Direct and correlated responses to recurrent selection for grain yield in maize breeding populations

Morphological and physiological traits associated with barrenness and grain yield in the maize breeding population, Iowa Upright Leaf Synthetic #1

“…Lonnquist and Jugenheimer (1943) and Sass and Loeffel (1959) suggested that the primary cause of barrenness is the lengthening of the anthesis-tosiIking period, i.e., by the time late silks emerge, the pollen is not viable. The interval between anthesis and silking lengthens as plant density is increased (Kiesselbach, 1950;Dungan et al 1958;Shav;and Thorn, 1951 ;Wool ley et al 1962;Cardwell, 1967;Meyer, 1970;Buren, 1970;Fakorede, 1977). Additionally, Kohnke and Miles (1951) reported that silking was delayed approximately one day for every increase of 7,000 -8,000 plants/ha.…”

“…Buren et al,(197^) reported that density-tolerant maize genotypes were characterized by rapid completion of silk extrusion, coincidence of pollen-shed and silk extrusion and rapid growth of the first ear and first-ear silk. Fakorede (1977) found that improved variety hybrids from advanced cycles of two recurrent selection programs exhibited earlier silking dates and reduced pol1en-shed-to-si1ki ng interval s.…”

“…Additionally, Kohnke and Miles (1951) reported that silking was delayed approximately one day for every increase of 7,000 -8,000 plants/ha. Maize hybrids intolerant of high plant densities have a longer delay in silk emergence than those that are tolerant (Moss and Stinson, 1961;Schwanke, 1965;Earley et al, 1967;Meyer, 1970;Buren, 1970;El-Lakany and Russell, 1971;Mock and Buren, 1972;Buren et al, (1974);Fakorede, 1977).…”

“…These authors mentioned that large tassels suppressed ear development of plants grown at high plant densities; and according to Anderson (1971), these repressive effects were probably caused by apical dominance created by increased auxin levels in the stem. Fakorede (1977) reported that maize variety hybrids from recurrent selection programs for grain yield displayed smaller tassels than their un improved counterparts. Mock and Schuetz (1974) found tassel branch number was highly heritable, and they suggested that progress from selection for low tassel branch number and short tassel heights should be possible.…”

Changes in physiological traits associated with grain yield improvement in single-cross maize hybrids from 1930 to 1970

“…Other studies (Duvick, 1977;Fakorede, 1977; found increased grain yields to be associ ated with delayed senescence. Increases in the length of the stay green period across the decades of hybrids likely have contributed directly to increases in dry matter production and indirectly to observed increased total and machineharvestable grain yields.…”