SEASON OF MANURE APPLICATION AND ROLE OF THE RECEIVING CROP
Michael Russelle
USDA-ARS-US Dairy Forage Research Center (Minnesota Cluster)
Manure is a natural byproduct of meat, milk, and egg production. Manure
contains many nutrients that crop plants can use. When properly collected, stored, and
applied, manure can substitute for nitrogen fertilizer. Nitrogen losses from livestock
manure, however, reduce its economic value, cause uncertainty about the need for
additional fertilizer for crop production, and may degrade the environment. The principal
forms that cause environmental harm are ammonia gas, nitrous oxide gas, solid and
dissolved forms in runoff to surface water, and dissolved nitrate in drainage water. This
discussion focuses on the gaseous losses.
Loss of ammonia gas is regulated by characteristics of the manure, the weather
during and after manure application, the presence of a growing crop, the condition of the
soil, and the length of time manure is exposed to the atmosphere. Manure composition
(ammonia and water content) is affected by animal diet, barn cleaning methods, the
presence of bedding, and storage conditions. Manure temperature changes with air
temperature, but also with solar radiation, which can warm the dark, manure-covered soil
several degrees. The warmer the manure, the greater is the ammonia loss. Rainfall tends
to reduce ammonia losses after application, whereas hot, dry, and windy conditions
increase losses. Conversely, more ammonia is conserved when liquid manure is spread on
dry soil. Ammonia can be captured by the leaves of a crop canopy, thereby reducing
losses and improving utilization of manure nitrogen. The best way to minimize ammonia
losses is to incorporate the manure during or immediately after application.
Although nitrous oxide is produced by naturally-occurring soil bacteria, it is a gas
that damages atmospheric quality. Its loss is regulated by the amount of nitrate and
available energy compounds in the soil, and by soil water content, oxygen level, and
temperature. Manures contain readily available energy compounds and some manure
nitrogen is converted to nitrate in the soil. Consequently, manure application can increase
nitrous oxide losses. These losses are higher under rainy conditions, unlike ammonia loss.
Mixing the manure into the soil uniformly usually reduces nitrous oxide losses more than
injecting it in bands.
Conserving manure nitrogen from gaseous loss can lead to greater losses in other
ways. It is important to grow a crop on land that receives manure to help prevent all
nitrogen losses – gaseous, soluble, and solid – because the crop uses the manure nitrogen
for growth. Cropping options are limited in each region. For example, it is difficult to
grow winter cover crops successfully in the northern USA to prevent nitrate leaching,
because of the cool, short autumn months after corn and soybean harvest.
Farmers face many problems in producing food, feed, and fiber in economically
and environmentally sustainable ways. Better manure use helps achieve both goals.
1991 Upper Buford Circle, Room 439
St. Paul, MN 55108-6028
voice: 612-625-8145; FAX: 651-649-5058
email: russelle@soils.umn.edu