Early fall is one of the best times to topdress maintenance fertilizer on perennial forages. Soils are usually firm in September, and autumn topdressing provides needed nutrients for good winter survival of the forage stand and vigorous regrowth the following spring. Remember that hay crops will remove about 50 lb of K2O and 12 lb of P2O5 per ton of dry hay harvested. Adequate amounts of soil P and K are important for the productivity and persistence of forage stands. However, nutrient over-application harms the environment and can harm animals fed those forages.
A recent soil test should always guide what nutrients to apply and how much. If nutrient deficiencies are suspected, then tissue tests combined with the soil test values can be helpful in the diagnosis of nutrient issues. When recommendations call for high rates of phosphorus and potassium, there is an advantage to splitting the application, with half applied this autumn and the remainder applied next spring after the first cutting when soils are firm.
Ohio State University Extension has an Excel tool to help you determine the right rates to apply based on your soil test report. The OSU Fertility Recommendation Calculator and a user guide are available at https://forages.osu.edu/forage-management/soil-fertility-forages. We highly recommend using this tool to check any fertilizer recommendations you receive, as we have seen some fertilizer recommendations that are too high.
It is crucial not to over apply P and K. Many dairy farms have high levels of soil P, making the expense of fertilizer P unnecessary. When soil test P exceeds the agronomic level of 50 ppm, there is increased potential of P losses into streams and lakes. Applying too much K will result in luxury consumption by the forage plants, leading to excessive levels of K in the forage that can cause animal health problems.