = 0.05) but neither improved glutathione activity nor antioxidant status (p > 0.75). In week 2 postpartum, POPLAR goats showed a lower osteocalcin-CTX ratio (p = 0.02) and serum Ca concentration (p = 0.04), which was likely due to tannins hindering intestinal Ca absorption. Conclusion It was concluded that black poplar leaves are a valuable feed source for non-lactating goats, but they are not recommended in early lactation.