Defining objectives

Introduction

Successful breeding programs have clear objectives. Usually, the objective is to produce a cultivar that is superior to farmers’ varieties in a particular target population of environments (TPE).  

But what exactly is meant by “superior”?

(roll your mouse over the pictures)

There is no absolute definition of a superior variety. In particular situations any of the criteria listed above may constitute superiority. However, if the goal of the breeding program is to have impact in farmers’ fields, breeders need to produce varieties that are better than the farmers’ current variety, under management that the farmer is likely to use, and in the opinion of the farmer.

Successful breeders therefore carefully assess the production, quality, and market requirements of the farmers they serve, and the characteristics of the environment in which their varieties will be grown. In this lesson, we will examine the factors that need to be considered in defining target environments and setting goals for breeding programs.

2. Factors used in delineating the TPE

There may be several TPE within a single region. Sometimes there is great environmental variability among locations, or even within single farms. This is particularly common in rice, because the fields in which rice is grown often vary greatly in hydrological and soil characteristics, depending on their position in the toposequence. Sometimes, there are big differences in the way farmers of different socioeconomic groups manage their fields.  

For example, wealthy farmers may use more fertilizer or pesticides than poorer farmers. If great environmental or socioeconomic variability exists within a region, then it may be best to divide the TPE  into separate breeding targets, and either serve each subdivision with its own breeding program, or focus resources on the most important TPE.

Defining the target environment is the thus first and most important decision in planning a breeding program. The target environment is defined in terms of environmental and socioeconomic factors.

Click on the icon to see a possible group exercise (10 minutes).

summary

3. Methods for grouping sites and farms into breeding target environments

The mandate of a rice breeding program is usually described in terms of:

1. a geographical region (for example, north-east Thailand),

and

2. an ecosystem (e.g., medium-deep rainfed lowland).  

As we discussed in the previous section, within this type of broad environmental classification, the breeder may still be faced with a range of soil types, management regimes, and quality preferences. He or she must therefore consider whether or not separate breeding targets exist within the TPE. Both quantitative tools and common sense are needed to determine whether to break a TPE into smaller breeding targets.

Although it may be possible to describe environmental or socioeconomic differences among farms or fields in a region, it is not necessarily the case that different breeding regions are needed for these different classes of environments. Separate breeding targets are only needed when cultivars perform differently (i.e., when there is rank change) in the different environments. Methods for defining target environments will be described in more detail in a later unit, but will be briefly discussed here.

A number of tools can be used to help define target environments, including:

Exercise: Group trials into two potential TPE

4. The relationship between the selection environment and the TPE

After the TPE has been defined, the breeder must design the SE. The SE should be chosen or constructed to maximize the power of trials and nurseries to predict performance in the TPE. If several “populations” of environments occur within the TPE (for example, a particular field may be subject to drought in some years and submergence in others), then several SEs may be needed to maximize overall progress from selection. For example, all lines may have to be screened both for submergence and drought tolerance.   

Although the SE is constructed to predict performance in the TPE, the breeder must always remember that the SE is not the TPE, and gains made in the SE will not necessarily be expressed in farmers’ fields. It is important for breeders to monitor the correlation between performance on-farm and performance in the SE.

There are 3 important features of a useful SE:

1. The SE must predict performance in at least some seasons and locations within the TPE. In other words the genetic correlation (rG) between TPE and SE must be high.

2. The SE must clearly and repeatably differentiate among genotypes under evaluation.  In other words, the heritability (H) for screening in the SE must be high.

3. The SE must permit relatively large numbers of genotypes to be screened at low cost. In other words, the SE must permit a high selection intensity (i) to be achieved.

For some stresses, the features of an effective SE are well known. For example, much research effort has been done to identify effective screening methods for seedling submergence tolerance. Methods have been developed that are known to be highly repeatable, predictive of performance under submergence in farmers’ fields, and appropriate for screening large populations (Mackill et al., (1996)  Chap. 6).  

For drought tolerance, the situation is quite different. Many screening methods for reproductive-stage drought tolerance have been proposed, but only recently has evidence begun to accumulate about the effectiveness of these methods in predicting performance under natural drought stress in the TPE.

5. Setting goals and prioritizing traits

It is important for rainfed rice breeding programs to have clear goals. These goals need to take into account the current priorities of farmers, but must also consider the way these priorities may change in the future.

Determining farmer preferences

Breeders should not assume that they know the priorities of farmers, or that selection on the research station will necessarily result in varieties that farmers prefer. There are many ways in which farmer trait priorities and varietal preferences can be studied, including:

• Focus group discussions in which farmers are asked about the positive and negative features of their current varieties.

• Preference analysis, in which groups of farmers are asked to rate a set of varieties in a demonstration plot, and to describe the good and bad features of the varieties in the plot.

(see also module on participatory approaches)

Breeders can learn a great deal by talking to as many farmers as possible about their production problems and varietal preferences. Many studies have shown that grain yield under their own management is the trait of most importance to rainfed rice farmers. However, farmers are also usually very concerned with cooking and eating quality, and will reject high-yield varieties that do not meet their quality requirements. Failing to focus on quality is a basic error that has led to the failure of many breeding programs to have an impact.

Occasionally, farmers may not express a preference for characteristics that breeders feel would be useful. This may be the case for major changes in growth or development permitting new cropping options. For example, a reduction in growth duration from 130 to 100 days for upland rice varieties may permit the sowing of a dry-season crop where none is currently produced, but farmers may not be aware of the existence of such early-maturing material, and may therefore not express a preference for it. Farmers may need to observe such new materials in on-farm demonstration trials before they recognize its potential benefits.

Setting goals

Successful rainfed rice breeding programs often have very specific objectives that are useful in planning and measuring progress. These objectives are often best expressed in terms of developing a replacement for a currently popular variety.  

For example, a program may wish to replace a traditional high-quality variety with one of similar quality but higher yield under farmer management.  

Several planning decisions follow logically from these objectives.

• A high-quality locally-preferred variety should be used as a parent in most crosses. Because it can be difficult to recover quality characteristics in a single cross, the high-quality parent may be used as the recurrent parent in generating a BC1-derived population

• Quality parameters should be the focus of early-generation selection, because they are highly heritable, whereas yield is not.

• The program should be structured to generate a large population of breeding lines with acceptable quality, which can then be evaluated for yield under farmer management.

Let's conclude

Summary

1. Breeders produce varieties for use by farmers in a set of fields and future years called the target population of environments (TPE).

2. Considerable season-to-season and field-to-field variability may occur within the TPE.

3. The TPE is defined in terms of both environmental factors )like soil texture and submergence risk ) and socioeconomic factors (like the ability of farmers to afford purchased inputs).

4. There are many ways to group trial sites into TPE. One of the simplest is to group together sites at which line means are highly correlated.

5. A selection environment (SE) is used by the breeder to predict performance of new lines in the TPE. If an SE is effective, the rank of breeding lines in it will be similar to their rank in the target environment. It will also permit large populations to be efficiently screened.

6. It is important to determine farmer preferences, especially for quality, and incorporate them as selection criteria. Failure to do so may result in non-adoption of a new line, even if its agronomic performance in the selection environment is excellent.