Corn (Zea mays L.) yields in Minnesota have increased from the 2,010 kg/ha yield level of the pre‐1930's to the current 6,290 kg/ha average. This increased yield can he attributed to a series of technological, cultural, and management practices adopted by farmers. My objective is to atlempt an analysis of the magnitude of the changes and the relative contributions to grain yield each practice has made over the 50‐year time period. Only simple linear responses were estimated. No attempt has been made to evaluate the nature nor the magnitude of the interactions among the various management practices. The change from open‐pollinated to hybrid corn combined with the estimated 36.5 kg/ha/year genetic gain has produced 58% of the yield increase. Although each kilogram of applied commercial N has been estimated to increase yields by 18.9 kg of grain per kg for 47% of the gain in yield, the effects of less manure, and reduced N from mineralized organic matter actually produces a net N effect of 19%. Improved weed control by the use of herbicides on 93% of the hectarage has increased yields 23%. Plant densities have increased by 19,130 plantdha contributing 47.4 kg/ha/1,000 plants for a total of 21% of the gain. Better soil drainage, fall plowing, and herbicides have permitted planting 10 days earlier for an average gain of 36.4 kg/ha/day and a total of 8% of the net increase. Drilling corn rather than hill dropping has contributed 8% and fall plowing has contributed 5% to current yields. Row spacing has decreased from 107 to 90 cm for a gain of 10.2 kg/ha/cm or 4% of the gain. Rotation changes have had no net effect on N balance with increases in soybean [Glycine max (L.) Merr.] offsetting decreases in forage legume hectarage. However, there has been a negative effect of more hectares of corn following corn resulting in 3% loss from corn rootworm (Diabrotica spp.) and 7% loss due to the interference effect of corn following corn. Corn borer (Ostrinia nubilalis Hubner) became a problem in the 1940's and has reduced yield potential an average of 5%. Soil erosion has reduced yield potentials 8% over the 50 years. Other negative and unaccounted factors reduce the potential yields by 23%.

An important question in current soybean (Glycine max (L.) Merrill) production is whether inoculation is beneficial when seed is planted in soils where effectively nodulated plants have been produced previously. These studies were conducted to measure the effective of inoculating soybeans with commercial cultures of R. japonicum on seed yield and protein percentage when soybeans were grown on soils which had grown nodulated soybeans previously. Paired samples of different lots of soybean seed taken before and after commercial inoculation with R. japonicum were planted at two Minnesota locations in 1967. More extensive studies were conductedi in 1968 at five locations in cooperation with various inoculant companies. Seed from a common lot was sent to the participating companies to be inoculated. Two treated samples and an uninoculated check were returned from each company. Two samples were treated immediately prior to planting with humus samples from each inoculant company. Three humus cultures of single strains of R. japonicum, selected from the more effective strains in an inoculation study on rhizobia free soils in Minnesota, also were used as inoculants. Soybean seed yields and seed protein percentage were not significantly increased by inoculating soybean seeds with Rhizobium japonicum at planting time. The uninoculated checks were adequately nodulated by rhizobia which were in the soil from previous soybean crops. Serotyping of single strain inoculants indicated a substantial range of recovery (0–17%) depending on the strain and location.

This study was conducted to ascertain the effects of temperature extremes on kernel characteristics, respiration, starch synthesis, soluble sugar, and protein content during grain filling in maize (Zea mays L.). Kernels cultured in vitro on a defined media at 15, 30, and 35 C were compared to kernels from ears developed in the field. Kernels cultured for 7 days at 35 C had higher dry weights than kernels from other treatments; however, their growth ceased by the 14th day. High soluble sugar content of aborted kernels suggests that inhibition of starch synthesis may have caused kernel abortion at 35 C. Respiration was higher for kernels from field‐grown ears. During the linear phase of dry matter accumulation, kernel growth rates were 0.65, 6.26, and 5.26 mg kernel‐1 day ‐1 for 15 C, 30 C, and field control treatments, respectively. The higher rate of kernel growth at 30 C was negated by a pronounced shortening of the duration of the grain filling period. Relative to the 30 C treatment, a three‐fold increase in duration of grain filling at 15 C was not adequate to compensate for the 90% reduction in kernel growth rate that occurred. Cool temperatures that occurred in the field or that were maintained in vitro caused soluble sugar levels to remain high and deposition of starch in the endosperm to lag several days behind that observed at 30 C.Cool temperatures did not affect the synthesis of proteins in general or cause differential synthesis of an individual or specific class of protein in the zein fraction. These data show that kernel abortion and shortening of the grain filling period at higher than optimum temperatures and severe reduction in kernel growth rates at lower than optimum temperatures are possible mechanisms by which temperature may mediate kernel growth and development and ultimately grain yield of maize.

No abstract

Freshly harvested seeds of wild rice (Zizania aquatica L.) require 3 to 5 months of moist storage at 1 to 3 to induce germination. Dormancy lasting more than 1 year has been noted. These periods of dormancy pose problems to plant breeders desiring multiple generations per year and to growers desiring to change varieties in establish fields. The purpose of this research was to determine the role of the pericarp in seed dormancy, the existence of germination inhibitors, and the influence of gibberellin and kinetin on germination of dormant wild rice seeds. The pericarp of seeds harvested the previous day was scraped off, punctured, or cut at several locations on the seed. Germination occurred only when the treatments were made directly over or very near the embryo, indicating mechanical resistance by the pericarp. Soil collected from fields 1 and 2 years out of production was screened for seed. The pericarps of nongerminating seeds were randomly punctured, increasing germination 33 and 79% respectively. This suggests an impermeable pericarp. Freshly harvested seeds from which the pericarp was either scraped or not scraped were germinated in aqueous extracts of the pericarp, and hulls (lemma and palea) from freshly harvested seeds. Aqueous extracts of the pericarp reduced germination 77% while aqueous extracts of the hulls reduced germination 84%, compared to scraped seed germinated in water. Aqueous extracts of the hulls from seeds stored for 1 year had little influence on germination when used as germination media for freshly harvested scraped seeds. The hulls were removed from some freshly harvested seeds and not from others before storing for 1, 2, and 4 weeks in water at 1.5 C. Seedling survival after 30 days was significantly reduced when lemmas and paleas were left on the seeds 1 or 2 weeks during storage in water at 1.5 C. These experiments support the contention of growth regulators in the hulls of freshly harvested seed. All combinations of 0, 0.01, 0.1, 1, and 5μM solutions of gibberellic acid (GA3) and kinetin were applied to germinated dehulled, punctured seeds. Before seeds were dehulled and punctured, they were stored in plastic containers at 1.5 C for 90 days. Germination increased from 36 to 51% as GA3 concentrations increased. Kinetin alone had little influence on germination except in combination with GA3. Less etiolated seedlings were obtained when kinetin was included in GA3 treatments. The addition of 5μM GA3 + 1μM kinetin increased germination of freshly harvested, scraped seeds from 29 to 76%. Wild rice appears to have multiple mechanisms of seed dormancy. The seed pericarp exhibits mechanical resistance and impermeability. Water soluble germination inhibitors appear to be present in hulls and pericarp, and gibberellic acid concentrations are low in freshly harvested seed. Freshly harvested seed can be germinated by dehulling and scraping, permitting multiple generations per year in breeding programs. Persistence of dormant seeds in fields will present problems in introducing new var...