No abstract

Current economic pressure, combined with growing public concern over nitrates in groundwater, has resulted in increased interest in managing N fertilizer in corn (Zea mays L.) production. The response of modern corn hybrids to N in a number of different field environments was investigated. Twelve elite hybrids were grown in 13 field environments and a number of other hybrids were evaluated in five additional environments. All locations were in the U.S. Corn Belt. Results indicated a statistically significant response of corn hybrid yield to N. The hybrids differed in average yield, however no consistent hybrid × N rate interaction was found in any of the experiments. This indicates that the hybrids responded to N in a similar way when averaged across the environments. While there may be differences in hybrid response in a particular type of environment, responses were not predictable across different weather patterns and soil types. Agronomic traits and leaf N were also measured and are discussed. Based on these findings, farmers should manage their N program in a similar manner for all hybrids, by efficiently applying an optimal rate of N and minimizing the possibility of nitrates leaching into groundwater.

No abstract

Maximum genetic gain from recurrent selection requires an efficient and effective strategy for selection of superior individuals from a population and intermating these selections to form a new population. This research was conducted to compare genetic response to alternative strategies of recurrent selection for seed yield in soybean [Glycine max (L.) Merr.]. Four strategies initiated in the AP6 soybean population represented a factorial arrangement of one or two stages of replicated yield testing before selection of superior F4‐derived or S2‐derived lines and one or three generations of intermating between cycles. A fifth strategy, representing a conventional breeding program, was initiated in a population formed by sampling the first of three generations of intermating used to synthesize AP6. The conventional breeding strategy utilized a two‐stage yield test of F4‐derived lines obtained from a limited number of single crosses each cycle. The response to selection was determined by evaluating composites of the parents of each cycle at three locations during 2 yr. The number of cycles completed for the strategies varied from two to five. The average genetic gain per cycle was 9.6 ± 1.1 g m−2 for the conventional breeding strategy, 6.0 ± 1.1 g m−2 for the twostage test and one intermating, 5.5 ± 1.8 g m−2 for the two‐stage test and three intermatings, 1.8 ± 0.6 g m−2 for the one‐stage test and one intermating, and 0.3 ± 1.1 g m−2 for the one‐stage test and three intermatings. The ranking of the five strategies for genetic gain per year was the same as for genetic gain per cycle. The results indicated that multiple generations of intermating did not enhance genetic gain and that the two‐stage test, as conducted in this study, was superior to the one‐stage test for yield improvement.

Four populations were studied to determine effectiveness of selection for low grain protein content in spring barley, Hordeum vulgare L. From 46 to 60 unreplicated F5 lines were grown for each of the four populations at Crookston, MN in 1980. Selection blocks based on check cultivars were used to aid selection of 20% of the lines for low grain protein and 10% of the lines for high grain protein. The selected lines were grown in replicated trials at St. Paul and Crookston, MN in 1981. Realized heritabilities for grain protein content for the four populations ranged from 18 to 100% with a mean of 54%. Grain protein content was independent of grain yield, whereas protein content showed a variable relationship in the four populations with percent plump kernels. These findings encourage selection for grain protein content in early generation barley populations using one row plots.