Soil properties and weather conditions are known to affect soil N availability and plant N uptake; however, studies examining N response as affected by soil and weather sometimes give conflicting results. Meta‐analysis is a statistical method for estimating treatment effects in a series of experiments to explain the sources of heterogeneity. In this study, the technique was used to examine the influence of soil and weather parameters on N response of corn (Zea mays L.) across 51 studies involving the same N rate treatments that were performed in a diversity of North American locations between 2006 and 2009. Results showed that corn response to added N was significantly greater in fine‐textured soils than in medium‐textured soils. Abundant and well‐distributed rainfall and, to a lesser extent, accumulated corn heat units enhanced N response. Corn yields increased by a factor of 1.6 (over the unfertilized control) in medium‐textured soils and 2.7 in fine‐textured soils at high N rates. Subgroup analyses were performed on the fine‐textured soil class based on weather parameters. Rainfall patterns had an important effect on N response in this soil texture class, with yields being increased 4.5‐fold by in‐season N fertilization under conditions of “abundant and well‐distributed rainfall.” These findings could be useful for developing N fertilization algorithms that would prescribe N application at optimal rates taking into account rainfall pattern and soil texture, which would lead to improved crop profitability and reduced environmental impacts.

No abstract

Glyphosate [N‐(phosphonomethyl)glycine] inhibits 5‐enolpyruvylshikimate‐3‐phosphate synthase, EC 2.5.1.19 (EPSPS), thereby blocking aromatic amino acid synthesis. While glyphosate‐tolerant (GT) soybean [Glycine max (L.) Merr.] contains resistant EPSPS, the N2‐fixing symbiont in soybean root nodules, Bradyrhizobium japonicum, does not contain a resistant enzyme, and glyphosate spray to GT soybean may interfere with the symbiotic relationship. Glyphosate‐tolerant soybean was treated with glyphosate at several different stages of development to evaluate N2 fixation, growth, and yield in a series of greenhouse, growth chamber, and field experiments. Early applications of glyphosate generally delayed N2 fixation and decreased biomass and N accumulation in the cultivar Terral TV5866RR (TV5866RR) harvested at 19 d after emergence (DAE), but plants had recovered by 40 DAE. The biomass and N content of GT soybean were also decreased by glyphosate in plants that were grown with available soil N. There were differences in sensitivity to glyphosate among GT cultivars, with biomass decreases in response to glyphosate ranging from 0 to 30% at 40 DAE for the most tolerant and sensitive cultivars that were evaluated. In growth chamber studies, N2 fixation was more sensitive to water deficits in glyphosate‐treated plants. In field studies, there was no measured effect of glyphosate on GT soybean at Fayetteville, AR where there was adequate soil water throughout the growing season. However, glyphosate tended to decrease biomass and seed yields under conditions of limited soil water at Keiser, AR.

Nitrogen available to support corn (Zea mays L.) production can be highly variable within fields. Canopy reflectance sensing for assessing crop N health has been proposed as a technology to base side‐dress variable‐rate N application. Objectives of this research were to evaluate the use of active‐light crop‐canopy reflectance sensors for assessing corn N need, and derive the N fertilizer rate that would return the maximum profit relative to a single producer‐selected N application rate. A total of 16 field‐scale experiments were conducted over four seasons (2004–2007) in three major soil areas. Multiple blocks of randomized N rate response plots traversed the length of the field. Each block consisted of eight treatments from 0 to 235 kg N ha−1 on 34 kg N ha−1 increments, side‐dressed between the V7–V11 vegetative growth stages. Canopy sensor measurements were obtained from these blocks and adjacent N‐rich reference strips at the time of side‐dressing. Within fields, the range of optimal N rate varied by >100 kg N ha−1 in 13 of 16 fields. A sufficiency index (SI) calculated from the sensor readings correlated with optimal N rate, but only in 50% of the fields. As fertilizer cost increased relative to grain price, so did the value of using canopy sensors. While soil type, fertilizer cost, and corn price all affected our analysis, a modest ($25 to $50 ha−1) profit using canopy sensing was found. These results affirm that, for many fields, crop‐canopy reflectance sensing has potential for improving N management over conventional single‐rate applications.