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Fertilization of forage crops

Doug Beegle, Penn State Soil Specialist Published on 15 June 2012

This year many areas of the state have been reporting good forage yields so far. As a result, we nutrient removal by the crop may be up significantly too. This combined with increasing fertilizer prices, especially for potassium (K) has raised questions about fertilization of forage stands.

If the field has not been soil tested recently, this should be the first priority to determine what is needed for the current crop and to maintain optimum levels in the soil. Also, if the soil reserves are still adequate the soil test can help reduce the cost of an unnecessary fertilizer application. Soil test kits from the Penn State Agricultural Analytical Services Laboratory are available from all Penn State Cooperative Extension offices.

The principal nutrients of concern for legume forages are phosphorus (P) and potassium (K). An alfalfa crop, for example, removes approximately 15 lbs P2O5 and 50 lbs K2O per acre per ton of yield and at a typical yield of six tons per acre it will remove a total of 90 lbs P2O5 per acre and 300 lbs K2O per acre, respectively.

This removal is built into the soil test recommendations. Recommended fertilizer P and K should be applied as soon after harvest as possible. However, if yields are higher than anticipated when the soil test was run, adjustments to account for the higher removal might be necessary. These adjustments could be made at a later cutting or better yet in the fall. Simply increase the recommendation by 15 lbs P2O5 and 50 lbs K2O per ton of yield above the yield on the soil test.

For grass hay, the removal of P and K are similar to alfalfa, but in addition grass hay needs a regular supply of added N for optimum production. The N recommendation for grass is 50 lbs of N per acre per ton of yield. The most efficient way to fertilize these grass hay stands is to split apply N based on the expected yield of the next growth cycle.

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 hay fields, 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 P and K will likely be applied over time.

Grass is a luxury consumer of excess K which can result in very high K levels in forage crops when excess K has been applied. This can lead to animal health problems, especially when fed to dry cows. Follow regular soil testing to monitor for excesses of these nutrients.

Applying manure between cuttings also provides another window to spread manure. This is typically manure that, if not spread on these hay fields, would be stored and spread in the fall when manure nutrient use efficiency is generally very low. Applying to these hay fields 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 hay fields. Liquid manure rather than solid manure is probably best on hay fields because there is less chance of smothering and you are less likely to gather up remnants of the manure in the next hay harvest.

Grass hay fields are a much better choice for manure than legume based hay fields because they need the N, whereas the legumes do not need the N, thus the N in the manure is wasted on legumes so this reduces the economic return to the manure application. Also, there are concerns with stand injury and the possibility of increased weed pressure. However, if it is necessary to apply manure to legume fields, fall is the best time to do it.

This is a way to supply the additional nutrients that might be required, there is less chance of stimulating weeds, and there is good cover in these fields in the fall, over the winter and early spring to minimize nutrient losses to the environment.

Usually we give priority to older alfalfa stands for manure application because they usually are more depleted in nutrients and if there are some negative effects from the manure application, there will be less long term impact on a stand that will soon be rotated compared to a new seeding.  FG

—Penn State Extension Field Crop News, Vol. 12, No. 13