Breeding widely adapted wheat (Triticum aestivum L.) genotypes with stable and high yields across environments is particularly important for developing countries since yield stabilizing inputs are often limited or not available. To evaluate the screening ability of locations for identification of such genotypes, data collected for 19 yr by the International Spring Wheat Yield Nursery (ISWYN) were analyzed; 1221 trials at 268 locations in 69 countries were involved. To compare single‐experiment parameters, i.e., genotypic variance (σ̂2g)k, error variance (σ̂2e)k, heritability (h2)k, and coefficient of variation (CV)k, trials without major biotic stresses were divided into three groups according to mean grain yield. Genotypic variance, error variance, and heritability increased and (CV)k decreased with yield. Group means for the four parameters were significantly (P = 0.01) different. A fourth group containing trials with major biotic stresses had the highest, but not significantly higher, average estimates for (σ̂2g)k, (h2)k, and (CV)k. The screening ability for each location was calculated as the correlation, rk, between mean grain yield of genotypes at each location and mean yield across locations. The screening ability was highest for locations with no major abiotic and biotic stress factors apart from leaf rust (Puccinia recondita Roberge ex Desmaz. f. sp. tritici) and stem rust (P. graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn.). CIMMYT's principal test site, Ciudad Obrégon, Sonora, Mexico, was most suitable for screening, with an average rk of 0.77. Sensitivity to photoperiod, cold tolerance, need for late maturity, tolerance to problem soils, and resistance to diseases other than rusts were the main adaptation‐limiting and location‐specific factors.

Hybrid combinations of 10 flint with nine dent inbred lines of maize (Zea mays L.) along with 10 standard cultivars were evaluated in two environments. Four flint and four dent inbred lines were the result of a breeding program for high protein percentage in the grain and good combining ability for agronomic traits. The materials were harvested at two stages, i.e. at the beginning of the grain‐filling period and at grain maturity. Data based on stover, ear, and grain were collected to study N translocation from stover to ear during the grain‐filling period and N uptake in two phases of plant growth: the pre‐grain‐filling period (phase 1) and the grain‐filling period (phase 2).Highly significant differences were observed due to general combining ability (GCA) and specific combining ability (SCA) for total N uptake, N uptake in phase I, N uptake in phase 2, and N translocation. Genotype ✕ environment interactions involving both GCA and SCA contributed significantly to the variation for these traits. A high N uptake in phase 1 accompanied by a low N uptake in phase 2 seemed to increase the N translocation, depending on the capacity of the sink. The experimental hybrids, with high protein percentage and protein yield of grain, generally silked later than the conventional hybrids, but were at least comparable for percent ear dry matter at maturity and grain yield. This indicated a high rate of ear dry matter production and drying of the high‐protein hybrids. It is hypothesized that a high protein percentage and protein yield of mature grain, may be due to an intensive N uptake, a prolongation of the N uptake phase, and a high N translocation. The relative importance of these factors may vary in different genotypes and environments.

In the cooler regions of the world, the area under forage maize (Zea mays L.) production has increased, and breeding for digestibility has gained importance. This study was conducted on digestibility traits of stover in maize forage, to determine variation, as analyzed by the combining‐ability approach, and correlation coefficients (r) in these traits and r between these and dry matter (DM) yield traits. Diallel crosses among 12 inbred lines were evaluated at silage and grain harvests for 2 yr at two agro‐climatically diverse locations in West Germany. Stover samples were analyzed for in vitro digestible organic matter (IVDOM), neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL) using laboratory and near infrared reflectance spectroscopy (NIKS) methods. Mean squares due to general (GCA) and specific combining ability (SCA) and their interactions with environments (GCA ✕ E, SCA ✕ E) were, for the most part, significant. At silage harvest, GCA was the most important source of variation in all traits, followed by SCA ✕ E interactions. At grain harvest, GCA and SCA ✕ E for IVDOM and ADL and SCA ✕ E for NDF and ADF accounted for larger variability than SCA and GCA ✕ E. The genetic ratio indicating the relative importance of GCA vs. SCA was > 0.81 in all cases, except that it ranged from 0.40 to 0.58 for NDF and ADF at grain harvest. The IVDOM had negative r with NDF, ADF, and ADL, and r values were stronger at silage harvest; IVDOM was not correlated with stover DM yield, and had negative r with ear DM yield. The results showed significant genotypic variation for several traits, with GCA the most important source of variation for most traits, and showed that selection for digestibility should preferably be made at the ensiling stage.

INDEX WORDSZea mays L., maize, reciprocal difference, protein content. SUMMARY Forty-two crosses and their reciprocals in maize (Zea mays L.) involving inbred lines highly diverse for protein content were evaluated in four environments. Data were recorded on crude protein content of grain, protein yield, grain yield, 1000 kernel weight, dry matter content of ear at harvest, days to 50~ silking, plant height, ear height, and early vigor. No significant variation due to reciprocal differences was observed for protein content and early vigor. For all other traits the variance component due to reciprocal × environment interactions was significant while the variance component due to reciprocal differences was significant only for kernel weight, dry matter content of ear, plant height, and ear height. The variance components due to nuclear differences and their interactions with environments were always highly significant and larger than the components due to reciprocal differences and reciprocal × environment interactions. The instability and low magnitude of reciprocal differences indicated that it might be difficult to exploit them commercially. However, seeing the presence of reciprocal differences for most of the traits studied, the evaluation of breeding materials for these differences seems to be important.

Genetic diversity study was done in 64 maize inbreds grown under excess soil moisture condition.The genotypes were grouped into eight clusters. It was observed that all intercluster distances were larger than all intracluster distances. Such results indicated that genotypes included within a cluster had less diversity among themselves but wider genetic diversity among the inbreds of different clusters. The highest inter cluster distance was observed between cluster IV & VII followed by IV & V , IV & VIII, II & VII and I & IV. The highest intra cluster distance was noticed in cluster V and the lowest was in cluster VII. The highest yield/plant, cob girth, number of rows/ear, number of grains/plant, SPAD value and number of nodes with brace roots were observed in cluster IV. The lowest mean value for yield/plant yield components and dwarf statured plant were observed in cluster VII. The plant height, ear height,days to 50% tasseling and silking, cob length, cob girth, 100 grain weight and SPAD value contributed considerably to total divergence. The genetically diverged genotypes in these distinct clusters could be used as parents in hybridization program for getting desirable hybrid(s).