Winter Nursery Research Articles

Evaluation of an Antibiotic Factor in Maize Silks as a Means of Corn Earworm (Lepidoptera: Noctuidae) Suppression1

The objective of this study was to further evaluate in vivo (field culture) the maize, Zea mays L., silk flavone glycoside, maysin, that has been shown to inhibit the growth and increase the mortality of corn ear worm (CEW), Heliothis zea (Boddie), larvae with in vitro experiments. Significant differences existed for both maysin concentration and CEW penetration among tested silks of maize genotypes. Although a nonsignificant phenotypic correlation between maysin concentration and CEW penetration was found, a negative trend was noted among the top cross progenies of ‘Zapalote Chico’ (ex MO) (resistant to CEW) and ‘Iowa Super Stiff Stalk Synthetic’ (susceptible to CEW). Two of five topcrosses with ‘Iowa Super Stiff Stalk Synthetic’ as the tester parent showed decreased larval damage, and three topcrosses had a higher level of maysin concentration relative to the susceptible tester parent. Larval penetration increased significantly only for one of the ‘Zapalote Chico’ (ex MO) topcrosses (A619 × A632), and Maysin concentration was unchanged relative to ‘Zapalote Chico’ (ex MO). Broad-sense heritability estimated for Maysin concentration was 79.8%. Predicted gain per cycle from S1 recurrent selection with a winter nursery based on this heritability estimate would be 25.7%. Although a non significant correlation was found in this study between Maysin concentration and CEW penetration, our results show that Maysin content could be increased through cyclic selection. Concentrations of Maysin might be elevated through breeding to a level that could be effective in CEW control.

Selection for Cold Germination in Two Corn Populations1

Tolerance of corn (Zea mays L.) to low temperature conditions after planting requires rapid germination, vigorous seedling growth, and resistance to disease and insect pests. A selection scheme designed to improve cold tolerance in breeding material was initiated in two breeding populations, Iowa Stiff Stalk Synthetic (SSCG) and Pioneer Cold Tolerant Synthetic (CTCG). Laboratory and field selection were employed in the procedure. Seedlings developed from the first seeds to germinate at 7.2 C in the laboratory (20% selected) were transplanted to the field, selected for agronomic traits, and self‐pollinated. The selfed progenies were recombined in winter nurseries to initiate the next cycle of selection in the laboratory.Regression analyses of the four cycles of selection indicated that cold germination under laboratory conditions (7.2 C) improved 8.8 and 9.9% per cycle in CTCG and SSCG, respectively. In the field, however, little improvement was observed in emergence or seedling vigor. The lack of correlation between laboratory and field results for these cold‐tolerant traits was attributed to mild spring weather during the 2 years of evaluation. Selection for germination at cold temperatures did not have any detrimental effects on other agronomic traits measured in the populations.

Relative Efficiencies of Population Improvement Methods in Corn: A Simulation Study1

The relative selection gains per year of three breeding methods (mass, modified ear‐to‐row, and S1 selection) for corn (Zea mays L.) population improvement were simulated for additive and complete dominance genetic models. The character was assumed to be controlled by 20 major (genotypic value of A = 2) and 20 minor (A = 1) genes with half of the genes of each type at an initial frequency of 0.5 and half at a frequency of 0.1. Genes were independently assorted. Selection intensities of 0.05 and 0.25 (or equivalent), and heritabilities of 0.2 and 0.6 were used~ The population size was 400. The half‐sib families in modified ear‐to‐row selection were evaluated in a four‐replication trial, and the S1 progenies were tested in a 10‐plant plot with four‐replications.Our studies indicated that S1 selection would be more efficient than the other two methods, provided that a winter nursery is used to shorten the cycle to 2 years. At the high heritability (0.6), mass selection was more effective than modified ear‐to‐row selection, but the latter was more effective under the low heritability. Genotypic variances decreased in all cases as selection proceeded, but the decrease was most rapid under S1 selection. About 14 cycles of S1 selection exhausted the genotypic variance at the high heritability and high selection intensity. The results also suggested that an S1 progeny evaluation trial with 10‐plant plots and four replications is adequate for improving population performance.

THE EFFICIENCY OF USING DOUBLED HAPLOIDS IN A RECURRENT SELECTION PROGRAM IN A DIPLOID, CROSS-FERTILIZED SPECIES

Mass selection in both doubled haploid and diploid populations was simulated for a 30-year period under additive and complete dominance models. The response to S1 selection was also included for the purpose of comparison. All selection programs were conducted at two selection intensities, 5% and 25%, under 0.2 heritability with a constant population size of 400. S1 lines were evaluated in 10-plant plots with four replications; use of a winter nursery was assumed. The character under selection was controlled by 20 major (A = 2) genes and 20 minor (A = 1) genes. The initial frequency of ten of the major genes and ten of the minor genes was 0.5; that of the remaining ten of each type was 0.1. The response to mass selection with doubled haploids was about 1.4 times faster than diploid mass selection and equal to S1 selection in terms of gain per year. If replicated trials were used to evaluate the doubled haploids, the rate of gain was faster than in S1 selection. The greater efficiency of using doubled haploids was due to doubling of the additive genetic variance and the elimination of dominance variance. However, significant loss of desirable genes occurred in the mass selected doubled haploid population under both the 5% and 25% selection intensities in the additive and complete dominance models.

Evaluation of Methods for Generation Advance in Bulk Hybrid Soybean Populations1

We evaluated four methods of generation advance in bulk hybrid soybean (Glycine max (L.) Merrill) populations for their ability to maintain superior genotypes and genetic variability for economic traits and for their efficiency in sampling time. The methods used were single seed descent (SSD), restricted cross‐bulk (RCB), maturity‐group bulk (MGB), and cross‐bulk (CB).Method means did not differ significantly at either location, and differences in genotypic variance among methods were not consistent in all crosses. SSD, RCB, and MGB maintained a similar number of high yielding lines, about twice as many as CB. SSD and MGB were the most effective methods in maintaining early lines. However, there were no differences among methods for the frequency of late segregates. SSD was about twice as effective for maintaining large‐seeded lines as the other methods. The four methods did not differ appreciably for the number of lodging resistant lines. RCB had the highest frequency of tall plants.Comparison of the harvest time spent in obtaining samples for the next generation showed that CB was the fastest and RCB the second fastest method. SSD was more time‐consuming than CB and RCB. MGB was the most time‐consuming, since it required pulling plants at several different times to maintain subpopulations restricted in maturity.SSD was the method least influenced by natural selection; therefore, it may be the most useful method in greenhouse or winter nursery environments where a genotype may perform differently than under field conditions in its area of adaptation.