Incorporation of exotic germplasm into the U.S. maize ( Zea mays L.) germplasm pool has often been proposed. Backcrossing and intermating were studied as techniques for incorporation of exotic germplasm using the populations AS‐A and MN‐ETO. Three levels of backcrossing (0, 1, and 2 backcrosses) and cycles of intermating (1, 3, and 5 cycles) were examined in all combinations by extracting 100 random S 1 lines from each treatment for field evaluation. All lines were evaluated for eight traits in six environments. Data for grain yield, grain harvest moisture, lodging, plant height, ear height, number of days to 50% silk emergence, number of days to 50% pollen shed, and a selection index (SI) [SI = grain yield (kg ha −1 ) −18.8 × harvest moisture (g kg −1 )] were analyzed. Analyses on trait means, genetic variances, correlated responses, selection differentials, and frequency distributions indicated that backcrossing generally shifted means and resulted in smaller genetic variances. Phenotypic correlations were both increased and decreased depending on the comparison examined. Changes of selection differentials of secondary traits were consistent with phenotypic correlations. The effect of backcrossing on the means of selected and unselected lines was very similar. Many changes were maturity related. Intermating levels used had no detectable effect on the populations. Significant differences were found, but these were isolated and did not form trends across intermating levels or backcross treatments. The results of this study suggest that backcrossing is useful in the incorporation of exotic germplasm, but results do not support the use of repeated intermating.
Substantial yield reductions in maize ( Zea mays L.) can be attributed to short‐duration drought in rainfed production areas. The objectives of this study were to investigate the effects of moisture stress in selection environments on response to mass and full‐sib recurrent selection and yield stability when selected materials were evaluated in environments with different levels of moisture stress. Five cycles of mass and full‐sib recurrent selection for grain yield were applied to population AS‐A under irrigated and dryland conditions on sandy textured soils. Selected cycles were evaluated as populations per se, S1 bulks, and in testcross combinations in 21 environments that included low, variable, and high moisture stress trials. Selection responses in populations and stability parameters for all materials were estimated. Under irrigated conditions, mass selection (MI) and full‐sib selection (FI), and full‐sib selection under dryland conditions (FNI) resulted in gains in AS‐A of 10.2, 11.0, and 5.8% per cycle, respectively. Mass selection under dryland conditions (MNI) did not result in significant gains per cycle except high moisture stress environments. Selection increased grain yield mainly by increasing responsiveness under low or variable moisture stress. The relative performance of S1 bulks and testcrosses were similar to relative performance of corresponding populations per se. On average, selection for grain yield under irrigated conditions gave results superior to those obtained from selection under dryland conditions. Selection under irrigation was as effective as selection under dryland conditions for increasing yield in moisture‐stressed environments and resulted in greater responsiveness of selected populations to favorable environments.
Breeders of maize ( Zea mays L.) have been concerned with relationships between yield and yield components and the effects of plant density on these relationships. We used three inbred lines (A251, A556, and A619) and backcross‐selection procedures for kernel depth, ear length, and ears per plant to develop six modified versions of each inbred: deep kernel (D), long ear (L), multiple ear (M), deep kernel and long ear (DL), deep kernel and multiple ear (DM), and long ear and multiple ear (LM). A normal (N) inbred plus its six modified versions were crossed with their counterparts in each of the two other inbred backgrounds to produce seven types of single‐cross hybrids: N, D, L, M, DL, DM, and LM. The 21 hybrids were evaluated at plant densities of 24,700, 49,400, and 74,100 plants/ha in 2 years at two locations per year. The objectives of this study were to compare the effects of backcross‐selection for one vs. two of the yield components and to estimate the relative effects of such backcross‐selection on response of hybrids to plant density. Expression of the selected yield component in single component types of hybrids usually exceeded that of their normal counterparts. Increase in expression of selected yield components in double component types occurred only for ears per plant at low plant density and ear length at higher plant densities. Yields of D, L, M, DL, DM, and LM hybrids were −4.2, +3.6, +6.6, −8.0, +0.4, and −2.2%, respectively, of yields of N hybrids when averaged over plant densities and genetic backgrounds. Modified hybrids, particularly double‐component types, did not respond well to medium and high densities. Within the highest yielding hybrid background (A619 ✕ A251), no modified hybrid significantly exceeded the yield of the normal hybrid at any density. Within the limits of our evaluation, backcross‐selection for yield components does not seem to be a useful procedure for improving high yielding hybrids grown at plant densities common in much of the Corn Belt. It might be a useful breeding procedure to improve yield in specific circumstances, e.g., in relatively low yielding hybrid backgrounds, especially at lower plant densities.
