Over the last century, we have not only lost valuable genetic diversity through modern agricultural practices, we have also lost much of the knowledge needed to steward our genetic resources. As farmers, gardeners, and plant enthusiasts, we all have an opportunity to shape sustainable agriculture in fundamental ways. By making new varieties available, we are able to give everyone new options.
Building a healthy, sustainable agriculture future requires farmer-centric seed systems at the regional level, where farmers and the communities they serve consciously choose which crop genetics they use, how they are maintained, and how these genetics are controlled. Done well, plant breeding can help ensure that we all control the seed we need. Breeding work can give us free access to genetic resources and the freedom to grow what we want.
All good farmers and gardeners who survived and flourished were, by necessity, plant breeders. They used observational skills to determine and select the best-adapted plants every year. This meant selecting the highest yielding, best tasting, and most disease-resistant plants. All of the heirloom crop varieties we know and enjoy today were developed over a long, rich period of our history, telling the tale of our plant breeding ancestors who were making selections and saving seed from the healthiest and most vigorous plants year after year. These farmers were constantly striving to improve varieties to better suit their needs, engaging in a constant dance of improvement and co-evolution with their food.
Modern farmers and gardeners who are choosing to produce and select crop varieties to flourish in their regional ecosystems are rubbing shoulders with the best plant breeders of the past who domesticated and continually improved our crop genetic resources.
There will always be a need to adapt new varieties to current challenges, such as climate change. Classical plant breeding allows us to select, adapt, and continually co-evolve with our food crops. This form of plant breeding is not to be viewed as “messing” with nature, and it is certainly not genetic engineering. The principles of evolution show us that there is always variation in biological populations, environmental conditions always change, and no biological entity ever stays the same in response to its environment. In other words, no organism is ever static in nature. Selection pressure and change over time are inevitable. We advocate for an evolutionary breeding model that allows the varieties we use to be part of the sustainable agricultural systems we are pioneering.
This article gives some basic steps to get you started on your plant breeding journey, introducing how to choose varieties to start with and the steps to improve them. However, there is more than one lifetime worth of learning to experience in this path. If you want to go further, take a look at some of the references at the end of the article.
Knowing what you want is critical to getting what you want. The most successful breeding projects have clearly defined traits that are being selected for or against. Prior to starting the breeding effort, it is important to take the broadly defined goals for the variety and dig down to define exactly what you will be looking for in the field. It may be very easy to come up with a list of the traits through brainstorming. Another way to understand which traits are important is by discussing the best currently available varieties. What are the traits that make certain varieties so great? What are they lacking? A useful concept is that of an ideotype. An ideotype is essentially an idealized crop variety that is perfectly suited to a particular set of production and market criteria. What would this ideal variety look like? How would it perform? What traits would it need to have to be successful? What are the traits that are important to the farmer and marketplace?
Some examples of traits that you may want to select for include:
- Plant height
- Plant stature
- Days to maturity
- Harvestable yield
- Storage life
- Seedling vigor
- Pest Resistance
- Disease Resistance
Once you have compiled a list of traits, the next step is to prioritize them. In general, a breeding project should not attempt to actively select for more than five or six traits at any given time. There may be more than five traits that are important, but it’s not feasible to work on everything all at once. Also consider that some traits may be easily fixed, or “locked in,” by choosing parents that share these traits. In this case, effective selection may be achieved simply by throwing out a couple of off-type plants here and there.
Germplasm is a general term that refers to the collection of genetic resources for a plant species. Varieties, landraces, collections, breeding lines, and unimproved material are all types of germplasm. Essentially germplasm is any form of living tissue (e.g., seeds, cuttings, roots, and tubers) from which plants can be grown. Germplasm is the parental material used to begin your breeding work. Sourcing quality germplasm determines how easily and rapidly you will be able to develop your desired variety.
There are two main factors to consider when choosing germplasm:
- Quality: the germplasm should include elements of the ideal variety you are striving to produce
- Variability: the germplasm should have variation for the trait(s) you want to improve
Consider varieties that:
- Perform well in your target environment
- Are considered to be commercial standards
- Contain unique traits that you want to incorporate into your project
Many valuable varieties for breeding work can be found in the catalogs of domestic and international seed companies. However, be aware of legal protections on some varieties that restrict their use for breeding or limit what you can do with them in other ways. These protections include plant patents, utility patents, plant variety protection certificates, and licenses. You can find more information about these protections on eOrganic’s intellectual property protection page at http://www.extension.org/pages/18449. Always investigate your germplasm sources to ensure there are no legal restrictions on breeding.
