Timing of Manure Applications to
Cropland to Maximize Nutrient Value

Karen A. Talarczyk
Regional Specialist, Nutrient Pest Management Program
University of Wisconsin-Madison

Timothy M. Wood
Ag Research Station Superintendent, Lancaster Agricultural Research Station

Livestock manure is a steady, dependable, year-around on-farm resource for those with animals on farms. It is also a resource with significant fertilizer value. However, lack of confidence in correctly crediting the nutrient value of the manure has led to avoidance in using the manure as a nutrient resource to the extent possible. The temptation for farmers is to over-apply nutrients to crops to insure that adequate nutrients re present. This adds expenses from purchasing commercial fertilizers, and increases environmental risk from the nutrients. Economically, dollars can be saved by avoiding nitrogen applications in excess of crop needs. However, research projects and on-farm usage have shown that there are many factors affecting the fertilizer value of animal manure including the rations of the livestock, the type of soil the manure will be applied to and the crop grown, the way manure is stored and applied, and the type and amounts of bedding and other additives.

On southwest Wisconsin dairy farmers, manure management questions are usually centered around uncertainties of application rate and nutrient content. Another area of ambiguity is the timing of manure applications. In much of Wisconsin, daily hauling of manure is the most common means of application. This manure management situation had led farmers to question whether unincorporated manure has the same nutrient value for the following crop regardless of the time of year applied. Fields may receive manure applications from late fall when the previous crop has been harvested until spring when the new crop is panted. A 1992-1995 study at the Lancaster Agricultural Research Station, Grant County, addressed the questions of timing of the manure applications on potential differences in nitrogen availability.

This experiment was designed to answer the question of potential differences in N availability as related to fall versus winter versus spring manure applications. In each of four years (1992-1995) manure was applied at a single rate (35 tons per acre fresh manure) in November of the previous year; in January and March, on top of the soil without incorporation; and in mid-April with incorporation by disking or chiseling within 4-12 days. Results from these treatments were compared with those from several rates of pre-plant applied urea fertilizer (75, 125, or 175 lb N/a) and an untreated control. Each treatment was replicated three times in randomized complete blocks. All plots received 140 lbs/a 8-32-17 starter fertilizer band applied at planting. Corn was planted between May 9 and 19, in 30-inch rows at 30,000 seeds per acre each year, and harvested as whole-plant silage or grain. The fields had been in corn the previous year, and had received no manure for two years before the study began. Therefore, the project field was located in a different area of the research station each year. Standard farm pest management practices were used as needed to all plots uniformly. A summary of treatments is shown below.

Dairy manure was obtained from a neighboring farm since the Lancaster Agricultural Research Station has no dairy cattle on the farm. The goal was to simulate, as closely as possible, the typical southwestern Wisconsin daily-haul situation. A New Idea box-end unloading manure spreader was used for the manure applications. By initially weighing the full spreader and calculating the manure load versus the plot size, we were able to apply the correct amount of manure needed to cover the plot at the desired rate (35 T/A). Application rates were determined by the plastic sheet method in which individual 5' x 5" plastic sheets were weighed after the manure was applied over them, and then the weights of the three sheets were averaged.

Nutrient content of the manure used in each of the years is shown in Table 1. Nutrient value of the manure was confirmed by lab analysis. The nutrient content varied relatively little from load to load although some variation is seen from on year to the next. The manure used for all applications was from the same farm except for the December 1992 application.

The soil type of each of the project fields (1992-195) was a Rozetta silt loam. This soil was formed in silt over limestone bedrock and has moderate permeability and high moisture-supplying capacity. Pretreatment soil tests for each field showed results in these ranges: pH 6.5 to 7.5, organic matter 2.3 percent to2.5 percent, P 23 to 40 ppm (EH), K 130 to 295 ppm (EH). Field slopes ranged from 2 to 6 percent in 1992 to 1994, and 2 to 10 percent in 1995 (EH = Exceptionally High).

Each year had its own weather challenges. 1992 was cool in April, and dry in May and June. 1993 had a cold, wet spring, and extreme wetness into June and July. 1994 experienced normal spring temperatures and a dry May. 1995 was cold and wet in April, and a cool May with normal rainfall.

