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Grass hay fertilization

Doug Beegle Published on 05 February 2010

As grass hay harvest gets underway, we need to be thinking about fertilizing those stands with nitrogen (N).

The most efficient way to fertilize these grass hay stands is to split-apply N based on the expected yield of the next growth. The actual rate should be 50 pounds N per ton of expected hay yield. Fertilizer should be applied as soon after cutting as practical. All of our common N fertilizer materials work well. If urea or UAN are used, applying these right before rain will help to minimize N volatilization loss.

Manure is an excellent choice for these grass hayfields, especially with current high fertilizer prices. Grass hay has a high demand for all manure nutrients so it will make good use of manure nutrients. Be aware that if manure is used as the sole source of N for a grass hay crop, excess phosphorus (P) and potassium (K) will likely be applied over time. Follow regular soil testing to monitor for excesses of these nutrients.

Grass hayfields are a much better choice for manure than legume-based hayfields because they need the N, whereas the legumes do not. Applying manure between cuttings also provides another window to spread manure. This is typically manure that, if not spread on these hayfields, would be stored and spread in the fall when manure nutrient use efficiency is generally very low. Applying to these hayfields can thus dramatically increase the economic return from manure nutrients compared to late-fall applications of the same manure for next year’s crops.

Be careful not to apply too much manure that you smother the hay. Also, apply as soon after harvest as practical to reduce potential injury to the regrowth. Finally, watch soil conditions so that you do not cause compaction by driving heavy manure spreaders on wet soils in these hayfields. Liquid manure is probably best on hayfields because there is less chance of smothering and you are less likely to gather up remnants of the manure in the next hay harvest.

Estimate the amount of N that will be available from the manure application to make sure it is adequate for optimum production. Depending on the rate applied, supplemental fertilizer N may be needed also. The availability of manure N applied between hay cuttings will range from around 50 percent if it gets significant rain (~0.5 inch) within a day of application to only 20 percent if there is no rain for a week.

For example, using book values, applying 5,000 gallons per acre (gal/A) of dairy manure and getting it rained-in right away will supply adequate N for most second cuttings of grass hay (5,000 gal/A x 28 lb N/1.000 gal x 0.5 = 70 lb available N). However, if you apply 5,000 gal/A anticipating rain and it does not rain, you may only get 28 lb available N/A (5,000 gal/A x 28 lb N/1000 gal x 0.2= 28 lb available N), which means you would need an additional 42 lb N/A, probably as fertilizer, to meet the crop needs for optimum production in this situation. The bottom line is that with good management, this 5,000 gal/A manure application to a grass hayfield could be worth as much as $150/A in fertilizer nutrient value. FG

—Excerpts from Penn State Field Crop News, Vol. 8, No. 10

Doug Beegle
Soil Fertility Specialist
Penn State