Besides seed companies, you can also source germplasm from:
- Seed exchanges
- Germplasm Resource Information Network: http://www.ars-grin.gov/
Unless you are already familiar with the germplasm you want to use in your project, it is wise to conduct variety trials to evaluate the available sources of germplasm. A guide to conducting variety trials can be found on Organic Seed Alliance’s website at: http://www.seedalliance.org/Publications/.
Creating a breeding timeline
Step 1. Conduct variety trials to identify the best potential parental germplasm.
Before committing to a long-term breeding project, carefully evaluate potential parental germplasm. This evaluation typically takes the form of a variety trial. A variety trial is a systematic way to evaluate a set of varieties using good trial design. The purpose of a trial design is to help minimize the environmental variation to ensure that the differences observed represent genetic differences between plants. For details on how to conduct variety trials refer to Organic Seed Alliance’s publication On-farm Variety Trials: A Guide for Organic Vegetable, Herb, and Flower Producers, which can be downloaded at: http://www.seedalliance.org/Publications/.
Step 2. If necessary, make crosses between parents. If adequate variation is present in a favorable existing variety, then no crosses are necessary.
To improve your population, some degree of variation for important traits is necessary. To have a variable population, you need to either start with germplasm that is variable for the traits you intend to improve, or you need to create variation by crossing two or more varieties together. These crosses can be made in a number of ways, depending on your goals and resources.
Types of crosses:
Controlled pollinations: Plant breeding might evoke an image of breeders making controlled crosses, perhaps with tweezers, magnifying glasses, and small paintbrushes. The general strategy will depend on whether the species you work with has perfect flowers or not. Perfect flowers have both male stamen and female pistils on the same flower. If the plant has perfect flowers, you will generally need to emasculate the female parent by removing the male stamen and then transfer pollen from the male parent. If the plant has only unisexual flowers – flowers that contain either all male or all female parts – there is no need to emasculate. You just transfer pollen from a male flower to a female flower. After pollinating, some sort of cover is often used to exclude other pollen from contaminating the cross.
Open pollinations: These crosses are also know as strain crosses or blind crosses, and are done by allowing one plant or a set of plants to cross with another plant or set of plants without making an attempt to ensure that pollen is only transferred between certain plants. This is accomplished by bringing all of the plants together and allowing them to freely cross-pollinate. Outside pollen is excluded either by isolation distance or by caging the plants inside a structure that prevents insects and pollen from entering.
Hybrids: These varieties can be used as pre-made crosses, saving a step in the breeding process. Keep in mind, however, that most hybrids are made by crossing two narrowly selected inbred parents and therefore a population formed from a single hybrid will have comparatively little genetic diversity relative to one created from a strain or blind cross between open-pollinated varieties.
Step 3. If you are working with a cross-pollinating crop and you made crosses, allow the offspring to randomly mate for at least two or three generations while removing obvious undesirable plants. In the case of self-pollinated crops, allow the plants to naturally self-pollinate for at least two or three generations after the cross while removing undesirable plants.
Prior to selection, grow your new breeding population for a few generations, allowing the plants to flower and freely cross-pollinate. During this time, you may practice negative mass selection (selecting based on how individual plants look) to rogue out the worst performing plants.
It is tempting to begin intensive selection in the generation after your crosses are made, as plant breeding can be such a long process. However, we recommend that you do not skip this step. In cross-pollinated crops, allowing a few generations of random mating breaks up the linkage between genes. Simply stated, linkage refers to how genes in a plant will tend to travel together. For example, if you cross a tall plant with dark leaves with a short plant with light leaves, you’re more likely to get either a tall dark plant or a short light plant than you are to get a tall, light plant or short, dark plant. Random mating for a couple of generations allows the genes to shuffle and makes it more likely that you will see new combinations of genes in the population.
For self-pollinated crops, it is also valuable to wait for at least two or three generations before making any significant selections. The reason for this is different than for cross-pollinated crops. As described in the previous discussion of differences in genetic structures between cross- and self-pollinated crops, random mating and the breaking of linkages does not occur in self-pollinated crops. Instead, each plant derived from the offspring of a cross has the potential to become a separate, independent family line with unique combinations of fixed pairs of genes. As each of these lines continues to self-pollinate over the course of a few generations, each line will become more uniform and the differences between lines will become more obvious. It will be easier to make distinctions between families if sufficient time has passed to allow the families to essentially become fixed and distinct from one another.
Additionally, the more generations you spend improving the population by this gradual process (mass selection), the better the genetics of the population will be, and the more likely you will be successful in the later phases of breeding.
These generations will also give you a chance to get to know the population, to see what kinds of variation exist in it, and to evaluate what its strengths and flaws are. Use these years to refine your goals, traits of interest, and evaluation techniques. Remember, when practicing selection, make sure the field location represents the location you want to select for and is relatively uniform throughout.
