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 lb N/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 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. Follow regular soil testing to monitor for excesses of these nutrients. Grass hay fields are a much better choice for manure than legume based hay fields because they need the N, where as the legumes do not need the N. 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 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. 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% if it gets significant rain (~1/2 in) within a day of application to only 20% if there is no rain for a week (See PA Agronomy Guide Table 1.2-14 for details).
For example, using book values, applying 5000 gal/A of dairy manure and getting it rained in right away will supply adequate N for most 2nd cuttings of grass hay (5000 gal/A x 28 lb N/1000 gal x 0.5 = 70 lb avail. N). However, if you apply 5000 gal/A anticipating rain and it does not rain you may only get 28 lb avail. N/A (5000 gal/A x 28 lb N/1000 gal x 0.2= 28 lb avail. 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 5000 gal/A manure application to a grass hay field could be worth as much as $150/A in fertilizer nutrient value.
Source: Doug Beegle, Soil Fertility Specialist, PA Extension