To minimize groundwater contamination by NO−3 and to maximize efficient crop N use, it is necessary to better characterize NO−3 leaching in various cropping systems. Our objectives were to compare the contributions of hairy vetch (Vicia villosa Roth) and NH+4 N sources to NO−3 leaching in corn (Zea mays L.) production, and to compare NO−3 leaching losses in a corn production system utilizing hairy vetch as a N source and winter cover crop with leaching losses in one utilizing an NH+4 N source and a rye (Secale cereale L.) cover crop. Nitrogen, sometimes enriched with 15N, was applied to corn grown on a Maury silt loam (fine, mixed, mesic Typic Paleudalf) in lysimeters as vetch‐ or NH+4‐N at rates between 10.5 and 14.0 g N m−2 yr−1. Treatments (N source/cover crop) were: NH+4‐N/fallow, vetch‐N/fallow, NH+4‐N/rye, and vetch‐N/vetch; mean 3‐yr cumulative NO−3 leaching losses were 6.11, 4.85, 0.35, and 2.51 g N m−2, respectively. Total recovery of added 15N in leachate was <5% for all treatments, and the percentage of leached NO−3 derived from labeled N sources was generally <15%. In the winter‐fallow lysimeters, effects of N source on the fraction of water input discharged, concentration of NO−3, and quantity of NO−3 leached varied with year and time of year; differences in N‐source availability and mulch effects were sometimes indicated. Generally, cover crop effects were larger than N‐source effects. The NH+4 N source/rye cover system leached consistently less NO−3 than the vetch N source/vetch cover system, even though the fraction of water discharged was not consistently different.

Remobilization of vegetative N during grain fill in wheat (Triticum aestivum L.) contributes significantly to final grain N content. To assess cnltivar variation for this trait in soft red winter (SRW) wheat, nine cultivars were evaluated in the field for 2 yr at one location on a Maury silt loam soil (fine, mixed, mesic, Typic Paleudalf). Plants were harvested at anthesis and maturity, and divided into spike, peduncle, flag leaf blade; and lower culm plus leaves. Significant cultivar differences in N remobilization from the tlag leaf blade, peduncle, and lower culm were detected. The proportion of N accumulated by the spike derived from vegetative N remobilization ranged among cuitivars from 51 to 91%. Approximately 83% of the total aboveground N at maturity was present in the plant at anthesis, in agreement with other studies. An analysis of cultivar differences indicated that all of the cultivar variation in final spike N could be associated with variation in total N uptake. Cuitivar differences in partitioning of vegetative N measured at anthesis did not contribute significantly to cultivar differences in final spike N. Postanthesis N uptake was associated with lower N utilization efficiency (spike weight/total plant N), higher grain N concentration, and lower grain yields. A controlled study with nutrient solutions subjected six wheat cuitivars to N deprivation at anthesis. The impact of N deprivation on grain N concentration varied among cultivars. The ratio of spike N to total N in all cultivars was increased by N deprivation from 0.69 in the presence of N to 0.82 in the absence of N. For the three cultivars common to both studies, field performance did not agree well with performance under controlled conditions.

Expanding production of early‐maturing soybean [Glycine max (L.) Merrill] cultivars in the southeastern USA has exposed such cultivars to a wide range of environmental conditions during seedfill. Temperatures during this growth stage influence levels of specific fatty acids, particularly of the unsaturated fatty acids. Our objective was to evaluate the grain quality responses of early‐maturing cultivars to the wide range of planting dates in the southeastern USA. Protein and oil contents, along with fatty acid profiles, were assessed for cultivars from Maturity Group (MG) 00 through IV using late April, mid‐May, early June, and late June planting dates on a well‐drained Maury silt loam (fine, mixed, mesic Typic Paleudalf) in 1990 through 1993. Across years and cultivars, delayed planting increased protein content and linolenic acid levels and reduced oil content and oleic acid levels, but had little or no influence on palmitic, stearic, or linoleic acid levels. The higher seed‐fill temperatures associated with early planting were strongly correlated with increased oil content and oleic acid levels and reduced linolenic acid levels. Increasing seed‐fill temperatures were closely associated with reduced linolenic acid levels for all six cultivars. However, the oleic acid response to seed‐fill temperatures strongly depended on cultivar maturity. Oleic acid levels of early‐maturing cultivars were much more sensitive to seed‐fill temperatures than were those of later maturing cultivars. While overall effects of environment on grain quality characteristics may be relatively small, perhaps the quality of new low linolenic acid cultivars could be amplified through culture under the warmer conditions of the southeastern USA.

Nitrogen use efficiency (NUE), defined as grain dry weight or grain nitrogen as a function of N supply, was evaluated in 25 soft red winter wheat genotypes for two years at one location. Significant genotypic variation was observed for NUE, nitrogen harvest index, and grain yield. Genotype x environment interaction for these traits was not significant. Several variables including N uptake efficiency (total plant N as a function of N supply), grain harvest index, and N concentration at maturity were evaluated for their role in determining differences in NUE. Nitrogen uptake efficiency accounted for 54% of the genotypic variation in NUE for yield and 72% of the genotypic variation in NUE for protein. A path coefficient analysis revealed that the direct effect of uptake efficiency on NUE was high relative to indirect effects.