Rotational vs. Nitrogen-Fixing Influence of Soybean on Corn Grain and Silage Yield and Nitrogen Use

Corn (Zea mays L.) grain yields generally are higher when planted following soybean [Glycine max. (L.) Merr.] as opposed to continuous corn. Many factors have been hypothesized to explain this phenomenon, including enhanced N availability following soybean. The purpose of this study was to separate the N₂-fixing contribution from the rotational effect of soybean on corn yield using nodulating and non-nodulating soybean and to determine whether increased rates of N fertilizer could compensate for a previous crop of soybean. A field experiment was conducted for 4 yr (1991–1994) at Arlington, WI, on a Piano silt loam soil (fine-silty, mixed, mesic Typic Argiudolls). Corn, nodulating soybean, and non-nodulating soybean were planted in 1991 and 1992 and corn was planted in all plots in 1993 and 1994. In 1992 and 1993, three N rates of 0,80, and 160 lb/acre were applied to both corn and soybean plots. Corn for grain and silage, without added N, preceded by either nodulating or non-nodulating soybean, outyielded continuous corn by an average of 46 bu/acre and 2.1 tons/acre, respectively. Nitrogen removal in corn grain and silage increased following nodulating and non-nodulating soybean as compared with continuous corn. Continuous corn yield with 80 lb N/acre was the same as corn following nodulating or non-nodulating soybean without added N, but 160 lb N/acre did not compensate for the crop “rotation effect.” Second year corn grain and silage yields following soybean averaged 15 bu/acre and 0.9 tons/acre greater, respectively, than continuous corn and the addition of 160 lb N/acre compensated for the previous crop advantage. When corn was planted the third year after soybean, the influence of previous crop was still present, but the rotation benefits to corn disappeared with added N. The results of this study demonstrate that a corn-soybean rotation enhances corn yields, however the N₂-fixing ability of nodulating soybean does not appear to be responsible for the observed rotation effect. Research QuestionCorn grain yields are generally higher when planted following soybean as opposed to continuous corn. The added N from biological fixation is most often assumed to be the reason for this increased yield. However, some studies show corn performing as well when following non N-fixing crops. The purpose of this research was to (i) separate the N-fixing contribution from the rotational effect of soybean on corn using nodulating and non-nodulating soybean and to (ii) determine whether increased rates of N fertilizer could compensate for a previous crop of soybean. Literature SummaryIt has been shown that corn yields increase when grown in crop rotations. Yield increases of 15.5 to 49.0 bu/acre for corn following soybean have been obtained compared with corn grown after corn. Fertilizer N rates needed to maximize corn yields are lower following soybean. The N-fixing ability of soybean has often been given credit for the reduction in required N. However, high-yielding soybean is being seen as a net consumer of N. Therefore, removal of the harvested grain may actually leave a net soil N deficit. This suggests that the yield increase of corn when following soybean in a rotation may not be due to the effect of N-fixation, but rather to a combination of factors generally referred to as the “rotation effect.” Study DescriptionField studies were conducted for 4 yr (1991–1994) under rainfed conditions at Arlington, WI, on a silt loam soil. To reduce the soil N reserve prior to the start of this study, this experimental site was cropped to corn for 3 yr (1988–1990) with no N fertilizer applications. Main plots planted in 1991 were corn, nodulating soybean, and non-nodulating soybean. Sub plots in 1992 were corn, nodulating soybean, or non-nodulating soybean. Sub-sub plots were three N rates; 0, 80, and 160 lb N/acre. Sub-sub-sub plots were planted to corn in 1993 and fertilized at N rates of 0, 80, or 160 lb N/acre. In 1994, all plots were planted to corn. Data collected from soybean plots included soil nitrate N, leaf N, plant height, plant lodging, grain yield and grain moisture and seed size. Data collected from corn plots included grain yield and grain moisture, grain N, biomass yield and biomass moisture, biomass N, soil nitrate N, plant and ear density, seed size, and bushel weight. Applied QuestionsWhat is the effect of nodulating and non-nodulating soybean on corn grain and silage yield? The cropping of soybean, or conversely, the absence of corn enhances corn yields in rotation. The benefit of a rotation with soybean was evident in the yield response of corn following a previous crop of soybean. However, based on these yield responses (Table 1, 2), the N2-fixing ability of nodulating soybean did not appear to be responsible. Corn yields were enhanced similarly by both N₂-fixing soybean and non-N₂-fixing soybean. Can N fertilizer rates be increased to compensate for a previous crop of soybean? Adding N to continuous corn could not compensate for the yield response of corn due to rotation, the first year following soybean (Table 1, 2). The second year of corn after soybean, yield advantages due to rotation began to diminish. Corn yields following soybean were still higher than those of continuous corn, but addition of high N rates to continuous corn could nearly balance out the yield effect of corn following soybean. The third year after soybean, rotation benefits to corn yield seemed to disappear. Yield responses due to added N were not different for corn following either soybean line. RecommendationsThe advantage of rotations of soybean with corn were demonstrated in this study. Cropping of soybean with corn reduced N fertilizer requirements for corn. As demonstrated with our results the N fertilizer requirement for corn was reduced regardless of the ability or lack of ability of soybean to fix N₂. However, continuous corn yields were never as high as corn grown in rotation with either N₂-fixing or non N₂-fixing soybean regardless of the N fertilizer level. We attribute this phenomenon to be due to the rotation effect, a term which refers to those elements that encourage enhanced corn performance in rotation. Degree of residue breakdown, disease and insect pest cycle interruption, soil texture, and altered water infiltration rate all contribute to this rotation effect. It is likely that all these elements work together rather than separately. Because of this rotation effect, the advantage of soybean in a cropping rotation with corn makes good economic sense both for yield advantages and reduction of fertilizer inputs. Table 1Corn grain yield at three N rates (0, 80, and 160 lb N/acre) 1 yr, 2 yr, and 3 yr following previous crops of corn, nodulating soybean, and non-nodulating soybean. 1992–1994. Arlington, WI. 1 yr after2 yr after3 yr afterPrevious crop 0† 80160 0† 80160 0† 80160--------------------bu/acre--------------------Corn 75 28 143 93 129 140 118 142 151Nodulating soy 123 154 158 105 137 146 133 152 155Non-nodulating soy 118 156 157 111 137 145 141 154 161LSD₀.₁₀ 8 7 13†N rate, lb/acre Table 2Corn silage yield at three N rates (0, 80 and 160 lb N/acre) 1 yr, 2 yr, and 3 yr following previous crops of corn, nodulating soybean, and non-nodulating soybean. 1992–1994. Arlington, WI. 1 yr after2 yr after3 yr afterPrevious crop 0† 80 160 0† 80 160 0† 80 160--------------------tons/acre--------------------Corn 4.3 6.9 7.7 5.8 7.5 7.9 7.3 7.9 8.3Nodulating soy 6.5 8.1 8.3 6.5 8.3 8.4 7.9 8.3 8.6Non-nodulating soy 6.2 7.9 8.2 6.8 8.1 8.8 8.1 8.4 8.7LSD₀.₁₀ 0.4 0.4 0.5†N rate, lb/acre