Nitrogen Fertilizer Management and Recommendations for Wheat Production in Central Mexico

Abstract

The current N fertilizer recommendation for wheat (Triticum aestivum L.) production in the central highlands of Mexico is to apply 71 lb N per acre. However, this N rate appears to exceed current crop needs. Therefore, research activities in farmers’ fields to estimate the optimum N rate and timing were initiated in 1999 and continued until 2002. A field trial was conducted at eleven location-years representing the major wheat production areas of central Mexico. Four N rates were applied (0, 36, 63, and 89 lb N per acre) under three N timing strategies; basal at planting, end of tillering-early jointing, between 30 and 60 days after planting, and split (one third at planting and the remaining two thirds at the end of tillering-early jointing). To estimate the critical N fertilization rates for optimum grain yield in each environment, yield data was fitted to a linear plateau model on N fertilization rates. Results indicated that the critical N rate for optimum wheat production is lower than current N fertilizer recommendation. According to the regression model used, predicted critical N fertilization, that explained 47% of the optimum grain yield variability, ranges from 28 to 66 lb N per acre. For grain yield goals below 45 bu/acre (± 3.1 bu/acre), farmers do not have to apply N fertilizer. For grain yield goals above this threshold, farmers have to apply 2.6 lb N per acre for every additional bushel expected. Nitrogen fertilizer application at the end of tillering-early jointing (Feekes scale 6) showed a tendency to increase grain yield by means of increasing the number of spikes per unit area. Introduction Wheat grain yields in farmers’ fields across the central highlands of Mexico average less than 45 bu/acre (8) and vary considerably throughout the region from 10.5 to 75 bu/acre. This is due primarily to a combination of variable rainfall conditions (15.8 to 39.4 inches) and variable management practices, mostly for weed control and N nutrition. Generally, the application of these inputs may exceed the recommended rates, contributing to lower potential yield and N use efficiency. Soil and plant tests to develop optimum N recommendations are often not available, or their use is not economic for growers in many less-developed countries. Under such conditions, empirical field experiments with N fertilizer rates are an option that can generally be used to generate fertilizer recommendations. However, this approach may provide varying results as different statistical models can be used to fit the response curve to N application. For example, results from a study (1) using a quadratic regression model for a year-location study with wheat in the central highlands of Mexico showed that economic optima for responsive sites to N varied between 68 and 192 lb N per acre. However, the current N fertilizer recommendation from the National Institute for Forestry, Agricultural, and Livestock Research (INIFAP) is to apply a flat rate of 71 lb N per acre. This contrasting information could be attributable to differing models the researchers might have used, as well as to the genotypes planted, as the N use efficiency varies among them (13). This is especially true 25 May 2006 Crop Management under rainfed conditions with variable water regimes (15). Subsequently, as more reliable models can be applied and more genotypes are released for the wheat production areas, current N management guidelines need re-examination due to the potential negative effects on the environment, crop development, and production costs. Most wheat growers in the specific case of the central highlands of Mexico apply an arbitrary N fertilizer rate that in many cases may not meet the crop N needs. In the case of excessive applications, the extra N can ‘leach’ as nitrate from soils to ground water reservoirs giving rise to environmental concerns (10). Such losses represent higher production costs since the N use efficiency is greatly reduced. Therefore, research in farmers’ fields is needed to adjust N rates to grow wheat using statistical models that predict lower critical N rates without reducing grain yield. As an alternative, the application of linear plateau models to N response curves developed from field experiments has been suggested to predict critical N fertilization rates. The advantage of this model over methodologies that use linear or quadratic regression procedures is that the predicted critical N rates for optimum yield are lower (3,18). A linear plateau model consists of two intersecting straight lines fitted to grain yield on N fertilizer rates. The transition zone where the two regression lines intercept represents the critical N rate that produces an optimum grain yield at an economically optimum rate of fertilization (6). The first regression line in a spline model is an upward line that goes from the intercept on grain yield (abscissa) at zero N application, to the point where it reaches the origin of the second regression line. Depending upon field conditions, the second regression line tends not to show major slope changes. This means that after the critical N rate has been reached, grain yield is not expected to change substantially with additional N fertilizer applications. However, even linear plateau models are sometimes criticized because the abrupt change in trend going from one segment to the next does not represent what would naturally occur (16); their application to N response curve in maize (3), barley (18), and wheat (12) has proved to be an adequate approach. The N fertilizer is often split applied to ensure maximum tiller development (11,17,19). It has also been reported that delaying the application of N fertilizer during the cropping season generally increases the content of grain protein (7,20). The objective of this study was to re-examine the critical N fertilization rate for wheat production by means of a linear plateau model that optimized grain yield and the effect of N timing on grain yield. Field Studies Field plots were established at eleven location-years in farmers’ fields from 1999 to 2002 in the states of Tlaxcala and Mexico of the central highlands of Mexico. A location-year combination is referred to as an environment. A brief description of each environment is shown in Table 1. All plots were established under rainfed conditions with the farmers controlling all of the production practices except for seeding rates, N application rates, and weed control. Plots were sown to the bread wheat (Triticum aestivum L.) variety ‘Nahuatl F2000,’ released in the year 2000 by INIFAP, at a seeding rate of 89 lb/acre. Plots were 4 m by 7 m and harvested by hand. Grain yield components were determined in four environments. Nitrogen treatments were 0, 36, 63, and 89 lb/acre. The N was applied with three different timing treatments, all at planting, all at the end of tillering-early jointing (Feekes scale 6), and a split treatment consisting of one third at planting and two thirds at the end of tillering-early jointing. A factorial arrangement with a RCB design and three replications was used. Urea was the N source used. At planting, fertilizer was broadcast and incorporated by disking along with the seed. At the end of tillering-early jointing growth stage, between 30 and 60 days after crop emergence, N fertilizer applications were surface applied without regard for soil moisture conditions. 25 May 2006 Crop Management Table 1. Summarized description of 11 environments in the states of Tlaxcala and Mexico. State Environ ment Locationyear Long-term precipitation Jun-Oct (in) Altitude (ft) Environment yield potential Soil OM (%) Soil units (FAO system) Planting date Tlaxcala 1 Nanacamilpa 1999 16.9 8968 High 1.34 Eutric Cambisols 09 June 2 Nanacamilpa 2000 16.9 8968 High 1.48 Eutric Cambisols 08 June 3 Nanacamilpa 2001 16.9 8968 High 1.48 Eutric Cambisols 01June 4 Nanacamilpa 2002 16.9 8968 High 2.15 Eutric Cambisols 10 June 5 Apizaco 2002 23.6 7898 Medium 1.61 Haplic Phaeosems 04 June