APEX FEEDLOT WATER QUALITY SIMULATION
A manure erosion equation was developed and added to the APEX model for use in estimating nutrient losses from 
feedlots and manure application fields. The modified APEX was validated with data from feedlots near Bushland, Texas, and 
Carrington, North Dakota. The model was used to investigate feedlot management options on a hypothetical feedlot with 
realistic data. Vegetative filter strip (VFS) characteristics including 10%, 25%, 50%, and 100% FLRs (flow length ratios, 
i.e., filter flow length/feedlot flow length) and slopes of 1%, 5%, and 10% were considered. Combinations of these VFS 
characteristics were compared on two soil types: clay loam and fine sandy loam. Management options included three stocking 
rates (10, 15, and 20 m2 head-1) and two clean-out intervals (90 and 180 d). Additionally, two climatic conditions 
(precipitation of 440 and 825 mm year-1) were simulated. Results from 50-year simulations indicate that a VFS downslope 
of the feedlot can greatly reduce nutrient loads. All three VFS characteristics (FLR, slope, and soil) were important in 
controlling organic N and P losses. The best organic N and P control was obtained from a VFS with maximum FLR (100%), 
minimum slope (1%), and a sandy loam soil. Results were similar for soluble N and P control except that VFS slope had little 
effect. The simulated management options (clean-out interval and stocking rate) were also effective in controlling nutrient 
losses. The climatic variable (annual precipitation) gave higher nutrient losses from the feedlot and the VFS with 825 mm 
than with 440 mm. Nutrient control efficiencies, CEs, 100.* (1.0 - nutrient loss from VFS / nutrient loss from feedlot) were 
calculated for all scenarios considered. The VFSs on sandy loam soil with FLRs equal to or greater than 50% gave the highest 
CEs for both soluble and organic nutrients. Other factors including VFS slope, clean-out interval, and stocking rate had 
marginal impacts on CE. For soluble nutrients, CE is inversely related to annual precipitation. Thus, it is important to locate 
feedlots in areas with low precipitation and to provide a well designed VFS. The APEX model with the new manure erosion 
equation provides a tool for designing VFSs for controlling nutrient losses from feedlots.