Effects of Fresh and Composted Dairy Manure Applications on Alfalfa Yield and the Environment in Arizona

The Unified Animal Feeding Operation Strategy requires that field application of animal waste, a common fertilization and disposal practice, may not exceed crop nutrient needs. Additional guidelines set forth by the Arizona Department of Environmental Quality state that animal waste applications on agricultural fields in designated Confined Animal Feeding Operations (CAFOs) must be made in a manner such that the total N applied to the field cannot exceed the uptake from the crop grown. Because alfalfa is grown year round and can take up large quantities of N, many operators of CAFOs apply animal waste to their production alfalfa fields as method of waste disposal. In this research, fresh and composted dairy manure was applied to plots in a production alfalfa (Medicago sativa L.) field to determine the impact on alfalfa yield, soil nitrogen (N), phosphorus (P), and electrical conductivity (EC) levels and the potential for nitrate (NO₃) and phosphate (PO₄) leaching. Unfertilized plots were maintained as controls. Fresh and composted manure was applied to fertilized plots after each harvest at a rate intended to replace N removed from the previous cutting. After 1.5 yr and 13 cuttings, soil analysis down to 150 cm depth showed no significant difference in soil N between treatments. At study end, NO₃–N made up 1.1% of total N in the fertilized plots but only 0.6% in control plots. Changes in soil N were not significant. Soil P content increased in fertilized plots but remained stable in control plots. Final soil PO₄ measurements were 16, 99, and 116 kg ha⁻¹ in the control, manure-treated, and compost-treated plots, respectively. Leachate from three drainage lysimeters contained no detectable NO₃ or PO₄ from any of the treatments. LSD showed no difference in EC between the beginning and the end of study, and alfalfa yield did not vary among treatments.