As May is upon us, many will be turning cattle out to pasture shortly or have done so already.  Cool-season grasses across the state have started to grow. Current moisture conditions will be a strong driver of cool-season grass growth. The month of April began drier for much of the state outside the panhandle but has been wet—very wet in some cases—for most of the state in the last two weeks. As such, a majority of locations in Nebraska are above average on precipitation for the water year, with much of the panhandle, Sandhills, and northeast sections coming in well above average. Only the southeast corner and pockets of southwestern Nebraska are below average. The recent precipitation has allowed for meaningful improvement in soil moisture and a majority of the state in the near-normal or above average range for 0-1 meter soil moisture. Note that Figure 2 is based on a robust model run by NASA and shows conditions relative to a climatology for a given point in time.

 

Figure 1. Percent of normal precipitation since October 1 for Nebraska as of April 29, 2024, based on observed and radar estimated precipitation.

 

Soil moisture in the top three feet of the soil indicates water available for plants to utilize at the start of the growing season. White areas are considered near normal, while warm colors represent drier and cool colors wetter conditions (Figure 2).

Figure 2. Soil moisture percentiles based on a land surface model within NASA’s Land Information System. Percentiles based on a 1981-2020 climatology.

 

The May precipitation CPC outlook favors above average precipitation for all but the panhandle. The ECMWF precipitation outlook for May shows above average precipitation in eastern Nebraska and near-average precipitation elsewhere. Looking out a little further, the CPC’s precipitation outlook is currently showing no strong signal for the period of May-July. The ECMWF shows wetter than average conditions in eastern Nebraska, near-average in north central Nebraska and below average elsewhere in the period from May to July. The CPC outlook for temperatures shows no strong signal in the month of May (Figure 3) and increased chances of warmer than average temperatures in the seasonal period (Figure 4).  The ECMWF outlook favors slightly above temperatures for May and for the May-July period, with warmest temperatures relative to normal in eastern Nebraska. The warmer temperatures are expected to be driven more by warm low temperatures than early summer heat waves, though those cannot be ruled out.

Figure 3. Map from NOAA Climate Prediction Center showing the probability of normal temperature and precipitation for the United States for the month of May 2024. Issued April 18, 2024.

 

Figure 4. Map from NOAA Climate Prediction Center showing the probability of temperature and precipitation being normal for the United States May through July 2024. Issued April 18, 2024.

Figure 5. U.S. Drought Monitor for Nebraska issued on April 23, 2024. Showing continued drought conditions in the southeast part of the state.

GrassCast

GrassCast is a forage production forecasting tool that can be useful when planning for the grazing season. When used in combination with season long forecasts, such as the ones provided by the Climate Prediction Center, this tool can help producers make stocking rate decisions informed by using predicted weather and forage production. It is important to note that GrassCast estimates total forage production changes which may not be the same as grazeable production in some areas. For this tool to be of the most value, a good understanding of average forage production over the last several years (the tool is using 36 years of data) as well as season long weather forecasts and a knowledge of local rangeland conditions and forage productivity is helpful.

Figure 6. Forecasted percent change in annual net primary productivity compared to the 36-year average assuming above normal precipitation through the growing season. Important Note: Grass-Cast estimates the change in Total Production NOT Grazeable Production. Forecast generated April 16, 2024.

 

 

Figure 7. Forecasted percent change in annual net primary productivity compared to the 36-year average assuming normal precipitation through the growing season. Important Note: Grass-Cast estimates the change in Total Production NOT Grazeable Production. Forecast generated April 16, 2024.

 

 

Figure 8. Forecasted percent change in annual net primary productivity compared to the 36-year average assuming below normal precipitation through the growing season. Important Note: Grass-Cast estimates the change in Total Production NOT Grazeable Production. Forecast generated April 16, 2024.

