Freeze Injury in Small Grains

By Daniela Carrijo

With the cold snap we had in mid-late April, there have been concerns with freeze injury in small grains. This may be counterintuitive but the mild winter we had this year may have contributed to this. With winter temperatures being above normal for a prolonged period, more growing degree days were accumulated, and crop growth was accelerated. With that, plants transitioned from the vegetative to the reproductive stage earlier in the spring, when temperatures (despite warmer than usual) were still too cold for the developing head to be exposed to. Overall, the closer the plant is to heading, the more susceptible it is to freeze injury. For example, in a wheat study, a 5% yield reduction was observed as a result of freeze injury occurring at the Feekes 6 stage (first node), compared to a 50% yield reduction when plants at the Feekes 10.5.1 stage (beginning of flowering) were exposed to similar cold stress levels (Lindsey et al., 2020).  

Among our small grains, barley was likely the most affected by this recent cold snap given that it is the least cold tolerant and is further along in developmental stages, compared to wheat and rye. In Centre County, we observed freeze injury symptoms in our barley variety trial after temperatures dropped below 30 F a few weeks ago. The cold snap coincided with the beginning of flag leaf expansion and caused the twisting of flag leaves (Figure 1). Before heading, there is little one can do to estimate the extent of freeze damage, except to extract the developing head from inside the boot and look for discoloration (browning) or deformation. If no severe symptoms appear at this stage, a more assertive examination can be done after heading.

Most barley in our state has started to head, so now is a good time to scout fields for freeze injury and estimate the impact on yield. Damaged heads may appear crooked as a result of being stuck inside a twisted flag leaf. That said, crooked heads can still bear healthy spikelets. Look closely at each spikelet and monitor fertilization and grain fill. Note that fertilization begins in the middle portion of the head and continues upward towards the top of the head and then downwards towards the base of the head. Unfertilized spikelets will appear translucid or bleached while healthy spikelets will appear green and will contain a clear liquid soon after anthesis (pollination).

Thinking ahead for the next season, if freeze injury is a concern, it is important to select a variety with good cold tolerance. Preliminary data on barley winter survival from this year’s Penn State Official Variety Trial is presented in Table 1. Winter survival was rated in late winter after a cold snap. Varieties were scored both visually (higher numbers indicate less cold injury symptoms) and using a canopy reflectance index (NDVI) given by a light sensor (GreenSeeker, Trimble). NDVI is not a direct measurement of cold injury but it is a measurement of plant greenness, with higher values typically indicating higher biomass, nitrogen content and overall plant health. Table 1 shows that that NDVI and score values are generally corelated, with Lightning and Hirondella being some of the most cold tolerant varieties in our Centre County trial. It is also worth checking the Cornell Small Grains Variety Trials which has winter survival data from the 2021-22 season. Stay tuned for the full variety trial reports at the end of this season.

Table 1. Winter survival ratings taken on 03/08/2023 from Penn State’s official barley variety trial at Rock Springs, Centre County.

¹Normalized Difference Vegetation Index: higher values typically indicate higher biomass, nitrogen content and overall plant health.

²Score ratings range from 1 (no plants survived) to 9 (no cold injury symptoms).

³Least Significant Difference: the smallest difference required for any two varieties to be considered statistically different at the 10% probably level.