High digestible dry matter yield is desired by maize (Zea mays L.) silage producers. The brown midrlb‐3 mutant (bm3) improves the digestibility of maize stover but reduces grain and fodder yields of homozygous bm3 genotypes. Our objective was to estimate the relative genetic potential for improvement of silage quality and yield in bm3 and normal maize populations. A total of 130 bm3 and normal 130 S1 lines were developed from three populations segregating for the bm3 allele. All 260 S1 lines were evaluated at two Minnesota locations in 1978. In 1979, 64 bm3 64 normal S1 lines, and 24 bm3 and 24 normal S1 ✕ S1 hybrids were evaluated at three locations.The bm3 genotypes averaged 77% of the grain yield, 90% of the stover yield and 84% of the fodder yield of the normal genotypes. Some bm3 genotypes produced as much stover as the best normal genotypes. However, no bm3 genotypes produced as much grain or fodder as the best normal genotypes. The normal genotypes yielded 16% more digestible dry matter than the bm3 genotypes. Estimates of genetic variability and predicted genetic gain for digestible dry matter yield were similar for bm3 and normal populations. Even though bm3 germplasm offers a substantial advantage in stover digestibility, our results indicate normal populations of maize may offer more potential for silage breeding programs.

Ten corn inbred lines were field evaluated for their tolerance to DPX-M6316 at 32 and 64 g ai/ha. Inbreds ‘A619′, ‘A641′, and ‘ND246’ were highly susceptible to DPX-M6316; inbreds ‘A671′, ‘A632′, and ‘B73’ were highly tolerant; and inbreds ‘A654′, ‘CM105′, ‘W153R’, and ‘M017’ were intermediate in their response. The basis for differential tolerance was studied by comparing the susceptibility of acetolactate synthase (ALS) to inhibition by DPX-M6316 in tolerant A671 and susceptible A619, and by examining the absorption, translocation, and metabolism of DPX-M6316 in both genotypes. I50values for DPX-M6316 inhibition of ALS activity in extracts from etiolated shoots of A671 and A619 were similar, 15.6 and 17.4 nM, respectively. There was little difference in14C-DPX-M6316 absorption by the two inbreds, but twice as much of the absorbed14C was translocated out of the treated leaf of A619 (13%) compared to A671 (6%). Differences in translocation may have been due to much more rapid DPX-M6316 metabolism in A671 than in A619. Extracts from treated leaves of A671 had only 23% DPX-M6316 remaining 5.5 h after treatment (HAT) compared to 78% DPX-M6316 remaining in extracts from A619 leaves. Therefore, rate of metabolism was the major factor involved in the tolerance of A671 and the susceptibility of A619 to DPX-M6316.

Identification of sources of favorable alleles to improve existing hybrids is one of the most important problems facing a maize (Zea mays L.) breeder. Previous work has demonstrated the effectiveness of a procedure developed by Dudley for identifying populations containing favorable alleles not present in an elite hybrid. However, previously reported work involved at most two elite hybrids. The objective of this study was to evaluate the potential of 20 improved populations to improve the three hybrids made from three inbreds in commercial use. Each of the populations was crossed to LH195, LH212, and LH216. The population × inbred crosses, the three hybrids among the inbreds, and the three inbreds were evaluated in seven U.S. midwestern environments in 1993 and four in 1994. Traits measured were grain yield, grain moisture, plant height, ear height, and concentration of protein, oil, and starch in the grain. For grain yield, 15 of the 20 populations had significant estimates of dominant favorable alleles not present in the highest yielding target hybrid (LH195 × LH212). None of the populations showed potential for reducing ear height. However, seven populations had more favorable recessive alleles than unfavorable dominants for plant height when LH195 × LH212 was the target hybrid. None of the populations tested appeared to have potential for increasing starch concentration in any of the target hybrids. Eight populations showed potential for increasing protein concentration in all three target hybrids. Assumptions required to identify parents were not met for grain moisture, oil concentration, and stalk and root lodging. Disciplines

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 S1 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.