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How 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 guidelines from the University of Minnesota.

Most university 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.

Field trials

To better understand how alfalfa stand age affects N availability and the fertilizer N requirements of first-year corn, field trials were conducted in Lamberton and Waseca 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 unfertilized plots, soil nitrate-N + ammonium-N at depths of 0- to 2-feet and corn N content were measured at the six-leaf (V6), ten-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 per pound of N and $3.50 per bushel corn.

All three stand ages at both locations had stand densities at termination greater than four 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 of N per acre from University of Minnesota guidelines. With this credit, guidelines indicate that less than 10 pounds of N per acre would economically optimize corn grain yield.

However, on medium-textured soils in 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 pounds of N per acre, respectively (Figure 1a).

In contrast, on fine-textured soil in Waseca, first-year corn required 85 pounds of N per acre following both 2- and 3-year-old stands and 105 pounds of N per acre following 1-year-old stands (Figure 1b).

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).

Graph with corn yield on y-axis and fertilizer N rate on x-axis. Lines are close together, with not much increase in yield with an increased N rate. They top out at 3 years with 95 lb N/ac, 150 bu/ac yield
Figure 1a: The response of corn grain yield to first-year corn following 1- to 3-year-old alfalfa stands at Lamberton. EONR = Economic optimum N rate.
Graph with corn yield on y-axis and fertilizer N rate on x-axis. Lines are close together, with not much increase in yield with an increased N rate. They trend up and even off  with 100 lb N/ac, 115 bu/ac yield
Figure 1b: The response of corn grain yield to first-year corn following 1- to 3-year-old alfalfa stands at Waseca. EONR = Economic optimum N rate.

Table 1: How stand age impacts alfalfa and soil N contents, stand density and corn N uptake

Shows 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. *n/a= data not available.
Location Stand age Alfalfa residue N Stand density Soil N content: V6 Soil N content: V10 Soil N content: R1 Corn N uptake: V6 Corn N uptake: V10 Corn N uptake: R1
Lamberton 1 year 45 lbs. N per acre 19 plants per sq. ft. 22 lbs. N per acre 22 lbs. N per acre 7 lbs. N per acre n/a* n/a n/a
Lamberton 2 year 97 lbs. N per acre 18 plants per sq. ft. 30 lbs. N per acre 26 lbs. N per acre 5 lbs. N per acre n/a n/a n/a
Lamberton 3 year 110 lbs. N per acre 11 plants per sq. ft. 35 lbs. N per acre 27 lbs. N per acre 9 lbs. N per acre n/a n/a n/a
Waseca 1 year 70 lbs. N per acre 34 plants per sq. ft. 31 lbs. N per acre 21 lbs. N per acre 17 lbs. N per acre 31 lbs. N per acre 66 lbs. N per acre 92 lbs. N per acre
Waseca 2 year 106 lbs. N per acre 18 plants per sq. ft. 29 lbs. N per acre 21 lbs. N per acre 18 lbs. N per acre 35 lbs. N per acre 67 lbs. N per acre 89 lbs. N per acre
Waseca 3 year 76 lbs. N per acre 7 plants per sq. ft. 30 lbs. N per acre 20 lbs. N per acre 18 lbs. N per acre 30 lbs. N per acre 71 lbs. N per acre 94 lbs. N per acre

Summary

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.

Matt Yost, former graduate student, College of Food, Agricultural and Natural Resource Sciences; Michael Russelle, emeritus soil scientist, USDA Agricultural Research Service; Jeff Coulter, Extension agronomist; Mike Schmitt, Extension associate dean; Craig Sheaffer, agronomist, College of Food, Agricultural and Natural Resource Sciences and Gyles Randall, emeritus soil scientist, College of Food, Agricultural and Natural Resource Sciences

Reviewed in 2018

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