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.
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.
The economic optimum N rate (EONR) was calculated at a 0.10 fertilizer N cost-to corn grain price ratio.
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).
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).
Table 1: How stand age impacts alfalfa and soil N contents, stand density and corn N uptake
|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|
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.
Reviewed in 2021