Do Mixed Variety Crops Have To Be A “Dog’s Breakfast”?

By Brenda Frick, Ph.D.

A recent Western Producer article suggested that using seed from hybrid plants would result in a “dog’s breakfast” crop. These crops would include characteristics from both parental lines, including differences that could make chemical management difficult. If most of the problems of using mixed varieties are associated with chemical use, perhaps mixed varieties are worth consideration for organic producers.

Our dependence on monocultures in modern systems is so complete that we often forget that mixtures have been common in early, traditional and subsistence systems. There is evidence that they give more stable yields, and are helpful in pest management.

The most spectacular demonstration of the benefits of mixtures is a study in Yunnan Province, China, where thousands of fields were planted to rice mixtures in an attempt to reduce blast, a devastating fungal disease. Blast was 94% less severe in the mixtures than in susceptible monocultures. Disease levels in mixtures were considered acceptable; fungicide use was eliminated. Yields were considerably increased. Similarly, wheat mixtures in research trials in the UK reduced disease levels from powdery mildew, Septoria and leaf rust.

There are several ways that mixtures may be less vulnerable to disease than monocultures. In a monoculture, susceptible plants are surrounded by susceptible plants, and diseases can spread easily. In a mixture of susceptible and resistant plants, the resistant plants act as a barrier to the spread of disease and susceptible plants are more widely spaced, making spread more difficult. If susceptible plants succumb to the disease, resistant plants may be able to compensate by growing larger and producing more seeds. In addition, plants that are exposed to diseases show a kind of immune response and become more resistant.

Mixtures can also slow disease by acting on the disease itself. A pathogen can adapt more readily to a uniform crop, becoming more able to cause disease. In a mixed crop, selection pressures are more varied and so evolution of the disease is slowed.

In the prairies we are blessed with a short season and a severe environment. Together these factors greatly reduce the impact of disease. The benefits of mixtures may be greater in areas where crops grow for a longer season and diseases can develop through many generations in a single crop.

Mixtures may also be more resistant to other types of stresses, such as drought or cold. Reviews of many studies show that mixtures usually yield at least as high as the average of the component crops and often they yield better. A mixture may include varieties that do well under moist conditions, under dry conditions, under nutrient stress, etc. In mixtures, yields are often more stable, as the stronger crop compensates for the yield lost by the weaker crop.

Why not simply grow the best variety? Which variety will perform the best can be difficult to determine in advance. A single type might be best if the environment is relatively uniform. Some would argue that high input systems create such a uniform environment, though they can not eliminate the variability caused by the weather. In practice, farm fields, perhaps especially organic farm fields, are highly variable environments, from year to year, and across the field.

Designing mixtures can be an art. Producers have used a small proportion of awned wheat in a largely awnless mixture, because the awnless variety was preferred, but the awned variety increased air flow through the swath, and prevented it from retaining too much moisture. Other producers have mixed short durum varieties with preferred taller varieties to help resist lodging. An ideal mixture may include varieties that differ in disease resistance, that have different desired traits, or that complement each other well.

There are some cautions in designing the mix. Matching the maturities of the varieties in the mixture will reduce the headaches at harvest. It is also very important to be sure that the buyer for the crop accepts the mixture. For hard red spring wheat and durum, or for crops used for feed, mixtures are not likely to be a problem. Other crops may be more variety specific. This is especially true of malting barley. Although a quality beer may be produced by combining malts of different varieties, the varieties must be malted separately to assure quality. Other end users may have similar concerns, preferring to blend their own mixtures of varieties. Mixtures of crops like pea or lentil which have visually different varieties, would have to be acceptable to the buyer as a mixture, or be so different in size that they could be effectively separated.

So do mixtures have to be a “dog’s breakfast”? A carefully planned mixture, with good harvestability and a good market could be an ideal crop for an unpredictable and variable environment. Increasing diversity, even at this level, can reduce risk and improve stability.

Brenda Frick, Ph.D., P.Ag., is the Prairie Coordinator for the Organic Agriculture Centre of Canada at the College of Agriculture, University of Saskatchewan. She welcomes your comments at 306-966-4975 or via email at brenda.frick@usask.ca . I thank Brian Rossnagel, Kirstin Bett and Brenda McCuaig for discussions on variety mixtures.

References

Gallandt, E.R., S.M. Dofing, P.E. Reisenauer and E. Donaldson, 2001, Diallel Analysis of Cultivar Mixtures in Winter Wheat, Crop Science 41:792-796

Phillips, S.L. and Wolfe, M.S. 2004. Plant breeding for agricultural diversity, Science and Practice for Profitable Livestock and Cropping Occasional Symposium No. 37, British Grassland Society. p.184-187

Wolfe, M.S. n.d. Perspective: diversity within crops resists disease. New Agriculturalist online http://www.new-agri.co.uk/01-1/perspect.html

Wolfe, M.S. 1985. The current status and prospects of multiline cultivars and variety mixtures for disease resistance. Ann. Rev. Phytopathol. 23:251-73

Zhu Y, Chen H, Fan J, Wang Y, Li Y, Chen J, Fan J, Yang S, Hu L, Leung H, Mew TW, Teng PS, Wang Z, and Mundt C.C. 2000. Genetic diversity and disease control in rice. Nature 406, 718-722

This article first appeared in The Western Producer, and is published here on the OACC website with permission.

Posted on the OACC website, July 2005