To begin to answer this question, let me first state that agronomists do not know the mechanism of the rotation effect. It remains a mystery. Farmers have long observed it and used it for managing crops, but we do not know what causes it. The purpose of this article is to provide a timeline regarding changes in thinking about the crop rotation effect.

Simply stated the rotation effect is the additional benefit of rotating crops when all production variables appear to be optimum, or when problems associated with monoculture are not apparent. The rotation effect was originally defined as the effect of all contributions, other than N, supplied by legumes in a rotation (Baldock et al., 1981). However, it is also recognized that other non-legume crops can provide benefits as well (Robinson, 1966; Crookston and Kurle, 1988).

Varro in the first century B.C. recognized that the rotation effect improved crop production (as cited by Baldock et al., 1981). Prior to the 1950s, farmers recognized the importance of rotation because there were few options for fertility and pest management. Between the 1950s and 1960s, the practice of corn and soybean monoculture became popular. Chemical fertilizers and pesticides were thought to be able to substitute for rotation. The attitude seemed to be "Continuous corn yields just as well, if not better than rotated corn" (Benson, 1985). The rotation effect was thought to be primarily N related.

In the 1970s, it became apparent that all rotation effects could not be overcome. Allelopathic effects from weed species were found to influence corn and soybean growth (Bhowmik and Doll, 1984). Research attempts were made to separate N effects and non-N effects (Baldock et al., 1981).

During the 1980s, the main question was: "What does crop rotation do in the system?" Researchers reported improvement of soil moisture (Roder et al., 1989), improvement of soil structure (Dick and Van Doren, 1985; Griffith et al., 1988), increases of beneficial soil microbes (Cook, 1984), decreases of pests (Cook, 1984; Dabney et al., 1988; Edwards et al., 1988), and decreases of phytotoxic compounds or growth promoting substances originating from crop residues (Yakle and Cruse, 1984).

During the 1990s, many experiments were designed to eliminate factors. Above-ground residue was found to have no rotation effect (Crookston and Kurle, 1989). Host-specific pathogens could not account for the rotation effect (Whiting and Crookston, 1993). Some root development differences were observed (Nickel et al., 1995). Grain nutrient composition was found to be unaffected by rotation (Copeland and Crookston, 1992). Many studies provided management recommendations and the rotation effect was better quantified (Meese et al., 1991; Porter et al., 1997; Porter et al., 1998; Adee et al., 1994; Porter et al., 1997; Lund et al., 1993).

Which brings us to the current state of knowledge regarding rotation effects. Probably the biggest change in thinking is recognition that the crop rotation effect on yield partially involves soil microbes and is specific to a location (Porter et al., 2001). The rotation effect may not be due to one mechanism; rather numerous factors acting alone or in combination chip away at yield and vary in importance from site to site within and between fields.

Many rotation studies begun in the 1980s are maturing and currently have the ability to generate long-term, longitudinal cropping sequence results. Next week some specific examples of new insights gained from these long-term rotation experiments.

Literature Cited

Adee, E. A., E. S. Oplinger, and C. R. Grau. 1994. Tillage, rotation sequence, and cultivar influences on brown stem rot and soybean yield. Journal of Production Agriculture 7:341-347.

Baldock, J. O., R. L. Higgs, W. H. Paulson, J. A. Jackobs, and W. D. Shrader. 1981. Legume and mineral N effects on crop yields in several crop sequences in the upper Mississippi valley. Agronomy Journal 73:885-890.

Benson, G. O. 1985. Why the reduced yields when corn follows corn and possible management responses? p. 161-174. Proceedings of the 40th Annual Corn and Sorghum Research Conference.

Bhowmik, P. C. and J. D. Doll. 1984. Allelopathic effects of annual weed residues on growth and nutrient uptake of corn and soybeans. Agronomy Journal 76:383-388.

Cook, R. J. 1984. Root health: Importance and relationship to farming practices. p. 111-127. In D.F. Bezdicek (editor). Amercian Society of Agronomy, Madison, WI.

Copeland, P. J. and R. K. Crookston. 1992. Crop sequence affects nutrient composition of corn and soybean grown under high fertility. Agronomy Journal 84:503-509.

Crookston, R. K. and J. E. Kurle. 1988. Using the kernel milkline to determine when to harvest corn for silage. Journal of Production Agriculture 1:293-295.

Crookston, R. K. and J. E. Kurle. 1989. Corn residue effect on the yield of corn and soybean grown in rotation. Agronomy Journal 82:229-232.

Dabney, S. M., E. C. McGawley, D. J. Boethel, and D. A. Berger. 1988. Short-term crop rotation systems for soybean production. Agronomy Journal 80:197-204.

Dick, W. A. and D. M. Van Doren. 1985. Continuous tillage and rotation combinations effects on corn, soybean, and oat yields. Agronomy Journal 77:459-465.

Edwards, J. H., D. L. Thurlow, and J. T. Eason. 1988. Influence of tillage and crop rotation on yields of corn , soybean, and wheat. Agronomy Journal 80:76-80.

Griffith, D. R., E. J. Kladivko, J. V. Mannering, T. D. West, and S. D. Parsons. 1988. Long-term tillage and rotation effects on corn growth and yield on high and low organic matter, poorly drained soils. Agronomy Journal 80:599-605.

Lund, M. G., P. R. Carter, and E. S. Oplinger. 1993. Tillage and crop rotation affect corn, soybean, and winter wheat yields. Journal of Production Agriculture 6:143-144, 207-213.

Meese, B. G., P. R. Carter, E. S. Oplinger, and J. W. Pendleton. 1991. Corn/soybean rotation effect as influenced by tillage, nitrogen, and hybrid/cultivar. Journal of Production Agriculture 4:74-80.

Nickel, S. E., R. K. Crookston, and M. P. Russelle. 1995. Root growth and distribution are affected by corn-soybean sequence. Agronomy Journal 87:895-902.

Porter, P. M., S. Y. Chen, C. D. Reese, and L. D. Klossner. 2001. Population response of soybean cyst nematode to long teerm corn-soybean cropping sequences in Minnesota. Agronomy Journal 93:619-626.

Porter, P. M., R. K. Crookston, J. H. Ford, D. R. Huggins, and W. E. Lueschen. 1997. Interrupting yield depression in monoculture corn: Comparative effectiveness of grasses and dicots. Agronomy Journal 89:247-250.

Porter, P. M., J. G. Lauer, D. R. Huggins, E. S. Oplinger, and R. K. Crookston. 1998. Assessing spatial and temporal variability of corn and soybean yields. Journal of Production Agriculture 11:359-363.

Porter, P. M., J. G. Lauer, W. E. Lueschen, J. H. Ford, T. R. Hoverstad, E. S. Oplinger, and R. K. Crookston. 1997. Environment affects the corn and soybean rotation effect. Agronomy Journal 89:441-448.

Robinson, R. G. 1966. Sunflower-soybean and grain sorghum-corn rotations versus monoculture. Agronomy Journal 58:475-477.

Roder, W., S. C. Mason, M. D. Clegg, and K. R. Kniep. 1989. Yield-soil water relationships in sorghum-soybean cropping systems with different fertilizer regimes. Agronomy Journal 81:470-475.

Whiting, K. R. and R. K. Crookston. 1993. Host-specific pathogens do not account for the corn-soybean rotation effect. Crop Science 33:539-543.

Yakle, G. A. and R. M. Cruse. 1984. Effects of fresh and decomposing corn plant residue extracts on corn seedling development. Soil Science Society of America Journal 48:1143-1146.