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 soils 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 (r2 = 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. Research Question Starter fertilizer applied at planting is a widely used practice, especially in northern corn production areas. Many soils have soil test P levels higher than needed to maximize corn yields, and the need for continued use of starter fertilizer on high‐testing soils is questioned for economic and environmental reasons. The purpose of this work was to: (i) determine the frequency of corn yield response to starter fertilizer on soils with excessively high P and K soil test levels; and (ii) identify factors affecting response to starter fertilizer on high testing soils to allow site‐specific prediction of profitable yield response to starter fertilizer. Literature Summary Numerous studies have shown increased early season plant growth, increased grain yield, and decreased grain moisture content at harvest from use of starter fertilizer in corn production. These responses have occurred in most regions of the USA, including northern, southern, and Great Plains states. Corn yield response to starter fertilizer placed near the seed at planting is probably due to positional nutrient availability, since profitable responses have been reported on soils with high available P levels. Increasing soil test levels in many fields used for corn production and concerns about water quality problems associated with P losses from agricultural land raise the question of whether additional fertilizer P should be applied to soils with P test values higher than those needed to maximize yields. Cool soil temperatures early in the growing season can reduce soil P availability, slow root growth, and reduce P uptake by roots. These findings have led to the perception that responses to starter fertilizer are more likely in cold soils at early planting dates, but the factors affecting response to starter fertilizer are not well known. If corn response to starter fertilizer depends on soil temperature, fewer responses should occur as planting dates are delayed. Recent studies of the effect of planting date on corn response to starter fertilizer either showed no effect or larger responses at later planting dates. Recent research has also shown that corn hybrids may differ in their response to starter fertilizer. The key issue in using starter fertilizer in a profitable and environmentally sound manner is improved identification of production situations where a beneficial response to these treatments will occur. The objective of this study was to determine whether applying starter fertilizer is economically justified for corn production under a range of soil and management variables such as soil test level, soil characteristics, tillage system, previous crop, manure history, planting date, and length of growing season. Study Description We evaluated corn response to starter fertilizer at 100 on‐farm sites, most having excessively high soil test P and K levels, over a 3‐yr period in Wisconsin. Trials were conducted using field‐scale equipment and had two basic treatments (with and without starter fertilizer). At each site, treatments (three replications) of none or an average of 15+26+32 (N+P 2O5+K2O) lb/acre were placed 2 in. below and 2 in. laterally from the seed at planting. Production practices were determined by the farmer‐cooperators, and detailed information on these practices and growing conditions were collected for each location. Field measurements included residue cover, plant height (8 wk after planting), preharvest plant density, and grain yield and moisture content. Soil tests for pH, available P, exchangeable K, and organic matter were also performed. Applied Questions Does corn respond to starter fertilizer on soils with excessively high P and K test levels? Results show that profitable responses to starter fertilizer can occur on soils with excessively high test levels for P and K. Statewide, starter fertilizer significantly increased yields by about 4 bu/acre each year. For individual sites, yield response to starter fertilizer ranged from −10 to +42 bu/acre and 40% of the sites had a positive economic response. Economic response to starter fertilizer increased with hybrid relative maturity (RM) and was more frequent at sites with soil K tests below 140 ppm. Can response to starter fertilizer be predicted before corn planting? The percentage of responsive sites was linearly related to the sum of hybrid RM and the Julian date (day of the year) of planting (PDRM). Responses are most likely at later planting dates using longer RM hybrids (PDRM >235) and where soil K levels are below 140 ppm. The starter fertilizer response with late planting dates and hybrids may be due to stimulation of early season growth rates by starter fertilizer, 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 starter fertilizer will enable corn producers to evaluate the potential for economic return on a site‐specific basis.