Reagan Noland – Assistant Professor & Extension Agronomist, Texas A&M AgriLife Extension Service, San Angelo, TX
Substantial wheat acreage in West Central Texas was planted later than normal, largely due to very dry fall conditions. Much of the region received less than 1 inch of rain through September and October combined, compared to the average >5 inches (Figure 1). Depending on the timing of planting relative to isolated rain events in November and December, some wheat did not emerge until late December or possibly early January. Average temperatures in December and January were a few degrees warmer than normal (Figure 1), and concerns have been indicated regarding vernalization of the late-planted wheat crop.
Figure 1. Recent environmental conditions compared to 30-year averages (San Angelo, 2019-2020)
Vernalization is a process that winter wheat undergoes through adequate exposure to cold temperatures, and is required for the timely and uniform onset of reproductive growth (grain production). Variety-specific information is lacking, but the vernalization requirement for hard red winter wheat varieties grown in Texas generally ranges from 3 to 6 weeks (~500 to 1000 hours) of temperatures between 32 and 45°F. This accumulation starts as soon as the seed imbibes with water to germinate, so vernalization can technically begin before emergence. Varieties adapted to South and Central Texas will have a shorter vernalization requirement, and varieties adapted to Rolling Plains and High plains will require longer vernalization.
Without adequate vernalization, head emergence can be sparse and inconsistent, or severely delayed. Poor vernalization can also result in reduced head size. In minor cases, ample heads may emerge, but could express inconsistent rates of maturity, posing potential challenges for harvest management and maintaining grain quality. In extreme cases, the entire crop may remain vegetative and never produce grain. Unfortunately, vernalization issues cannot be confirmed until wheat begins jointing (after the optimum timing for top-dress N application). Hollow stems in non-vernalized plants will not contain developing wheat heads. A more obvious tell-tale sign is erratic timing and distribution of elongating stems throughout the field. Reports often involve seeing “a head here and there” among an otherwise vegetative crop.
We can forecast the risk of a vernalization problem based on cumulative chilling hours from the time of germination. Vernalization issues occurred across much of Central and South Texas in the 2016-2017 production season. Viewing wheat variety trial results from a known “bad year” alongside cumulative chilling hours can help inform relative thresholds and identify differences between varieties. Table 1 shows a range of common wheat varieties and likely vernalization issues relative to chilling hours in 2016-2017.
Table 1. Wheat yields relative to location and cumulative chilling hours in the 2016-2017 season. (Red cells indicate likely vernalization issues, and marginal or partial vernalization issues are highlighted in yellow.)
Among the varieties compared, TAM 401 has a notably low vernalization requirement, and TAM 114 and SY Grit appear to require greater vernalization (~500 hours). Note that differences in yield should not be attributed entirely to vernalization, as these values represent a range of genetics in different environments. Rather, extreme differences should be identified between varieties that otherwise perform consistently similar within a location. For example, compare TAM 114 to Gallagher at McGregor vs. Brady and Abilene. The severe yield penalty to TAM 114 at McGregor is likely due to insufficient (~400) chilling hours. It is also important to note that low vernalization requirements may influence earlier maturity, which introduces higher risk of frost injury after jointing, so varieties should not be selected based on vernalization requirements alone. Consult variety testing results from your region when making variety selections. With some general thresholds in mind, Figure 2 shows cumulative chilling hours to-date (February 7th) relative to a range of germination dates. This is illustrated for site-years with known vernalization issues in 2017, and for the current year at San Angelo.
Figure 2. Cumulative wheat chilling hours relative to germination date in three selected site-years.
Cumulative chilling hours are reduced at later germination dates (Figure 2). Wheat that germinated on or before December 15th in the San Angelo area has accumulated at least 360 chilling hours to-date. As more chilling hours will be accumulated through the remainder of February and into March, vernalization will not likely be an issue for the range of germination dates prior to December 15th (Table 2). Wheat that germinated as late as January 1st may be at greater risk of inadequate vernalization, as it may accumulate <400 chilling hours before March 1st and <500 before April 1st. Be aware of fields with potential for poor vernalization and watch them closely when wheat in the area begins jointing and elongating. Timely identification of the issue can allow greater flexibility with management options. If failed vernalization is confirmed, growers with livestock may consider grazing the wheat crop out, or it may be terminated in time to establish a productive warm-season crop. Always consult your crop insurance agents before grazing out or terminating a crop to determine the insurance implications of your management decisions.
Table 2. Projected chilling hours for 2020 wheat crop based on current accumulation and 5-year averages (San Angelo, TX).