Step 4. Grow a large population and save seed separately from individual plants. The seed from individual plants become families.
During this generation you will be selecting your best plants to serve as parents for families. Evaluate your plants multiple times throughout the season, considering the goals and traits you have prioritized. Since there may be some variation in the soil quality, weed pressures, and other factors in your field, select the best plants from all parts of the field equally. If all of the plants in one corner of your field look worse off than the rest, still try to pick some of the best looking plants in that patch – those are plants that can survive under some environmental stress or potentially suboptimal conditions.
If your crop is cross-pollinating and is a crop where you have an opportunity to evaluate and select the best plants before pollination occurs, you will make faster progress by removing all inferior plants from the field before pollination occurs. In this way, any cross-pollination that occurs will only be between the selected plants.
When you harvest seed from selected plants, put seed from each plant into a separate bag. The seed in each bag represents a family. Label each family with a unique number. Along with the number on each label, you may want to make notes about why you selected that plant. These plants will represent families for the next generation’s round of selection. In the case of cross-pollinated crops, try to save seed from at least 20 to 50 individual plants. In the case of self-pollinating crops, save seed from at least 30 to 100 plants.
Step 5. The following generation, grow family plots by planting each separate plot with seed harvested from an individual plant. Evaluate and select the best performing families.
This generation you will be looking at the progeny of your selected plants as families. In breeding terms, this is the process of family selection. This means you will plant seed from each plant that you selected in separate plots or rows, just as if you were conducting a trial, following the best practices of trial design as outlined in OSA’s On-farm Variety Trials: A Guide for Organic Vegetable, Herb, and Flower Producers.
Examine each plot as a whole. How does this family of related plants look? On average, which families look best given your goals and traits? Select the best families based on their overall quality, and eliminate the other families, preferably before they have a chance to cross-pollinate with the selected families. Look for families where at least 60 to 80% of the plants are acceptable.
Next, select the best plants within the best families. Aim to eliminate 30 to 40% of the plants within a family, keeping in mind the minimum population size you want to maintain to keep the population healthy.
Step 6. Harvest seed from the best plants within the best families.
At this point, save the seed from each family. In other words, combine the seed from all of the selected plants within each family so that you end up with a separate bag for each family.
Step 7. Repeat the process of growing and selecting family plots until you are satisfied with the performance of all selected families.
The following generation, continue to plant the selected families in separate plots and evaluate them again throughout the season. If, during the second round of evaluations, some of the families no longer seem acceptable, discard those families. Also eliminate all inferior plants within the selected families that you decide to carry on to the next generation.
Step 8. Once the selected families are all acceptable, combine the seed of these families together into a bulk population.
For these past few generations, you have been maintaining the population as a set of separate families. Once all of the remaining families meet your goals, you can combine them together to reform the population.
Step 9. Grow the population again and remove the least desirable plants. Repeat as necessary.
Once the families have been reformed into a single population, you can maintain this population in subsequent generations through mass selection (i.e., selecting the best individual plants to reproduce each generation). If at any point you believe the variety needs to be more intensively selected, you can complete another cycle of family selection by saving seed separately from individual plants and planting them in individual plots the following generation, as previously described.
Step 10. Once the population is uniformly acceptable, increase seed for release. As you increase the seed, continue to rogue out any undesirable plants as they arise.
I hope that this brief introduction gives you some of the tools to think about developing your own unique varieties to meet your needs and the needs of your community. Below are some resources for further exploration.
Allard, R.W. 1964. Principles of Plant Breeding. John Wiley and Sons, New York, NY.
Bassett, M. J. 1986. Breeding vegetable crops. AVI Pub. Co., Westport, Conn.
Deppe, C. 1993. Breed Your Own Vegetable Varieties. Chelsea Green, White River Junction, VT.
Fehr, W. R. 1987. Principles of cultivar development.
White, R., and B. Connolly. 2011. Breeding Organic Vegetables: A step by Step Guide for Growers [Online]. Available at http://www.nofany.org/sites/default/files/BreedingOrganicVegetables-2011.pdf (verified 20 Apr. 2012). NOFA – NY, Rochester, NY.
Zystro, J., A. Shelton, and S. Snapp. 2012. Participatory Plant Breeding Toolkit. Organic Seed Alliance, Port Townsend, WA.
Zystro, J. and J. Navazio. Introduction to On-farm Organic Plant Breeding. 2014. Organic Seed Alliance, Port Townsend, WA. This article was largely excerpted from this publication.
Jared is Organic Seed Alliance’s research and education assistant director. Jared has a graduate degree in plant breeding and has worked in the organic seed industry for over 15 years, managing seed production at two farms and conducting research and education projects with OSA. He lives in the coastal town of Arcata, CA, with his wife and son.