Site-specific factors affecting corn response to starter fertilizer

The need for starter fertilizer (SF) In corn (Zea mays L.) production is questioned for economic and environmental reasons where sails have phosphorus (P) test levels above those needed to maximize yields, We evaluated corn yield response to SF at 100 on-farm sites, most having excessively high (EH) soil test P and K levels, over a 3-yr period in Wisconsin. Treatments (three replications) of none or an average rate of 15+26+32 (N+P2O5+K2O) lb/acre were placed 2 in. below and 2 in. laterally from the seed at planting. Statewide, SF significantly increased yields by about 4 bu/acre each year. For individual sites, yield response to SF ranged from -10 to +42 bu/acre and 40% of the sites had a positive economic response to SF. Economic response to SF increased with hybrid relative maturity (RM) and was more frequent at sites with soil K tests below 140 ppm. The percentage of responsive sites was linearly related (r(2) = 0.51) to the sum of RM and planting date (PD) in Julian days (PDRM). Results show that profitable responses to SF can occur on soils with EH test levels for P and K, Responses are most likely at later PDs using longer RM hybrids (PDRM >235) and where soil K levels are below 140 ppm. The SF response with late PDs and hybrids may be due to stimulation of early season growth rates by SF resulting in a realization of more of the crop's yield potential by the end of the growing season. The use of PDRM values in conjunction with soil test K information to predict response to SF will enable corn producers to evaluate the potential for economic return on a site-specific basis.