A Maize Breeding Methods Study in Kenya¹

Abstract

Four years of reciprocal recurrent selection has produced rapid improvement in one of the parental varieties (‘Ecuador 573’), in the variety cross, and in a commercial topcross maize (Zea mays L.) hybrid. The improved strain of Ecuador 573 is now used as the male parent of the current commercial hybrids, ‘H611C’ and ‘H613C,’ and they yield approximately 25% more than the original versions.Estimates obtained from seven years of the selection trials showed no changes in genetic variances for yield. Hence, this rate of improvement is not expected to change in the near future. Progress from modified ear‐to‐row selection was greater in the introduced Ecuador 573 variety and in KCA, (‘Kitale II’ ✕ Ecuador 573)syn‐4, than in the local Kitale II variety. No improvement was detected in KCA from mass selection. An increased number of ears per plant was associated with yield improvement by reciprocal recurrent selection and modified ear‐to‐row selection. Selection for lodging resistance and blight resistance in the breeding nurseries resulted in slight improvements in the varieties under reciprocal recurrent selection and in the variety cross. Greater attention to ear height and lodging resistance is required to reduce harvesting losses in the commercial maize crop in Kenya.

Open-Pollinated vs. Hybrid Maize Cultivars

“…Sprague noted "it appears that the ear-to-row method of selection for yielding ability was discredited not because of genetic limitations of the method, but because of the inadequate field plot technique" [16]. Research after Richey's 1922 conclusion showed recurrent selection and varietal hybrids could be very successful at increasing maize yields after all, although they could not provide cultivars with the yields of the best hybrids [35,47,81,95,119,120,[126][127][128][129][130][131].…”

“…Modified-ear-to-row selection has demonstrated average gains per year of about 2.1-3.5% [10,35,47,48,133]. These and other recurrent breeding techniques have been very successful in breeding cultivars for farmers in the tropics [120,128,131,132]. Conservative estimates of average potential annual yield gains of 82 kg/ha for mass selection and 83 kg/ha for modified-ear-to-row selection, largely from North American data, are comparable to those experienced in an open-ended inbred hybrid breeding program from the United States, where yields have increased by about 66 kg/ha each year since 1930 [9,29,35].…”

“…Gardner hit a yield plateau with Hays Golden after 12 cycles of mass selection; modified ear-to-row selection reached the same plateau in only six cycles [47]. Yield plateaus can be overcome by outcrossing, and then proceeding anew with the resulting composite population [131,139]. Eberhart et al promoted the idea of cooperative work with many populations [128].…”

“…Would many farmers be interested? Some answers come from the tropics where maize breeding has embraced population improvement and inbreeding and breeders develop inbred hybrids, varietal hybrids, synthetics and other OP cultivars to best fit local needs [120,128,131,[152][153][154][155][156][157]. In those regions synthetic cultivars have proven useful for farmers by providing higher yields and low seed costs, although for long term improvement synthetics may require more genetic diversity than that provided by only 8-10 lines and they may require more intensive selection schemes for further improvement [133,155].…”

Selection Indices for the Improvement of Opaque‐2 Maize1

“…The tendency for genetic variance to decrease with time has been noted in several recurrent selection programs (Hallauer, 1970;Burton et al, 1971;Penny and Eberhart, 1971), though not in others (Darrah et al, 1972); this may be largely the result of restricted population sizes. In the long term, selection uses up genetic variation, although the persistence of variation has been noted after 4 0 and 76 generations, respectively, in the in vestigations of Frankham et al (1968c) and Dudley (1976).…”