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The N Game

Life is a series of building blocks. We learn to crawl, then walk and eventually run. Joe Lally, an Iowa State University nutrient management specialist, likens that progression to the succession of steps in manure containment and management in Iowa's feedlots, with the latest step being a move beyond the environmental benefits to those of cost savings and sustainability.

Lally, along with USDA Agri-cultural Research Service (ARS) researchers Tom Moorman and Jeremy Singer of the National Laboratory for Agriculture and the Environment in Ames, IA, have worked with several Iowa producers over the past couple of years to improve their use and efficiency of feedlot manure in crop production. One of the things they've learned is that timed application of feedlot effluent on a cover crop of cereal rye not only captures feedlot manure's nitrogen (N) to provide another supply of harvested forage in the spring but also retains N in the soil for use by the summer crop (Tables 1 and 2).

“The use of a cover crop appears to be a useful method of capturing manure N and recycling it into silage,” the researchers said in a progress report released this summer. “Based on these preliminary data, producers may be able to increase annual silage production (both corn and cover crop) by including the cover crop in their crop-production system. The rye cover crop will also protect the soil from erosion and help maintain soil organic matter.”

Joe Lally puts it another way: “We could potentially take $100/acre off the cost of production. We don't get many opportunities like that in life to ratchet down costs. Usually it's a penny here and a penny there.” He says it's a practice that could be applicable west to Nebraska, east to Ohio and south to northeast Kansas and northwest Missouri.

New structures

Surface runoff ponds are a relatively new feature of the Iowa feeding landscape, brought on by new Environmental Protection Agency requirements on concentrated animal feeding operations (CAFO). Most Iowa feedlots of more than 1,000 head have constructed such containment facilities and irrigate the liquids onto nearby cropland, generally using center-pivot irrigation systems, Lally says.

The collection and storage facilities utilize pickets to strain off liquids from the solids. The liquids are irrigated onto cropland; the leftovers are field spread.

“Our first serious effluent applications were in 2007,” Lally says of his Iowa CAFO work. “That's when we learned how to manage the water, how the basins filled and didn't fill.”

Once the facilities and their management were in hand, the next logical step was to maximize the value of the manure.

“The reason for cover crops is for nutrient retention, minimizing soil erosion and adding organic matter to the soil,” says Singer, a research agronomist. “So, typically the cover crop is sprayed with herbicide in the spring and tilled under with the cash crop to follow.”

But one of the traditional problems with production of cover crops is getting them started in the fall. They're generally planted late in the year after the summer crop has been harvested and soil moisture is lagging. So, the cover crop goes in the ground and just sits there.

“With center pivots, it's a whole new ballgame,” Lally says. “If you put the water down as soon as you put down the cover crop, germination is no longer an issue and it provides a stronger stand going into the winter.” Plus, the head start offers great potential for a crop in the spring.

A unique project

While considerable research has been conducted over the years on cover crops, this project was unique, Lally points out.

“We really wanted to use cover crops to capture existing nutrients and keep them there for further use in the next crop,” he says. “Our goal is to capture the N that we're putting down with these systems. We don't want it to leach out; we want to capture it in the root zone until we get a growing crop of corn there to use it.”

So, working with an Iowa feedlot, a 7,000-head-capacity operation on which the ARS researchers had conducted effluent analysis and soil tests, rye was drilled on 141 acres following harvest of corn silage. Half of those acres were center-pivot irrigated with effluent.

The corn silage harvest for the 2008 crop year — 32 tons/acre — had been completed on Sept.17, and the rye planted two days later. The center pivot was operated on Sept. 22, Oct.19, Nov. 19 and Nov. 24 to empty the feedlot's runoff detention basin. Altogether, the applications of effluent delivered 100-150 lbs. of N/acre.

On Nov. 25, soil samples, root and shoot growth measurements were taken at four locations. The sampling process was repeated on May 14, 2009, except that an area of the field under the center pivot without a cover crop was sampled, the researchers report.

By May, the cover crop shoots had accumulated 200 lbs./acre of N, which was mostly captured by harvesting the cover crop.

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Despite at least 60 lbs./acre of N uptake by the cover crop in the fall, there was some residual soil N in the soil (Table 2), the researchers report, with slightly more N present under the center pivot. By May, the cover crop reduced available N in the top 1 ft. of soil compared to the fall concentrations or to the soil without the cover crop. The soil under center pivot with a cover crop had 33 lbs. of available N/acre with the cover crop, compared to 146 lbs. of available N/acre without the cover crop. These samples were taken from the same part of the field and the difference was likely due to the cover crop rather than differences in soil or topography, the researchers add.

The rye cover crop was harvested for silage on May 30 and June 1, 2009. Total production from 137 acres was 900 tons at 45% moisture.

“This corresponds to 7,200 lbs./acre of dry matter (DM) and compares well with the DM data in Table 1, considering that sampling was conducted on May 14 and the field wasn't cut until May 26,” the researchers say.

The rye registered a 16.4% crude protein (dry weight basis). It's still too early to determine how much seasonal DM and feed nutritive value will differ than corn silage alone. The next task is to quantify the total silage biomass this fall, Singer says.

“Planting the rye and harvesting the rye silage delayed planting of the corn to June 4, so using a shorter-season corn hybrid to get the silage harvested in a timely fashion in the fall with the delayed planting in the spring is something we need to consider,” he adds.

But the system appears to be a win-win for production costs and the environment, Singer says.

“I think there's little doubt that energy prices will continue to increase. So, if producers can retain and recycle their nutrients on-farm, it will have a large economic impact in the future. Thus, this kind of system can be quite attractive both in terms of the economics and the environment.”

Table 1. Nutrient uptake and dry matter (DM) biomass of a rye cover crop following corn silage
Irrigation Shoot DM Root DM Shoot N Root N Shoot K Shoot P

Fall 2008
Under CP 1,939 432 76 7 54 15
No CP 1,257 648 58 10 39 11
Spring 2009
Under CP 5,800 2,232 200 39 278 36
No CP 5,240 2,767 195 46 214 33
Samples were obtained from under a center pivot (CP) irrigation system or not under the CP on Nov. 25, 2008 and May 14, 2009. Root data are from the 0- to 12-in. soil depth. Spring root biomass from fallow area under CP, without cover crop was 724 lbs. DM/acre. The center pivot is used to irrigate effluent from feedlot footprint.
Table 2. Soil nitrogen (0- to 12-in. depth) in the field under center pivot (CP) irrigation or not at the time of cover crop sampling

Available N (lbs./acre)
Fall 2008
Under CP 96
No CP 82
Spring 2008
Under CP 33
No CP 14
Under CP, no cover crop 146