Potassium and nitrogen management during the rotation from alfalfa to corn.

Abstract

High K and N fertilizer prices in recent years have made it imperative for growers to apply optimum fertilizer rates in alfalfa (Medicago sativa L.)–corn (Zea mays L.) rotations. Although adequate K is needed for both yield and persistence of alfalfa, current University K fertilizer guidelines in the Corn Belt do not change for the last production year, when alfalfa stand persistence is typically not a concern. Furthermore, little is known about carryover of K applied to alfalfa on first-year corn grain and silage yields. In 2008 to 2010, on-farm research was conducted on 10 fields with medium soil test K (STK) to determine response to K for alfalfa yield and quality in the last production year, and to estimate K carryover to first-year corn. Alfalfa yield and relative feed value (RFV) and quality (RFQ) did not improve with K fertilization. Herbage K concentration and K uptake increased with K fertilization across sites, indicating that applied K was available during the season of application. Regardless of K rate applied to alfalfa, additional K applied to corn increased corn stover and silage yields by 10 and 8%, respectively. However, when K was not applied to the corn, each 100 kg ha-1 increase in the index of available K increased corn grain yield by 0.5 Mg ha-1, decreased stover yield by 0.4 Mg ha-1, and did not affect silage yields. This suggests that carryover K was less available than K applied to corn. Therefore, on medium STK soils going into the last year of alfalfa, applying fertilizer K to first-year corn rather than alfalfa may enhance economic returns. Compared to corn following corn, N guidelines for corn following alfalfa in the Corn Belt suggest that N rates for first-year corn after alfalfa be reduced by 168 kg N ha-1 when ≥43 to 53 alfalfa plants m-2 are present at termination, however, these guidelines have been questioned as corn grain yields have increased. In addition to the 10 N response trials in the K experiment, experiments were conducted at another six locations in Minnesota to address questions regarding N availability to first-year corn after alfalfa that relate to amount and timing of alfalfa regrowth incorporation. Corn yield and fertilizer N uptake were not affected by regrowth management, tillage timing at six locations, or carryover K at 10 locations. Corn grain yield ranged from 12.1 to 16.0 Mg ha-1 among 16 site-years, but responded to N fertilizer on just one location. At this location with above-average rainfall and inadequate soil drainage, the economic optimum N rate (EONR) was 85 kg N ha-1 (assuming prices of $0.87 kg-1 N and $132 Mg-1 grain) and grain yield was 15.8 Mg ha-1. Assuming the same N price and $39 Mg-1 silage, the EONR for silage yield across the additional six locations in 2010 was 31 kg N ha-1. These results demonstrate that on highly productive medium- to fine-textured soils in the Upper Midwest with ≥43 alfalfa plants m-2 at termination, first-year corn grain yields are usually maximized without N fertilizer, regardless of alfalfa regrowth management or timing of incorporation, but that small N applications may be needed to maximize silage yields.