homogeneous yield-limiting factors, for which a single rate of a specific crop input is appropriate (Doerge, 1999). The spatial variation of productivity across farm fields can be classi-Various techniques of delineating management zones fied by delineating site-specific management zones. Since productivity are currently being investigated in different parts of the is influenced by soil characteristics, the spatial pattern of productivity USA (Fraisse et al., 1999; Fleming et al., 2000; Khosla et could be caused by a corresponding variation in certain soil properties.

Outbreaks of plague in wildlife are sporadic and spatially dispersed, and they depend on coincidence of susceptible hosts, flea vectors, the plague bacterium (Yersinia pestis), and environmental factors that support pathogen transmission. We fit spatial models of plague outbreaks to a long‐term data set (1981–2005) of towns of black‐tailed prairie dogs (Cynomys ludovicianus) on the shortgrass steppe of northeastern Colorado. We investigated the effects of spatial distribution (town area and connectivity to other prairie dog towns), climate (spring and summer precipitation and temperature), and soil moisture‐holding capacity. In logistic regression models, plague epizootics were predicted by connectivity to other towns experiencing plague during periods with relatively low temperatures, in soils with high moisture‐holding capacity. After accounting for connectivity between prairie dog towns and current‐year climatic conditions, little additional spatial or temporal autocorrelation was detected. Spatial logit association models provided evidence for localized epizootic hotspots and that greater summer rainfall predicted plague events. Plague outbreaks were not predicted by precipitation in the previous year. As such, no evidence was found to support an indirect cascade model of plague outbreaks for black‐tailed prairie dogs on the shortgrass steppe in Colorado. Instead, the models suggest that plague occurrence depends upon direct climatic effects on flea vectors and the plague pathogen.

Numerous techniques of management zone delineation have been studied; however, few comparisons between techniques exist in the literature. The objectives of this study were: (i) to determine how consistently two management zone delineation techniques (a soilcolor-based management zone [SCMZ] technique and a yield-based management zone [YBMZ] technique) characterize regions of high, medium, and low grain yield; and (ii) to compare the relative accuracies with which the two management zone delineation techniques characterize the grain yield within low, medium, and high productivity potential management zones. This study was conducted for three site years in northeastern Colorado. Management zones were delineated before planting. The SCMZ technique used: (i) bare-soil imagery, (ii) topography, and (iii) farmer's experience. The YBMZ relied on: (i) bare-soil imagery, (ii) soil organic matter, (iii) cation exchange capacity, (iv) soil texture, and (v) the previous season's yield map. Grain yields ranged from 6.9 to 15.5 Mg ha 21 across all site years. Grain yields were significantly different between SCMZ zones for all site years. Grain yield in the SCMZ high zones were up to 1.88 Mg ha 21 higher than YBMZ high zones. Areal agreements for the SCMZ technique were 37, 41, and 45% for Site Years I, II, and III. Based on the approaches used in this study to classify grain yield patterns, the SCMZ technique was found to be relatively better than the YBMZ technique.

Development of improved fertilizer management practices has the potential to increase fertilizer use efficiency and improve environmental quality. The objectives of this study were (i) to characterize the within field spatial variability of N uptake across irrigated corn production fields, (ii) to quantify and compare N uptake and grain yield across three site specific management zones (SSMZs), and (iii) to compare grain yield response to applied N between management zones. This study was conducted on continuous corn (Zea mays L.) in irrigated fields in northeastern Colorado. Fields were classified into high, medium, and low site specific management zones. Treatments consisted of a control and two uniform N application rates over 2 site‐years (one field over 2 consecutive yr and another field over 1 yr). Nitrogen fertilizer rates varied with site‐year and ranged from 56 to 268 kg N ha−1. Aboveground biomass samples were collected at physiological maturity and analyzed for total N. Between management zones, N uptake, grain yield, and grain yield response to applied N were found to be statistically different (p < 0.05). Management zones were found to be less spatially variable than the whole field. The SSMZs accurately characterized variability in N uptake as well as grain yield response to applied N. Thus, variation in N uptake and grain yield can potentially be managed using SSMZs.

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