As shown by the grain and silage yields in table 2, when comparable nutrient rates of manure and commercial fertilizer are applied, equal yields are obtained. Grain and silage yields in 1992 and 1994 shoed that manure provided enough nutrients for top yields and were comparable to those achieved with the commercial fertilizer applications. Furthermore, in these years, no differences in yields were seen between manure application dates. In 1993, the nutrient availability from the manure is substantially less than it was in the other two years. This poorer crop performance was probably due to slower-than-normal manure nitrogen mineralization, since, if it were because of nitrogen losses, we would expect the fall application to be the worst. Since the winter and spring applications actually showed the lower yields, we attribute this to a nitrogen availability problem. In 1993, top yields from manure treatment did no equal those achieved with commercial fertilizer. In 1995, the generally poor yields did not appear to follow a pattern. However, in both 1993 and 1995, fall manure applications appeared to perform as well as any treatment.

The total average nitrogen content per acre of the manure applications was about 406 pounds of nitrogen (35 tons per acre x 11.6 pounds nitrogen per ton) averaged from the 48 manure analysis 1992-1995. By using an equivalent yield basis between the fertilizer response curve and the manure treatment, it appears the manure supplied about 125 pounds of available nitrogen in 1992, 1994, and 1995. In 1993, there was only about 75 pounds per acre of available from the fall manure applications and 40 pounds available from the winter and spring applications. The 1992, 1994, and 1995 data provide confirmation of the 30 percent first-year availability values used by Wisconsin. The lower availability in 1993 was accurately predicted by other techniques such as the pre-sidedressed nitrate test (PSNT).

This study provides evidence that the time of manure applications is apparently not the critical issue inmost years. In 1993 and 1995, weather conditions especially during planting and the early growing season appear to be the important factors in the manure's fertilizer value. In 1992 and 1994, when spring temperatures were warmer, it didn't matter when manure was applied. Its nitrogen was released quickly and sufficiently so the crop could use it. But in 1993 and 1995, when spring temperatures were cool and wet, organic nitrogen was probably not being released in time from the winter and spring applications to be used by the corn. Corn fertilized with winter and spring manure applications yielded 20 bushels per acre less than corn receiving a fall manure application.

These data consistently show that fall application of manure does best in southwestern Wisconsin. However, given the regional topography and geology, environmental risks are associated with application at this time of year. Time management of farming activities is somewhat dictated by the weather. But if fall conditions are conducive to early crop removal, it is a best management decision to manure as many of the cropland fields as possible in the fall. Nutrient losses are not excessive because more than half of the nitrogen in fresh manure is not available until microorganisms breakdown the manure, and these microorganisms cannot live at low temperatures. Manure spread in the fall continues to hold the nitrogen until the soils dry out and temperatures warm up in the spring. However the high permeability of sandy soils is not conducive for retention of nutrients from either fall applied manure or commercial fertilizers.

Although these data show that fall application of manure was most consistent in southwestern Wisconsin, some environmental risks are associated with application at this time of year. To help minimize these nutrient runoff and leaching risks, efforts should be made to spread the manure as evenly as possible at a rate which does not exceed the nitrogen needs of the crop to be grown and to incorporate surface-spread manure within three days of application following the land contours whenever possible and using a form of reduced tillage. Residue preservation should be a goal of whatever practice is used. Manure spreading should be directed to fields that do not discharge unfiltered runoff to streams and lakes.

Based on this study, the book values for nutrient levels of semi-solid dairy manure are valid and dependable. Using these values eliminates the challenge of obtaining a representative manure sample. However, if you have any reason to question whether your manure is atypical, we recommend the manure be analyzed.

Table 1: Levels of Nutrients Determined by Analyses Conducted at the UW Soil and Forage Analysis Laboratory, Marshfield, WI (Nov 1991 to April 1995)

Table 2: First-year corn grain and silage yields as affected by time of manure applications compared to commercial fertilizer on nitrogen availability, Lancaster, Wisc, 1992-1995.

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