The degradation of tropical pastures sown with introduced grasses (e.g., Urochloa spp.) has dramatic environmental and economic consequences in Latin America. Nitrogen (N) limitation to plant growth contributes to pasture degradation. The introduction of legumes in association with grasses has been proposed as a strategy to improve N supply via symbiotic N₂ fixation, but the fixed N input and N benefits for associated grasses have hardly been determined in farmers’ pastures. We have carried out on-farm research in ten paired plots of grass-alone (GA) vs. grass-legume (GL) pastures. Measurements included soil properties, pasture productivity, and sources of plant N uptake using ¹⁵N isotope natural abundance methods. The integration of legumes increased pasture biomass production by about 74%, while N uptake was improved by two-fold. The legumes derived about 80% of their N via symbiotic N₂ fixation. The isotopic signature of N of grasses in GA vs. GL pastures suggested that sources of grass N are affected by sward composition. Low values of δ¹⁵N found in some grasses in GA pastures indicate that they depend, to some extent, on N from non-symbiotic N₂ fixation, while δ¹⁵N signatures of grasses in GL pastures pointed to N transfer to grass from the associated legume. The role of different soil–plant processes such as biological nitrification inhibition (BNI), non-symbiotic N₂ fixation by GA pastures and legume–N transfer to grasses in GL pastures need to be further studied to provide a more comprehensive understanding of N sources supporting the growth of grasses in tropical pastures.

The aim of this research was to evaluate dietary inclusion of Leucaena diversifolia into a grass monoculture (GM) diet and its influence on enteric methane emissions, animal live weight gain and meat fatty acid content of steers. Two GM diets comprised Urochloa hybrid cv. Cayman or U. brizantha cv. Toledo; silvopastoral (SPS) mixed diets included an association of either grass with L. diversifolia in a 74:26 (grass:legume) planting ratio. The experiment was conducted over 16 months with 14 Brahman x Angus crossbred steers (374±7.5 kg body weight), divided into two diet groups. Steers were weighed periodically during the experiment, and methane emissions were quantified with the polytunnel technique. The meat quality and fatty acid profile were measured after slaughtering the steers. Energy density, total and digestible dry matter intake and crude protein were higher in steers fed SPS than GM (P≤0.05). At the end of the experiment, animals from SPS weighed on average 63 kg more than animals offered GM (535 vs. 472 kg; P=0.025). Net methane emitted by steers offered SPS was 142 g per day, this is equivalent to 15% less than emissions from animals that were offered GM (168 g; P=0.007). The meat fatty acid profile was similar between diets, except for cervoniv-DHA and omega-6: omega-3 ratio, which were higher in steers consuming GM diet (P≤0.05). The results of this study suggest that the inclusion of L. diversifolia improves nutrient intake, increases animal productivity, reduces daily enteric methane emission, and decreases the omega-6: omega-3 ratio in meat.

In the low fertility acid soils of the Orinoquian savannas of Colombia, Urochloa humidicola cv. Tully or Humidicola is one of the most widely planted tropical forage grasses for improving livestock productivity. Low nutritional quality of this grass limits sustainable livestock production in this region. In this study, we conducted a phenotypic evaluation under field and greenhouse conditions of one of the first hybrid populations of U. humidicola generated from the forage breeding program of CIAT. Our objective was to identify a set of new hybrids of U. humidicola that combine improved productivity and nutritional quality plus the biological nitrification inhibition (BNI) trait/ability to reduce nitrogen (N) losses via leaching and nitrous oxide (N₂O) emissions. To this end, we tested 118 hybrids (planted in pots) in the greenhouse for over 6 months and measured potential nitrification rates (NR) using soil microcosm incubation. NR values observed ranged from 0.27 to 5.75 mg N‐NO₃⁻ kg soil⁻¹ day⁻¹. Later, 12 hybrids with different levels of NR were selected and field‐tested in the Orinoquia region over a 4 years period (2013–2017) for dry matter production, nutrition quality (crude protein, in vitro digestibility and fibres content) and NR in each year. In the rainy season of 2018, two hybrids with superior agronomic performance and contrasting field level NR (Uh08/1149 and 0450) were subjected to analysis of soil‐borne N₂O emissions after fertilization during 13 days. The NR values recorded were not directly correlated with the forage quality parameters evaluated, however, the two grasses with the lowest NR values were among those with the highest biomass production, crude protein content, and N uptake. The grass hybrid Uh08/1149 and the germplasm accession CIAT 16888 were found as materials with superior forage value, with production of 14.1 and 14.6 tons dry matter ha⁻¹ year⁻¹ (up to 8% higher than the cv. Tully), crude protein of 11.5 and 9.1% per cut (up to 20% higher than the cv. Tully), and N uptake of 31.6 and 25.7 kg N ha⁻¹ cut⁻¹ (up to 30% higher than the cv. Tully). Additionally, these two grasses are likely to exhibit high‐BNI ability, with potential to improve N use efficiency in managed pastures.

