Heterosis in compositional, physical, and wet-milling characteristics of hybrids from exotic introgressed by adapted inbred lines in corn
Corn (Zea Mays L.) is one of the most important cereal crops in the world and the main crop in the United States. Starch is the most abundant component of the corn kernels and the most valuable fraction recovered by wet milling of corn. The amount wet milled from a sample is an indication of the millability or ease with which kernel components are separated. New hybrids with high grain yield and higher starch, protein, or oil contents have been developed and are available to corn growers. However, these hybrids are the result of crossing adapted inbred lines and rarely have corn lines from exotic germplasm been crossed with elite inbreds to develop new and useful breeding lines. Studying the physical, compositional, and wet-milling properties of new inbred lines and their hybrids, as well as the proximate composition of recovered fractions, needs to be done at the laboratory level before these materials are of value to the corn processing industry. The present study was conducted to: (i) determine whether Corn Belt lines introgressed with exotic materials have appropriate wet-milling characteristics, and (ii) determine the expression of heterosis in physical, compositional, and wet-milling characteristics of the F1 and F2 generations of hybrids from their crosses with adapted inbred lines. Ten inbred lines from the Germplasm Enhancement of Maize (GEM) project introgressed with exotic germplasm from Argentina, Chile, Cuba, Florida, and Uruguay were grouped as high starch and low starch exotic lines (HSEL and LSEL) according to the starch yield produced and were crossed to three commercial adapted inbred lines used as testers. B73, Mo17, and their hybrid were used as controls. The 10 inbred lines from GEM and the F1 and F2 generations of these 30 experimental hybrids were analyzed using both Near-Infrared Transmittance (NIT) and a 100-g wet-milling procedure. AR227 and CU562, two lines of exotic origin, had similar or better starch yield and starch recovery than B73 and the other adapted inbred lines, which indicates that these lines can potentially be used to improve the proportion of extractable starch present in the kernels of their hybrids. There was no significant difference between groups of hybrids from high and low starch lines, but the effect of lines and testers, as well as the interaction effect of lines by tester (hybrids), were statistically significant in both the F1 and F2 generations and produced great variation among physical, compositional, and wet-milling characteristics of the experimental hybrids. Exotic inbred lines from Argentina, Chile, Cuba, and Florida had better wet-milling characteristics when crossed to Tester 3, which indicates that the original accessions (AR16035, CH05015, CUBA117, and FS8B(T)) used in GEM can be a valuable source of value-added characteristics if a breeding program to increase the extractable starch of corn hybrids incorporates breeding lines from these accessions as part of it s germplasm base. Negative heterosis for starch content and positive heterosis for protein content was expressed, which led to negative heterosis values for starch yield and starch recovery and poor wet-milling properties of the F1 generation of experimental hybrids. Positive mid-parent and high-parent heterosis for starch content and negative values for protein content were expressed by the F2 hybrids. This led to positive heterosis values for starch yield and starch recovery and better wet-milling properties for some hybrid grain samples with exotic germplasm compared to B73xMo17 hybrid grain. There was a positive correlation between starch content and starch yield and starch recovery and a strong negative correlation between protein content and starch yield and starch recovery. This indicated that genotypes with high starch and low protein contents produce hybrids with better millability. NIT may be used as a predictive tool to screen early progeny for high starch and low protein contents, which are associated with higher starch yield and starch recovery, to save time and costs in a corn breeding program directed to the improvement of the wet-milling efficiency of corn.