Mechanical detasseling is a common procedure in hybrid seed production of maize ( Zea mays L.) and is often accompanied by loss of leaves. No one has examined the potential of husk leaves, i.e., laminae extending from the husk, to compensate partially for leaf area lost during detasseling. We investigated the contribution of husk leaves to grain yield in three environments under normal conditions and under stress induced by partial defoliation prior to anthesis. The study involved four inbreds and two hybrids of early Corn Belt dent maize with high husk leaf area. Treatments consisted of a check, removal of the tassel plus the top two leaves prior to anthesis, and removal of all tissue above the uppermost ear prior to anthesis. Also, the husk leaves were either left intact (check) or removed when they are fully extended. Reduction in grain yield due to the removal of only the tassel and the top two leaves averaged 9.2% over all genotypes. Removal of all tissue above the uppermost ear caused an average yield loss of 50.2%. The decrease in yield due to complete husk leaf removal was significant for four of the six genotypes, and the average reduction was 2.6% (1.2 q/ha). The contribution of the husk leaves to grain yield was similar across the various detasseling treatments. Therefore, selection of seed parent inbreds for husk leaf areas similar to those we studied would not greatly offset yield reductions caused by mechanical detasseling.
Corn (Zea mays L.) can be a serious weed in soybean [G/yc/ne max (L.) Merr.] fields.A new herbicide, diclofop, has selectively controlled corn in soybeans when applied as a spray over the top of both soybeans and corn.Corn inbreds have shown differing degrees of susceptibility to diclofop.This report gives the response to diclofop by F 2 generation plants (simulated volunteer corn) from 240 Corn Belt hybrids.F 2 generation plants showed differing degrees of susceptibility to diclofop, suggesting that volunteer corn from some hybrids might be unsatisfactorily controlled especially in cases where diclofop was not applied at the proper time or at the proper rate or where environmental factors were unfavorable for herbicidal activity.
High digestible dry matter yield is desired by maize ( Zea mays L.) silage producers. The brown midrlb‐3 mutant ( bm 3 ) improves the digestibility of maize stover but reduces grain and fodder yields of homozygous bm 3 genotypes. Our objective was to estimate the relative genetic potential for improvement of silage quality and yield in bm 3 and normal maize populations. A total of 130 bm 3 and normal 130 S 1 lines were developed from three populations segregating for the bm 3 allele. All 260 S 1 lines were evaluated at two Minnesota locations in 1978. In 1979, 64 bm 3 64 normal S 1 lines, and 24 bm 3 and 24 normal S 1 ✕ S 1 hybrids were evaluated at three locations.The bm 3 genotypes averaged 77% of the grain yield, 90% of the stover yield and 84% of the fodder yield of the normal genotypes. Some bm 3 genotypes produced as much stover as the best normal genotypes. However, no bm 3 genotypes produced as much grain or fodder as the best normal genotypes. The normal genotypes yielded 16% more digestible dry matter than the bm 3 genotypes. Estimates of genetic variability and predicted genetic gain for digestible dry matter yield were similar for bm 3 and normal populations. Even though bm 3 germplasm offers a substantial advantage in stover digestibility, our results indicate normal populations of maize may offer more potential for silage breeding programs.
We conducted experiments to determine the inheritance of tolerance in corn ( Zea mays L.) to HOE 23408 (methyl‐2‐[4‐(2,4‐dichlorophenoxy) phenoxy]propanoate), an herbicide that has been effective in controlling volunteer corn in soybeans [ Glycine max (L.) Merr.]. Three tolerant and three susceptible inbred lines, their 30 singlecross hybrids (Fl's) including reciprocals, and the sibbed Fl's (F2's) were grown in two adjacent experiments planted 4 weeks apart in 1975 near Rosemount, Minn. Each experiment contained three replications of pairedrow plots, and one row of each pair was sprayed over the top with 0.86 kg/ha of HOE 23408 when the corn had six to seven fully‐expanded leaves. Percent control was rated 3 weeks after spraying. Percent control did not differ significantly between experiments or among reciprocals. Genotype ✕ experiment interaction was significant but accounted for less than 5% of the variation among genotypes and experiments. Gene action for percent control was primarily additive, with 91% of the variation among F1 means accounted for by fitting only general combining ability effects. Analysis of F2 data also indicated a preponderance of additive gene action. Significant heterosis and nonadditive gene action were present, but they were not of major importance. Broad‐sense heritability values (individual F2‐plant basis) averaged 95.3%, and relative frequency distributions of percent control ratings on F2 plants indicated several loci controlled tolerance to HOE 23408. More than half of the F2 plants from tolerant ✕ tolerant and tolerant ✕ susceptible F1's were less than 75% controlled by HOE 23408. Therefore, satisfactory control of volunteer corn in soybeans by foliar application of HOE 23408 depends in part on the pedigree of corn hybrids grown the previous year.
