Agronomic factors affecting dryland grain sorghum maturity and production in Northeast Colorado

Grain sorghum [Sorghum bicolor (L.) Moench] is an important crop in southeast Colorado, but little sorghum is produced in the northeast part of the state. Grain producers in northeast Colorado are interested in growing grain sorghum due to its drought tolerance and lower cost of inputs compared to corn. The main reason sorghum is not widely produced in northeast Colorado is because the shorter growing season reduces the chance that sorghum will reach physiological maturity before the first fall frost, which can significantly decrease grain yield and test weight. The main objective of this thesis was to determine if dryland grain sorghum could be produced in northeast Colorado by incorporating certain agronomic practices and selecting hybrids with the appropriate maturity. We hypothesized the agronomic treatments would have a significant impact on the cumulative GDD to maturity. A second objective of the study was to calculate the probability of the hybrids reaching maturity at different hypothetical planting dates at Akron and Fort Collins. We hypothesized the probability of the hybrids reaching maturity at Akron would be high, and the probabilities would be lower at Fort Collins based on the shorter growing season. The last objective of the thesis was to determine the effects of two different water treatments on grain yield of four hybrids, where we hypothesized the limited irrigation treatment would yield lower than the full irrigation treatment.;Three agronomic factors, row spacing (0.76 and 1.5 m), seeding rate (3, 11, and 20 seeds m-2), and row orientation (north/south and east/west) were studied along with hybrids (88P68-early maturity, DKS29-28-early maturity, and 5745-medium-early maturity) differing in maturity class. The maturity study was conducted at two locations in 2010 (Akron and Stratton) and two locations (Akron and Fort Collins) in 2011. In the maturity study, growing degree-days (GDD) were used to measure the amount of thermal time required from planting for the plants to reach physiological maturity in each treatment. Grain yield, test weight, plant population, and flowering measurements were taken in the four trials in addition to the maturity measurements. Probabilities of hybrids reaching physiological maturity at Akron and Fort Collins were calculated. Additional trials were conducted at Greeley in 2011 to determine the effects of two irrigation treatments (full and limited) on hybrid (CGSH-9-early maturity, 88P68, DKS29-28, and 5745) grain yield.;The maturity study results from treatments in the north/south row orientation (hybrid, row spacing, and seeding rate) showed hybrid selection, seeding rates, and the production environment effects were all significant. The two earlier maturing hybrids (88P68 and DKS29-28) required significantly fewer GDD to reach maturity than the medium-early hybrid (5745). As seeding rates increased from 3 to 20 seeds m-2, significantly fewer GDD were required for the hybrids to reach maturity. The cumulative GDD required for maturity was significantly lower in 2011 than in 2010. The hybrid and environment results from treatments in the east/west row orientation (row spacing and hybrid) showed the two early maturity hybrids required significantly fewer GDD to reach maturity than the later maturity hybrid. The row orientation treatment did not have an effect on maturity. The expected probability of the hybrids reaching maturity at Akron was high for all three hypothetical planting dates (max of 97% for May 15, 91% for May 23, and 81.5% for June 1). At Fort Collins, the probabilities were much lower (max of 73.5%), especially for the medium-early maturity hybrid, 5745 (max of 57.5%). The earlier the sorghum GDD accumulation start date, the greater the probability the sorghum would reach maturity at both locations, showing the importance of planting date and hybrid selection.;Yield results for the maturity study showed hybrid, seeding rate, and environment main effects were all significant. The environment by row spacing two-way interaction was significant due to different optimal row spacings in different environments. The 88P68 hybrid yielded significantly lower than the other two hybrids. The seeding rate of 3 seeds m-2 yielded significantly lower than the two higher seeding rates. Fort Collins (2011 only) yielded significantly higher than the other three trial environments. At Greeley in 2011, the two irrigation regimes had a significant effect on the yield of all hybrids. The full-irrigation yield was significantly higher than the limited-irrigation yield under high drought and heat conditions in 2011. Grain test weight from experiment 1 (north/south row orientation with hybrid, seeding rate, and row spacing treatments) in 2011 at Akron and Fort Collins showed that interactions among all trial factors were significant. Environment, row spacing, and hybrid were each significant effects. Test weight was significantly lower at the 0.76 m row spacing than in the 1.5 m row spacing. The Akron location had significantly higher test weight than Fort Collins.;Overall results from the grain sorghum studies showed grain sorghum could be reliably produced in northeast Colorado. The maturity study provided valuable information that could apply to specific situations (such as re-plants or late-plantings) in other areas where early maturity in grain sorghum may be important. Hybrid selection and planting date are two of the most important factors that can affect the cumulative GDD required for grain sorghum to reach physiological maturity. Different hybrids can take significantly different amounts of cumulative GDD to reach maturity depending on the genetics. Hybrid selection should be based on the relative maturity and yield of the cultivar, along with other characteristics depending on the growing conditions. The planting date of grain sorghum can have a substantial impact on the probability of grain sorghum reaching maturity as the probability of reaching maturity can be increased by planting as early as possible in the growing season. Less important factors affecting the cumulative GDD to maturity are seeding rate and environment. Results from the study showed that as seeding rates increased, cumulative GDD to maturity decreased. The row spacing and orientation factors did not affect the cumulative GDD to maturity.;Grain yield and test weight were both significantly affected by growing environments and hybrid factors. Water availability in the environment is the most important factor as it can significantly affect both grain yield and test weight. Yield can also be significantly increased by using the appropriate row spacing depending on water availability. A notable trend from the yield and test weight results showed there was a trade-off between yield and test weight.;The results from the thesis project show the importance of agronomic and environmental factors in grain sorghum production in Colorado. Our hypothesis that the agronomic factors would have a significant impact on the cumulative GDD to maturity was partially correct since seeding rate and hybrid main effects were significant. We found that earlier maturity cultivars can be produced at Akron with reasonable confidence (minimum probability was 81% for the early maturity hybrids) they will reach physiological maturity before the first freeze. Future research on factors affecting grain sorghum maturity could include additional hybrid and seeding rate treatments as well as a planting date factor. The creation of a map of northeast Colorado with hypothetical probabilities of grain sorghum hybrids reaching physiological maturity could also be done to give producers a useful production tool. The row orientation factor does not warrant further research for its effect on grain sorghum maturity and additional growing environments would not be necessary for future research since environments could be modeled to determine adaptability.