My April issue column discussed how effectively pastures can run on legume nitrogen (N) alone. While legumes can put quite a bit of N into the pasture, the next question is how effectively are you recycling N in your pasture system?
Whether it comes from legumes or fertilizer, N in pasture can be recycled for new pasture growth. The more effective job you do of managing the natural N cycle, the less money you will spend on N fertilizer.
While cattle consume a lot of N as protein in the forage, less than 5% of the N is retained in their bodies. The rest is excreted as either dung or urine.
When the diet protein level is close to what the animal needs, the excreted N is split equally between dung and urine, with the fecal N being slowly released as manure decomposes. Almost all urinary N is readily available in the soil.
As the protein content of the pasture increases, most of the extra N passes through the urine, making urine a potent N fertilizer. If you've noticed dark green patches of green in your pasture, it's because urine has the N-fertilizer equivalent of 200-1,000 lbs. N/acre in that little patch.
Because most of the N in urine is in a urea form, N can be lost to the atmosphere as ammonia gas, just as with urea fertilizer. Hot, dry soils lose a lot more ammonia than cooler moist soils.
As the urine remains in contact with the soil, the ammonia is converted to ammonium, a positively charged ion, and it becomes bound to the soil. However, as ammonium in the soil is converted to nitrate, it can also leach out of the soil, especially on sandy soils. So even though most of the N consumed passes through the animal, more than half of it easily can be lost from any further potential as a fertilizer. These loss pathways are why we have to continually add N to soils.
Capturing more N
Grazing management that leaves more of the soil covered with green plant residual or dead litter keeps the soil cooler and enhances urine infiltration rate. Good grazing management traps a lot more N in the soil and reduces the ammonia loss, leaving more N in the soil to support the next plant growth cycle. Short grazing periods that leave taller residuals after grazing result in a much more effective nutrient cycle, compared to grazing shorter through a longer period.
Changes in grazing management can make big changes in the effectiveness of the N-cycle. On continuously stocked pastures you may have noticed the urine spots seem to be scattered and don't really affect large areas of the pasture. Nutrient cycling research has shown that as little as 2-5% of the pasture area may be affected by cattle urine in a single grazing season. No wonder urine spots just serve as a reminder of how desperately the pasture needs N.
Graziers using high-intensity, short-duration grazing notice much more uniform pasture growth following a grazing period of just a few days. Research shows that in a short-duration grazing system, as much as 50% of the pasture surface area may be affected by urine in a single year. This produces a much more uniform pasture; and when cattle pass through a pasture the next time, they tend to graze more uniformly.
In a continuously grazed pasture with typical stocking rates for the Midwest, the effective N application rate from cattle urine is less than 1 lb./acre/day. This level does little more than feed the soil microbes.
At the opposite end of the spectrum is a grazing system where cattle are moved to a new paddock every day. In this scenario, the effective N-fertilizer equivalent from urine is around 50 lbs./acre/day — a fertilizer rate that will really make grass grow! Other stocking scenarios fall between these two extremes.
A twice-weekly rotation puts about 20 lbs./acre of readily available urinary N on the pasture. If the pasture has 30-40% legume in the pasture, the combined effectiveness of legume-fixed N and recycled urine can support a relatively high level of productivity. Several university studies around the U.S. indicate a well-managed legume pasture with effective nutrient cycling produces yields comparable to applying 100-200 units of N/acre.
With cost management an ongoing concern for most beef producers, taking nutrient-cycle management seriously is a key step to reducing or eliminating fertilizer costs.
Jim Gerrish is a grazing management consultant based in May, ID, and a former lead pasture researcher at the University of Missouri's Forage Systems Research Center in Linneus. Reach him at 208-876-4067, firstname.lastname@example.org, or visit http://americangrazinglands.com.