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Managing manure application and soybeans

Moving away from corn on corn could mean more manure has to be applied before planting soybeans

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Corn is the perfect crop for receiving the nutrients from manure.

Liquid manure applied to meet two-thirds of the nitrogen needs will generally provide a good balance of the macro and micro nutrients needed for a highly productive corn crop.

Using commercial starter and nitrogen for the balance of the crop’s needs ensures nutrients are available early in the growing season when manure nutrient mineralization may be slow.

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In some areas of the province, there are fields that have grown continuous corn for more than 20 years. With the increase of corn rootworm resistance, these fields may be forced into a more diverse crop rotation. These farms may also be forced into changing manure application routines.

Crop rotation has proved to increase yields and increase options for weed and pest control. Crop rotation has also provided additional seasons and opportunities for manure application.

Still, there are fields where continuous corn supplies the feed needed for livestock while also providing a place for manure application. How will manure management planning be affected if these fields are rotated to soybeans?

Soybeans have never been the primary crop for manure because as a legume in inoculated soils, the nodules provide all the nitrogen required. The cost of commercial nitrogen doesn’t pay for the increased yield benefit unless there is a problem with inoculation.

Soybeans consistently have better yields when grown in fields with good fertility levels, but unless soil fertility is low, they don’t respond to applied nutrients in the same way as corn or wheat.

A 50 bushels per acre soybean crop removes about 190 pounds per acre of nitrogen (produced by the nodules), 40 lb. per acre of phosphorus and about 70 lb. per acre of potassium. A 185 bushel per acre corn crop will remove about 150 lb. per acre of N, 80 lb. per acre of P205 and 55 lb. per acre of K2O.

A typical dairy manure with five per cent dry matter, applied at 5,000 gallons per acre (6,000 U.S. gallons per acre) will provide about 75-55-114 lb. per acre of N – P2O5 – K2O.

Where manure has been applied yearly there is also a nitrogen credit from previous applications that provides an additional three to five lb. for every 1,000 gallons.

With application of 5,000 gallons per acre of finisher hog manure (five per cent dry matter) approximately 140-140-145 lb. per acre of N – P2O5 – K2O would be applied with about the same N credit from previous applications.

A rate of 5,000 gallons per acre of dairy manure applied to continuous corn would closely match crop removal. However, when applied to soybeans, it would slowly increase soil fertility levels over time.

With hog manure applied yearly at 5,000 gallons per acre, the P and K applied would exceed the nutrients removed and soil fertility levels would increase rapidly, resulting in increased risk to soybeans from lodging and white mould and increased risk of nutrient loss to the environment.

Benefits

Manure provides nutrients and micronutrients to the soil, especially potassium that improves soybean stalk strength and helps prevent lodging. Manure also provides organic matter to the soil, which helps sustain microbial populations and improve water holding capacity.

There is a large variation in manure composition, which results in differences in nutrients added, nutrient availability and the speed of improvements to soil health.

If manure is being applied to a soybean crop for the first time, a manure analysis is important to determine the appropriate application rate.

White mould often occurs in portions of the field where yield potential is highest, and this often coincides with high fertility, adequate moisture and a robust soybean stand.

Application of manure increases fertility and plant growth and creates a microclimate that is ideal for white mould. White mould can decrease yield potential in affected areas by more than 50 per cent. Manure application should be avoided in areas of the field with history of white mould.

Where fertility levels are high and increasing from regular manure application, the risk of nutrient loss from run-off water and from water leaving through field tile increases. As soil test phosphorus increases over 30 parts per million, the risk of phosphorus loss increases rapidly.

Since most of the phosphorus in run-off and tile water is in the soluble form, it contributes to eutrophication and algae blooms. Nitrogen loss can also be significant. Any nitrogen not used by the crop will remain in the soil as nitrate, and during the fall and winter will move with soil water below the root zone and into tile water. Cover crops planted after harvest help tie up left-over nitrogen in the biomass.

If manure application is planned for soybeans:

  • A manure analysis will help determine application rates that will minimize over-application of nitrogen and phosphorus
  • Keep the crop-available ammonium nitrogen to less than 75 lb. per acre. Too much nitrogen will significantly increase the risk of lodging and white mould
  • Select a variety that is physically shorter and has a good score for lodging (performance trials at gosoy.ca)
  • Plant a variety that has some tolerance to white mould as provided in some seed catalogues
  • Reduce plant population to the lower end of the ideal range for field location and soil texture
  • Wider row spacing will increase air movement through the canopy and may help reduce risk of white mould
  • Practice 4R nutrient management (right source, right rate, right time, right place) to maximize nutrient utilization.
  • If soil fertility levels are increasing rapidly, consider manure application to different fields that require the nutrients or consider selling or trading manure to a neighbour that does not have access to manure.

Chris Brown is a Sustainability Field Crops specialist with OMAFRA. This article first appeared at fieldcropnews.com.

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