D. C. Rasmusson

Breeding programs in major crops normally restrict the use of parents to those improved for a variety of traits. Gain from utilizing these good × good crosses appears to be high, and improvements are sufficient to encourage continued breeding within narrow gene pools even though each cycle is expected to lead to reduced genetic variability. These finely tuned programs have gradually limited the amount of new diversity introduced into the breeding gene pool. This breeding strategy has led to a genetic gap where there is a large difference in the favorable gene frequency between the improved and unimproved lines and to a narrowing of genetic diversity within elite gene pools. At the same time, evidence has accumulated in plant breeding programs and long‐term selection experiments in several organisms that the genome is more plastic and amenable to selection than previously assumed. In the barley ( Hordeum vulgare L.) case study reported here, incremental genetic gains were made for several traits in what appears, based on pedigree analysis, to be a narrow gene pool. Given this situation, we call for an examination of the generally held belief that the variation on which selection is based in elite gene pools is provided almost exclusively from the original parents. Classical and molecular genetic analyses have shown that many mechanisms exist to generate variation de novo , such as gene amplification and transposable elements. Accordingly, we put forward the hypothesis that newly generated variation makes an important contribution. We also hypothesize that gene interaction, epistasis, is more important than commonly viewed and that it arises from de novo generated diversity as well as the original diversity.

Plant breeders have attempted to enhance yield by selecting for individual traits since the beginning of plant breeding. This approach has been broadened to encompass the breeding of model plants or ideotypes. An ideotype is a hypothetical plant described in terms of traits that are thought to enhance genetic yield potential. Ideotype breeding is defined as a method of breeding to enhance genetic yield potential based on modifying individual traits where the breeding goal (phenotype) for each trait is specified. The purposes of this paper are to elaborate on and evaluate ideotype breeding as a method to enhance genetic yield potential and to describe an ideotype for barley ( Hordeum vulgare L.). Successes that have occurred in breeding to enhance yield with individual traits, the value of genetic diversity for individual traits, and benefits from goal setting are presented as arguments in support of ideotype breeding. Alternatively, information is presented on the requirement of symmetry in size among plant parts, compensation among plant parts, pleiotropy, and genetic background, all factors that may slow progress in ideotype breeding. Ranges of genetic diversity, heritability estimates, and introgression information are presented for 27 barley traits. A barley ideotype consisting of 14 traits and the target or goal for each trait are described. Ideotype breeding is recommended as a methodology to augment traditional plant breeding, when the breeding goal is enhancing genetic yield potential. Breeding experience and research to date suggest that ideotype breeding is not a suitable substitute for traditional yield breeding.

The constraints imposed by the requirement of acceptable malting and brewing quality in new barley( Hordeum vulgare L.) cultivars, and the resultant use of a narrow germplasm base make the genetic gains in breeding of Midwestern malting barleys of special interest. In this study cultivars which have dominated the malting barley acreage in the tri‐state area of Minnesota and North and South Dakota since 1920 were evaluated to determine the amount of genetic gain which has been made in yield, agronomic and quality traits. Six cultivars were evaluated for 18 traits in seven trials conducted at Crookston and St. Paul, Minn. during the years 1978 to 1982. Four of the six cultivars—‘Kindred’ ‘Traill,’ ‘Larker,’ and ‘Morex’—were in turn the dominant cultivar in the tri‐state malting barley area. Cultivars differed significantly for 15 of the 18 traits. Grain yield of Morex was 51% greater than that of ‘Manchuria,’ the oldest cultivar; the average annual increase in grain yield measured from Manchuria to Morex was 0.9%. However, regression analysis indicates that for the past 40 years, the time period in which serious breeding efforts have been made in the tri‐state area, yield gains have been nearly linear with an average annual increase of 2%.During this time grower yields have nearly doubled and 73% of this increase can be attributed to improved cultivars or genetic improvement. Increases in grain yield of the cultivars were attributed to improvements in lodging resistance, an increase in harvest index( HI) from 0.31 to 0.40, and increases kernel weight and spike number. Significant improvements were achieved for plump kernels, malt extract, alpha amylase, diastatic power and disease resistance. Nitrogen harvest index also was higher in the newer cultivars. The results indicate that sizable yield gains, as well as gains for other traits, can be made within a relatively narrow germplasm base.