 

Setting trigger dates for range and pasture health

Pasture and native rangeland forage production fluctuates as the growing season progresses and is influenced by precipitation, temperature, range health, previous years precipitation and management, as well as soil nutrients. The amount and timing of spring and early summer precipitation, as well as temperatures, is an important factor in determining total forage production. Using trigger dates can help producers adjust stocking rates if a lack of precipitation is expected to result in below average forage production. Trigger dates will vary depending on the dominant species of grass present in pastures and rangelands.

Available soil moisture is a major driver of plant growth. Cool- and warm-season grass species have rapid-growth windows that occur when optimum air temperature, day length, and soil moisture are all present for plants to fully express their growth potential. Once that window of opportunity has passed for a particular grass species, it is too late to get significant growth for that season, even if additional rain comes. For example, cool-season grasses produce most of their growth in late spring with some limited growth again in the fall.  Warm-season grass growth occurs primarily during the middle part of the growing season.

Precipitation during May, June and July are strongly correlated with total forage production of warm-season species and season-long total forage production in the Nebraska Sandhills. In the Nebraska Panhandle, many range sites are dominated by cool-season grass plants, and forage production is influenced by April, May, and June precipitation. This same timeframe also applies to pastures dominated by smooth bromegrass; an introduced cool season grass common in eastern Nebraska. Smooth bromegrass is a sod-forming grass and can be very drought-tolerant. Many smooth bromegrass pastures will have some growth again in the late summer and early fall when day length shortens and cooler nights return (Figure 5).

Areas affected by drought during the previous growing season will likely experience lower forage production even if adequate moisture is available the subsequent year. It is important to give range and pasture plants the chance to recover from drought and rebuild root carbohydrate energy reserves. Maintaining a lower stocking rate post-drought can help accomplish this.

 

Figure 9. Annual growth curve of warm- and cool-season grasses. From NebGuide G1502, Perennial Forages for Irrigated Pastures

 

Suggested trigger dates

Trigger dates for reducing stocking rates in an operation will depend on the grass species present and available grazing resources. Here are some key trigger dates to consider for Nebraska cool- and warm- season dominated range sites.

• April 15 to May 10. By this time, 30–45-day precipitation forecasts have a moderate level of reliability. If above-average temperatures with average to below-average precipitation is predicted, plan for reductions in stocking rates. In smooth bromegrass pastures with below-average precipitation, total annual production may be reduced by 25-50%.
• May 20 to June 10. Needlegrasses species will be completing their forage production and western wheatgrass is in its rapid growth window. If March-May precipitation was 50-75% of the long-term average, reduce stocking rates by 30-40% or more depending upon grass species and plant health. Warm-season grasses such as prairie sandreed and little bluestem are just starting to grow.
• June 15 to June 30. Approximately 75 to 90% of grass growth on cool-season dominated range sites and 50% of grass growth on warm-season dominated range sites will have occurred by this time.  Rainfall after late June usually has limited benefit for cool-season grass production but could still benefit warm-season grasses.

• June 15 to July 15. Precipitation and available soil moisture is critical for warm-season grass growth during this time frame.

• July 15. Precipitation after this date will have limited benefit to warm-season, tallgrass production but can still result in some forage growth from shortgrass, warm-season species such as buffalograss and blue grama.

• September 1 to September 15. Smooth bromegrass or other cool-season-dominated pastures need adequate available soil moisture by these dates for enough fall forage growth to occur to be grazed.

Resources

• The National Weather Service Climate Prediction Center provides short- and long-term precipitation and temperature forecasts.
• The NebGuide, Skillful Grazing Management on Semiarid Rangelands, describes the growth pattern of grass species and provides information on grazing management practices.
• The UNL Beef Team’s Drought Management Planning page provides a wide range of information related to drought including Managing Drought Risk on the Ranch from the National Drought Mitigation Center. This resource provides additional information on trigger dates and a planning guide.
• GrassCast is a forage forecasting software that uses remotely sensed data, past weather, and other information to give an estimate of expected forage production for the growing season. https://grasscast.unl.edu/
• Weekly Weather Update | Nebraska State Climate Office | University of Nebraska–Lincoln (unl.edu)