Tree density effects on soil, herbage mass and nutritive value of understory Megathyrsus maximus in a seasonally dry tropical silvopasture in Panama

Silvopasture, an agroforestry practice combining forages, livestock, and trees, has been shown to optimize productivity and mitigate the effects of environmental stress on livestock, yet few ranchers in Panama deliberately incorporate trees into pastures. Panama's Azuero Peninsula experiences a five-month dry season that results in substantial losses in grass productivity. We studied three cultivars of Guinea grass (Megathyrsus maximus cv. Massai, cv. Mombaza, and cv. Tanzania) under open (O), moderate (M), and dense (D) tree canopies. We hypothesized that changes in soil properties, herbage mass, and nutritive value would be optimized in M in agreement with the stress gradient hypothesis, which predicts an increase in facilitation with increasing environmental stress. Results after two years showed few changes in soil chemical properties, but bulk density decreased with increasing tree cover. Annually, O produced more herbage mass than M and D and Massai produced more herbage mass than Mombaza, but not compared to Tanzania. Nutritive value improved under tree cover, with D and M showing higher relative feed values than O. Massai demonstrated the greatest tolerance to drought in M, producing more herbage mass in February when compared to D and O. Results partially support the hypothesis, suggesting that herbage mass is greatest in M for Massai during the dry season, but greatest in O over the year. The observed productivity under moderate tree densities could result in improved animal welfare between the months of December and May, the critical dry period for this region.