Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2002
Publication Date: N/A
Citation: Interpretive Summary: Farmers and researchers have long recognized that grain sorghum yields are not uniform within fields in the Rio Grande Valley of south Texas, but little quantitative information is known about the variability. This study aimed to characterize the spatial variability in yields and profits within ten grain sorghum fields in south Texas. A yield monitor integrated with a aglobal positioning system (GPS) receiver was used to collect yield data, and yield maps and profit maps were generated to show spatial patterns in yields and net returns within the fields. Results indicate that large yield variability along with high production costs and low grain prices result in large profit variability and low economic returns for grain production in south Texas.
Technical Abstract: Various degrees of spatial variability in crop yields have been documented, but seldom has this been associated with spatial variability in profits. This study examined spatial variability in yields and profits within ten grain sorghum (Sorghum vulgare Moench) fields, with areas of 7 to 26 ha, in south Texas in 1998. A yield monitor integrated with a global positioning system (GPS) receiver was used to collect yield data from these fields. The yield data were then aggregated into grids with a cell size of 9 m, equivalent to the combine's effective cutting width. Statistical analysis showed that grain yields had coefficients of variation within fields ranging from 32% to 57%. Variograms showed strong spatial dependence in yields with an influence range of 122 m to 300 m among the fields. Yield maps were generated using the variograms within a localized neighborhood to show spatial patterns. Net profits were calculated based on the differences between the gross returns from the yields and total production costs on a cell by cell basis for each of the ten fields. Profit maps were generated to show the spatial variability in net returns within the fields. Average net profits among the ten fields ranged from -$137/ha to $65/ha at a grain price of $0.09/kg, and from -$63/ha to $281/ha at $0.15/kg. The percentage of land areas with positive returns among the ten fields ranged from 0.4% to 76% at $0.09/kg, and from 16% to 91% at $0.15/kg. These results indicate that large yield variability, high production costs, and low grain prices result in large profit variability and low economic returns for grain production in south Texas.