By Jameson Brennan
Spring precipitation (April – June)
When dry conditions continue to persist on an annual basis, it can be easy to get excited when spring precipitation comes. Spring precipitation is often the focus of forage production models, with the months of April, May and June having a key role in predicting annual forage production in the central and northern Great Plains (Smart et al. 2020). This is true for several plant communities: mixed-grass (e.g. western wheatgrass, blue grama, and buffalograss – a mix of cool and warm-season grasses), shortgrass (e.g. blue grama and buffalograss dominated – warm-season grasses) and midgrass (e.g. western wheatgrass dominated – cool-season grass) (Smart et al. 2007).
Note that mixed-grass and midgrass communities have a large proportion of western wheatgrass, a native cool-season grass, which typically initiates growth in the spring in mid-April and reaches peak production by the end of June.
Warm-season grasses that dominate shortgrass plant communities begin growth in mid-to-late May and have active growth that carries into July. Thus, spring precipitation might be stored in the upper 10 centimeters of the soil profile, which is where most shortgrass roots occur (Coffin and Lauenroth 1991). Therefore, if there is adequate spring precipitation available, the warm-season grasses, like blue grama and buffalograss, will be able to use that moisture, as long as it is not depleted by cool-season grasses.
Days until last spring freeze
Some research (Smart et al. 2007) suggests that days until last spring freeze (days since Jan. 1 to the last freeze date) can impact cool-season dominated plant communities. Temperatures below freezing in the spring can impact plant growth and development by rupturing plant cell walls and damaging meristemic tissue (i.e. tissue where growth originates from). The “chill tolerance” that grasses typically possess decreases in the spring, when grasses focus their energy on developing stem structure. Thus, if a freeze does occur while grasses are in a rapid growth phase, they will be more susceptible to the cold temperatures. Consequently, forage production for the entire growing season could be impacted in cool-season grass pastures.
The reason days until last spring freeze impacts cool-season grasses more than warm-season grasses is because warm-season grasses don’t begin their rapid growth phase until June – when we typically don’t have freezing temperatures.
Growing degree days
As demonstrated on the grazing readiness map, growing degree-days can play an important role in plant development. Growing degree days are the temperature or heat units that a plant needs to accumulate to begin leaf development. When spring temperatures are cooler than the long-term average, forage growth will likely be delayed developmentally throughout much of the state. For example, instead of western wheatgrass reaching grazing readiness on a long-term average spring turnout date of June 3, it might not reach grazing readiness until June 18.
Conclusion
Year-to-year variability in climate can result in large swings in annual forage production, making it difficult for livestock producers to match animal demand to forage production. Key to mitigating the effects of this variability is utilizing available climate tools, developing a drought management plan, and identifying key trigger dates for management actions.
Source : sdstate.edu