Stand age affects alfalfa nitrogen credits to first–year corn
Alfalfa typically contributes large amounts of nitrogen (N) to subsequent corn crops. The size of this N contribution is affected by the age of alfalfa stands at termination; however, alfalfa stand age is not used in current university guidelines. Most guidelines are based on stand density at termination and corresponding book–value N credits that should be subtracted from guideline rates for corn following corn.
In order to better understand how alfalfa stand age affects N availability and the fertilizer N requirements of first–year corn, field trials were conducted at Lamberton and Waseca, MN over three years. In each year, no–tillage corn was grown following fall–terminated 1–, 2–, and 3–year–old alfalfa stands. Fertilizer N was applied to corn at planting as broadcast ammonium nitrate. In nonfertilized plots, soil nitrate–N + ammonium–N in the 0– to 2–ft depth and corn N content were measured at the 6–leaf (V6), 10–leaf (V10), and silking (R1) corn growth stages. Corn grain yield was determined at "black layer" and the economic optimum N rate (EONR) was calculated at $0.35/lb N and $3.50/bushel corn.
Figure 1. The response of corn grain yield to first–
year corn following 1– to 3–year–old alfalfa stands
at Lamberton (above) and Waseca (below).
EONR – Economic optimum N rate
All three stand ages at both locations had stand densities at termination greater than 4 plants per square foot, except in one case (3-year-old stands at Waseca in one year). Therefore, almost all stands qualified for the highest N credit of 150 pounds N per acre from University of Minnesota guidelines. With this credit, guidelines indicate that less than 10 pounds N per acre would economically optimize corn grain yield.
However, on medium–textured soils at Lamberton, only first–year corn following 3–year–old stands needed no N fertilizer, whereas the corn following 2– and 1–year–old stands required 55 and 85 lb N per acre, respectively (Figure 1 – top).
In contrast, on fine–textured soil at Waseca, first–year corn required 85 lb N per acre following both 2– and 3–year–old stands and 105 lb N per acre following 1–year–old stands (Figure 1 – bottom).
Therefore, stand age should be considered when utilizing alfalfa N credits because first-year corn following 1– or 2–year–old stands can often require N even though stand densities are high. The greater N contribution of 3–year–old stands relative to younger stands may be due to soil quality enhancements because stand age had no or minimal impacts on soil nitrate–N + ammonium–N content and corn N uptake during the V6 to R1 corn growth stages (Table 1).
Table 1. Location, alfalfa stand age (including establishment year), alfalfa residue N content (herbage and roots to a 1–foot depth), alfalfa stand density at termination, and available soil N content (nitrate–N + ammonium–N) to the 0– to 2–foot depth, and corn N uptake at the 6–leaf (V6), 10–leaf (V10), and silking (R1) corn growth stages.
|Soil N content||Corn N uptake||Location||Stand age||Alfalfa residue N||Stand density||V6||V10||R1||V6||V10||R1|
|year||lb N/ac||plants/sq ft||lb N/acre||lb N/acre|
|†n/a, data not available.|
This demonstrates that stand age effects on first–year corn N requirements are difficult to detect with early–season soil and plant N indicators, so improved predictions are necessary. Our ongoing efforts are focused on developing field– and site–specific predictions of when and to what extent corn following alfalfa will respond to N, using combinations of crop management practices, soil characteristics, and weather conditions.
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