Association mapping in maize (Zea mays L.) has been effective at identifying major quantitative trait loci (QTL) for less‐complex traits, and historical inbreds are a potential source of useful variation. Our objectives were to (i) characterize genomewide linkage disequilibrium, (ii) assess variation for flowering time, kernel composition, and resistance to northern corn leaf blight (NCLB) (caused by Setosphaeria turcica) and Goss's wilt and blight (GWB) (caused by Clavibacter michiganensis subsp. nebraskensis), and (iii) identity major QTL for these traits in a collection of 284 historical maize inbreds mostly developed at the University of Minnesota. Linkage disequilibrium (LD) was high, decaying (r2 = 0.1) in 802 kb, among 39,166 single nucleotide polymorphism (SNP) markers on the Illumina MaizeSNP50 Beadchip. The mean LD between adjacent markers was r2 = 0.38. The A321 subpopulation, to which 60% of the Minnesota inbreds belonged, had inbreds with either or both the minimum and maximum inbred mean value for all traits except protein concentration. We identified QTL for days to anthesis (eight QTL), days to silking (11 QTL), oil concentration (11 QTL), starch concentration (two QTL), NCLB (13 QTL), and GWB (nine QTL). The Minnesota inbreds were enriched for favorable QTL alleles for days to flowering and oil concentration but not for starch concentration and disease resistance. These QTL identified in historical maize inbreds may be useful in modern breeding programs.

The University of Minnesota has developed more than 600 maize (Zea mays L.) inbreds since 1915. Our objectives were to (i) characterize the population structure and relatedness in the historical Minnesota maize inbred collection in comparison with inbreds of known heterotic groups, (ii) identify any unique germplasm groups among the historical Minnesota inbreds, and (iii) characterize the genetic diversity of the historical Minnesota germplasm. We used the Illumina MaizeSNP50 Beadchip to analyze 284 inbreds with 43,252 single nucleotide polymorphism markers: 143 inbreds developed at Minnesota (referred to as “A” inbreds), 26 inbreds developed by public breeding programs, and 115 inbreds developed by seed companies. Model‐based clustering divided the inbreds into K = 5 subpopulations represented by the B73, Mo17, Oh43, A321, and PH207 genetic backgrounds. More than 60% of the A inbreds clustered in the A321 subpopulation; many of these inbreds were derived from the open‐pollinated cultivar Minnesota 13. The A321 subpopulation had higher mean dissimilarity (0.384) and gene diversity (0.377) than the other four subpopulations. Subclusters in the A321 and Oh43 subpopulations represented groups of inbreds unique to the historical Minnesota germplasm. Our results may help provide maize breeders with information needed to effectively use historical genetic resources while maintaining heterotic patterns necessary in hybrid breeding.

Conventional, nondwarf corn {Zea mays L.) hybrids grown in the northern U.S. Corn Belt are typically >2 m tall and have a 75-to 100-d relative maturity (RM). Our objectives were to assess the potential of open-poHinated C0P0P1 semidwarf corn for grain and forage production, estimate genetic variances and heritability in C0P0P1, and develop COPOP1 subpopulations that exhibit heterosis. In 2005 and 2006, we evaluated C0P0P1 with four commercial, nondwarf hybrids at three plant population densities. Grain yield of open-pollinated COPOP1 was about half of the mean yieid of the conventionai hybrids. However, C0P0P1 had lower grain moisture (equivalent to 62 d RM) and better forage quality than the commercial hybrids. In 2007, C0P0P1 was evaluated for performance when crossed to two divergent inbred testers, LH227 and LH295. Heritability was significant for grain moisture, plant height, and ear height in testcrosses to both testers but was significant for grain yield only in testcrosses to LH227. In 2009, bulk Cycle 0 (i.e., original C0P0P1) and Cycle 1 testcrosses, nine semidwarf hybrids, C0P0P1, and three commercial hybrids were evaluated at two plant population densities. Grain yieid and most agronomic and forage traits did not improve between Cycle 0 and Cycle 1 in either testcross population. None of the semidwarf hybrids had higher grain yieids than C0P0P1. While semidwarf C0P0P1 can serve as a souroe of useful variation for improving elite germplasm in the northern U.S. Corn Beit, further selection in C0P0P1 must be done to deveiop improved semidwarf populations. Abbreviations: ASI, anthesis-silking interval; BSSS, Iowa Stiff Stalk Synthetic; IVTD, in vitro true digestibility; NDF, neutral detergent fiber; NDFD, neutral detergent fiber digestibility; NIRS, near-infrared reflectance spectroscopy; RM, relative maturity. I N CORN {Zea mays L.), genes such as reduced plant 1 {rdl), brachytic 2 {br2), and compact plant {ci) cause dwarfing (Nelson and Ohlrogge, 1957;Scott and Campbell, 1969;Modarres et al., 1997b;Neuffer et al., 1997). Semidwarf corn may have characteristics that make it suitable for grain and forage production in short-season environments (Dijak et al., 1999;Coors and Lauer, 2001). In particular, the limited heat units in short-season environments reduce grain yield and lessen the probability of the crop reaching physiological maturity in a given year. Semidwarf hybrids have required 10% fewer heat units than conventional hybrids to reach anthesis (Dijak et al., 1999) and generally reach physiological maturity earlier than conventional cultivars (Modarres et al., 1997a). Like semidwarf wheat {Triticum aestivum L.; Vogel et al., 1963), semidwarf sorghum {Sorghum bicolor L.; Windscheffel et al., 1973), and semidwarf rice {Oryza sativa L.; Hedden, 2003), semidwarf corn may be less prone to stalk and root lodging due to the shorter stalks and lower ear placement (Dijak et al., 1999). Semidwarf corn has the potential to produce high-quality forage due to its high ear-to-stover ratio, although dry matter y...