Should I apply fall N to pastures?

Published: August 29, 2008

We are already well into the optimum nitrogen application window for Missouri pastures. Optimum application window for fall nitrogen in fescue pastures starts around August 1 and response to fall-applied nitrogen decreases incrementally after September 1.

High fertilizer prices have producers reconsidering all aspects of their forage fertilization program. Should I apply fall nitrogen? And if yes, what rate should I apply?

Average yield response to fall-applied nitrogen typically is less than response to spring-applied nitrogen and the fall response can be quite variable from year to year. The big unknown is the amount of fall rainfall. A favorable distribution of precipitation can lead to a large yield response. Unfortunately, you cannot know the how much rain we will get in fall when you need to apply fall nitrogen.

How much nitrogen should I apply? Research from Missouri and other states shows a positive response to rates of nitrogen over 100 lbs/acre when yields are averaged over multiple years. But the amount of response decreases at higher rates. Higher rates are also more likely to reduce legumes in mixed fescue-legume swards. A rate between 50 and 100 lbs nitrogen per acre is justified depending on how much you want to stake on having good fall rainfall with the lower rate favored on pastures you are trying to maintain a high legume component. Rates above 100 lbs/acre are likely to have lower yield response per unit of N applied.

If a pound of nitrogen increases yield 25 lbs and a pound of nitrogen costs $0.75 then the nitrogen cost of the feed is $0.03/lb ($60/ton). Rob Kallenbach found 25 lbs fescue dry matter was the average response to 50 lbs N/acre in a four-year Missouri study on fall-applied nitrogen response in pastures.

The argument supporting fall nitrogen applications is the high quality and high utilization of properly managed stockpiled production. The ability to stockpile fall forage production and then strip graze it in late fall and winter insures most farmers will make good use of any increased yield in response to applied nitrogen. Rob Kallenbach has shown stockpiled fescue maintains quality through March while the toxic element in tall fescue (ergovaline) decreases by half by mid January.

One alternative to feeding stockpiled grass is purchasing hay or other supplemental feeds. Purchase of supplemental feed has the added benefit of importing nutrients onto the farm. A ton of lower quality hay contains about $30/ton of fertilizer nutrients at today's fertilizer prices and the value of high quality hay can exceed $45/ton. To calculate these values we assumed fertilizer nitrogen cost $0.75/lb N and 30% of nitrogen in hay is available as fertilizer; we also assumed phosphate fertilizer cost $1.10/lb and potash fertilizer cost $0.55/lb and 100% of the phosphate and potash in the hay is available as fertilizer. To insure hay nutrients have value on your farm you must commit to distributing manure nutrients around your paddocks through practices such as unrolling hay.

In our example, if you buy low quality hay for less than $90/ton or high quality hay for less than $105/ton you are saving money compared to buying fertilizer for early winter stockpiling of forage. The savings from purchasing hay include saving $60/ton by not purchasing nitrogen fertilizer plus $30 to $45 in fertilizer value in the purchased hay in our example. The quality of the purchased hay, particularly low quality hay, will be less than stockpiled forage. If you are feeding well conditioned cows this may not be a problem. If you are feeding thin cows or growing cattle, poor quality hay will likely increase the cost to balance the ration or reduce animal performance.

Both purchased hay and stockpiled forage will suffer storage losses during winter. If you delay grazing until late winter, stockpiled forage may decrease in quantity by up to 50% (12 lbs increasing the nitrogen cost of feed to $0.06/lb ($120/ton) in our example). Losses in poorly stored hay can be similar.

The decision to purchase nitrogen fertilizer or purchase hay will depend on the type of operation you have and what type of risk you want to take:

• If you need high quality forage and can maximize utilization through good pasture management while managing year-to-year variation in the quantity of available forage then a stockpile forage system is ideal.
• If you can purchase sufficient quality hay at a competitive price, have a way to store it without comprising quality and have a feeding system promoting nutrient distribution around your farm a purchased hay system has merit.