Managing maize production in shifting cultivation milpa systems in Yucatán, through weed control and manure application

Milpa cultivation involving cutting an area of forest, burning, and planting crops has existed in the Yucatán Peninsula for more than three millennia. Fallow periods are short and decreasing, leading to a productivity collapse of the system. Technologies that increase yield and maintain plots under cultivation have the potential to decrease the land area needed for family food production, resulting in more mature forests. This study was undertaken to examine the relative importance and potential interactions of declining fertility and increasing weed pressure in reducing maize (Zea mays L.) yields, and to examine the effectiveness of combinations of weed control and sheep manure fertilization rates in sustaining productivity. The study sites were located near Merida, Yucatán, Mexico. The experimental design with three replicates consisted of two sites, 2 cultivation years (1 or more than 1 year of cultivation), three sheep manure application rates (none, 4MgDMha⁻¹, and 8MgDMha⁻¹), and three weed control treatments (none, traditional hand weeding, and herbicide). Measurements included labor required for weed control, weed cover, and maize leaf, stem, and grain harvest and quality. Considerably more labor was needed for hand weeding than for chemical control. At harvest, grass and woody weed cover was greatest for plots with hand or no weed control. Herbicide and, to a lesser extent, hand weeding were effective in controlling herbaceous weeds after the first year of cultivation. Manure applications of 4MgDMha⁻¹ and 8MgDMha⁻¹ increased grain yields by one half (415kgDMha⁻¹ and 425kgDMha⁻¹), stem yields by 36% (549kgDMha⁻¹) and 50% (758kgDMha⁻¹), and leaf yields by 40% (386kgDMha⁻¹) and 45% (431kgDMha⁻¹). With increasing cultivation year, chemical weed control was more effective than hand weed control in maintaining yields. Treatments had minor effects on protein and fiber concentrations of leaf, stem, and grain. Modest manure fertilization combined with chemical weed control has the potential to maintain or increase yields in repeatedly cultivated plots. Manure application could be implemented and maintained on smallholder farms with sheep.