Biological nitrification inhibition and forage productivity of Megathyrsus maximus in Colombian dry tropics | Juliana Carvajal-Tapia, Sandra Morales Velasco, Daniel M. Villegas, Jacobo Arango, Nelson José Vivas Quila | Agricultural Journals

In tropical regions, cattle production often relies on grass monocrops (GMC), which limit productivity and land-use efficiency. Sustainable intensification of pastures is needed to meet rising demand for animal-source foods. Silvopastoral systems (SPS) with nitrogen-fixing legume trees, such as Leucaena diversifolia, offer a potential solution but may increase nitrous oxide (N₂O) emissions due to higher nitrogen (N) inputs. We conducted a one-year study comparing Urochloa GMC versus SPS with L. diversifolia on a fertile Pachic Haplustoll in Valle del Cauca, Colombia. Two grass cultivars were evaluated: Urochloa hybrid cv. Cayman and U. brizantha cv. Toledo. In SPS, tree rows created two zones: grass-only (SPS/GO) and under-tree (SPS/UT). We measured forage biomass, grass N uptake, N₂ fixation by L. diversifolia, and soil N₂O emissions under two conditions: no additional N input (+water) and after urine deposition (+urine). SPS produced 30% more forage dry matter than GMC. Leucaena diversifolia obtained 54% of its N from atmospheric fixation. The grass in SPS absorbed 8 kg N ha⁻¹ more per grazing cycle than GMC. Under no additional N inputs, soil N₂O emissions were higher in SPS/UT, likely due to greater N recycling. With urine addition, emissions were variable, with higher values in SPS/GO, but no statistical difference was found between SPS/UT and GMC, possibly due to inappropriate urine-N overdosing in GMC. Although SPS showed higher N₂O emissions under specific conditions, their greater forage yield and quality, plus potential co-benefits like lower enteric methane emissions and higher carbon capture, indicate a favorable production-environment trade-off.

Background and aims In Amazonia pastures are usually established after deforestation by sowing Urochloa grasses. Productivity declines due to nitrogen (N) limitation. Integrating N₂-fixing legumes and grasses with Biological Nitrification Inhibition (BNI) capacity are considered as options for improving pasture sustainability. Methods We studied grass-alone (GA) and grass–legume (GL) pastures on seven farms in Colombian Amazonia. Each treatment included grass with either low (Urochloa brizantha), intermediate (U. decumbens), or high (U. humidicola) BNI capacity. Biomass yields were measured over six harvests. Total N and δ¹⁵N of grasses and legumes and mineral N in topsoil (0–0.1 m) were analyzed in the first harvest. Results GL pastures with high BNI U. humidicola showed a biomass yield benefit of 1.6 t ha⁻¹ over six harvests and 9.3 kg N ha⁻¹ higher N yield for the first harvest compared to GA pastures. More than 70% of the legume N was derived from atmosphere, irrespective of Urochloa grass present. Plant δ¹⁵N indicated that Urochloa grasses obtained 20% less atmospheric N₂ in GL than GA pastures. In contrast to U. brizantha, soil mineral N under U. humidicola did not increase significantly in GL pastures, indicating that BNI reduced nitrification and enhanced N use efficiency. Conclusion Under conditions in Amazonia, legumes enhance the yield and N supply when associated with high BNI grass. The importance of different N sources and isotopic fractionation during N uptake of Urochloa grasses requires further investigation. These results highlight the potential of GL–high BNI systems to support pasture sustainability. Graphical Abstract The alternative text for this image may have been generated using AI.