Managing a mixed sward to maintain species and functional diversity

Abstract

The intensification of animal production systems to meet the increasing demand for animal products may undermine environmental sustainability. One solution is to establish pastoral environments with diverse plant functional groups, promoting sustainable intensification by enhancing ecosystem services such as nutrient cycling and soil health. However, maintaining the species diversity in such systems, particularly those with grasses and legumes, remains a challenge. Although using the critical leaf area index (LAI) and its corresponding height has proven useful in managing single-species stands, there is a need for further investigation of the criteria for managing more complex swards. This study aimed to identify the management strategies for coupling species diversity and forage production in mixed pastures containing grasses and legumes. The pastures included five species: Tifton 85 (Cynodon spp.), kikuyu grass (Cenchrus clandestinus), pintoi peanut (Arachis pintoi), white clover (Trifolium repens) and birdsfoot trefoil (Lotus corniculatus). Two management heights (17 and 23 cm) and three nitrogen fertilizer rates (50, 150, and 250 kg ha−1) were combined in a randomized complete block design, with a factorial arrangement and three replicates per treatment. The pastures were subjected to intermittent grazing with a 40% defoliation severity, and the experimental period lasted for three warm seasons (November – April) from 2020 to 2022. Forage accumulation was similar for both management heights; however, shorter heights (17 cm) reduced species diversity and higher nitrogen rates increased forage production. Tifton-85 and kikuyu grasses dominated the canopy and coexisted in similar proportions across all treatments. However, higher nitrogen application rates decreased species diversity and dissimilarity in community traits. Our findings suggest that grazing pastures at shorter heights (17 cm) can be detrimental to canopy diversity, whereas managing mixed swards at a pre-grazing height close to the critical LAI (approximately 23 cm for a predominantly Tifton 85 and kikuyu grass mixture) can promote greater species diversity without compromising forage production. Additionally, our data suggest that functional diversity is enhanced when grasses experience suboptimal nitrogen nutrition indices (NNI no greater than 0.8). However, further research is required to fully understand this relationship.