Using In-Situ Measured Soil Temperature to Predict First Hollow Stem, a Critical Growth Stage for Removal of Cattle in Dual-Purpose Winter Wheat

Wheat producers in Oklahoma have historically circled March 15 on their calendars as a date to pull cattle from their fields. However, rather than using a fixed calendar date, past research at Oklahoma State University has shown that the optimal time to pull cattle off wheat is when ungrazed wheat reaches a particular growth stage called First Hollow Stem (FHS). Leaving cattle on wheat past this growth stage can result in grain yield losses of anywhere from 1 to 5% per day, so grazing one week past FHS could reduce grain yield by as much as 35%. Economic analysis has shown that in most years the added cattle weight gains associated with grazing past FHS are insufficient to offset the lost grain yield.

This presentation will describe the development of three soil temperature based models (one for each of three FHS categories depending on wheat variety) to assess the probability of FHS currently across the state of Oklahoma and throughout the next two weeks, thus giving producers a tool to know when to begin scouting for FHS and a heads up as to when to begin moving cattle off wheat fields. These models have been incorporated into an operational product on the Oklahoma Mesonet called the First Hollow Stem Advisor, a product which is updated daily from late December through mid April and which features maps as well as site-specific charts and tables. Examples from this operational product will be shown after discussion of model development.

Using 10 years (1995-2004) of FHS occurrence dates for a number of wheat varieties, correlations of various weather and soil variables from the Oklahoma Mesonet with FHS dates showed early on the importance of 4” soil temperature under vegetative cover (soil heat units) to this particular growth stage (the growing point of the wheat plant is still underground at FHS). Wheat varieties were grouped into three FHS categories (early, middle, late), depending on the relative occurrence dates of FHS. Three models (one for each category) were developed based on this 4” soil temperature variable after investigating various start dates for the soil heat unit accumulations and various temperature thresholds for the daily heat units. Using the mean number of heat unit accumulations to FHS as the FHS predictor for each model, predicted FHS dates were compared to observed FHS dates as were the average observed FHS date (static date) for each wheat category and the March 15 historical removal date. Across all three wheat categories, the soil temperature based predictive models outperformed the two static dates, with the soil based models having the lowest mean absolute errors (days from observed FHS) and lowest standard deviation of errors (days from observed FHS) compared to the two calendar date methods (with the exception of late varieties, March 15 is almost always too late in estimating FHS). In 2013, in order to fine tune the models, additional years of FHS occurrence dates through 2012 (and soil heat unit accumulations to FHS) were incorporated into the databases and a variety of cumulative probability distributions were fitted to the FHS soil heat unit accumulations for each wheat category. The log-logistic distribution was chosen for each model, enabling the mathematical calculation of FHS probability for any heat unit total.

These three probability distributions along with the three soil temperature based models were then incorporated into an operational product (First Hollow Stem Advisor) on the Oklahoma Mesonet in January 2014. Maps showing soil heat unit accumulations and FHS probability levels for each of the three FHS categories of wheat (early, middle, late) are updated daily for the current date as well as for 1 and 2 weeks into the future. For purposes of calculating future daily soil heat units, climatological 15-year daily average 4” soil temperatures under vegetative cover at each Mesonet site are used. Charts and tables at specific Mesonet sites are available as well, showing the progression of soil heat unit accumulations from model start date through the current date as well as through the next two weeks. Producers of dual-purpose wheat are encouraged to begin scouting for FHS once the 5% FHS probability level is reached and to remove cattle by the 50% probability level.

The First Hollow Stem Advisor constitutes the first applied Oklahoma Mesonet product that relies on in-situ measured soil temperature, showing the benefit of including soil measurements (temperature and moisture) in automated weather station monitoring networks. In another paper given at this conference, in-situ measured soil moisture was seen to be critical in the assessment of wildfire danger during the growing season in Oklahoma, showing again another benefit of taking soil measurements.