Growing state regulatory pressure and difficulty in managing earthen storage structures are two reasons pork producers in Iowa increasingly use deep manure pits.
“Those producers who want to maximize manure retention, thereby reducing time spent pumping out pits, have chosen to install wet-dry feeders,” says Kris Kohl, Iowa State University (ISU) Extension agricultural engineer. An added incentive is that less pumping also reduces nuisance complaints.
“We have had a lot of producers with deep-pit buildings move to using wet-dry feeders,” he says. These feeders are easier to clean up because there are usually less moving parts than conventional dry feeders, and they don't need to be removed from the gating during washing, he adds.
But he notes a more common reason for the switch: Wet-dry feeders seem to produce about two-thirds as much manure by volume, meaning that the manure is more concentrated.
“In some of these deep pits, we are running up to 80 lb. of nitrogen per thousand gallons of manure,” observes Kohl. “That makes that manure fairly valuable as a fertilizer source and allows producers to extract more value.”
And that value is enhanced when fuel and fertilizer (nitrogen) costs skyrocket as they did in 2001, he points out.
Also, it makes the manure more attractive for neighbors who might need it for crops. Kohl explains: “With wet-dry feeders, if farmers look at the nutrient value vs. the application costs, they are looking at probably three times as much value as the manure costs to apply (return ratio of about 3:1).”
In northwest Iowa, where Kohl is located (Storm Lake), it is common practice for farmers who receive “free” manure to pay all pumping and hauling costs.
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Kohl says that 3:1 return ratio is based on wet-dry feeders tested this past year on about 65 sites with deep manure pits. He compared results with the going rate for nitrogen, phosphorus and potassium at the local co-op elevator.
Differences in manure nutrient value can vary greatly between individual pits. It depends on how much pressure washing a producer does. The more washing, the more water in the pit, he explains.
“Modern wet-dry feeders greatly reduce water wastage and also do not need much washing,” states Kohl. In comparison, nipple waterers that are properly maintained and adjusted for pressure waste much less than those that constantly leak and are set at a high pressure. Swinging waterers typically waste about half as much water as standard nipple waterers, he suggests. Most of the wet-dry feeders used in the study were Aqua Tube feeders, sold by Swine Service Specialists of Lyons, NE. Call (800) 654-1378 for more information.
Figures 1 and 2 detail the results of the tests conducted by Kohl on swine finishing manure in both wet/dry and dry feeder operations. The manure samples were collected from pits two weeks prior to land application by probing the pit or taking a sample off the top. Samples were also collected during pumping and hauling from the first, middle and last loads.
Both Kohl and fellow ISU Extension agricultural engineer Greg Brenneman analyzed data for the best sampling method for deep manure pits.
Brenneman's results are based on 10 finishing pits (see Tables 1 and 2). Five samples were collected from each pit to test for nitrogen and phosphorus. A profile or probe sample, as well as a surface sample collected with a pail, were both taken at least two weeks prior to land application. The remaining three samples were collected from manure tank wagons: one from the first load applied to land, one from the middle load when the pit was half full, and one from the last load applied to the land.
Brenneman concludes finishing pit testing discounts a couple of major assumptions.
“There is less variability in the nutrient content of manure in deep-pit barns than was previously thought,” he says. “The profile or probe sample was assumed to be the superior way to collect a sample. Since most producers are applying swine manure based on its nitrogen content, and the top sample is a better predictor of nitrogen content, producers should use this sampling technique.”
Prior to land application, the pit should be agitated to suspend solids and produce a uniform slurry, notes Kohl. Care must be taken when agitating a pit to prevent gas releases that can kill pigs or humans.
“The results of this study should be good news for producers, because it only requires a pail to collect the samples,” concludes Kohl. “Collecting and analyzing for nitrogen content weeks before the application can help pork producers fine-tune their manure management.”
These two research projects were supported by the Leopold Center for Sustainable Agriculture based in Ames, IA.