As the middle of the grazing season approaches now is a good time to look at weather conditions over the past couple of months and the forecasts for the rest of the grazing season to gain some insight into the forage supply for the rest of the year. At this point most native cool-season range grasses have reached their peak standing forage and won’t put on more growth unless conditions are favorable this fall. Most warm-season grasses are in the middle of their rapid growth stage currently. Current moisture conditions as well as continued precipitation are important for warm-season growth. Over the past year most of the state has seen above average precipitation (Figure 1), including the vast majority of the Sandhills. Root-zone soil moisture has seen significant improvements across most of the state over the last year, a trend that has accelerated since early May. The exception is the western panhandle (and the broader Northern High Plains region) which has seen drought conditions develop. Portions of the southwest were drier than normal in the month of June as well (Figure 2), though precipitation has returned to portions of the southwest since the beginning of July.

Figure 1. Percent of normal precipitation since over the last 365 days. Map generated from the Southern Integrated Water Portal using AHPS precipitation data.

Figure 2. ACIS Climate Map from the High Plains Climate Center showing percent of normal precipitation for June in Nebraska as of July 1, 2024.

Soil moisture percentiles from NASA’s SPoRT LIS (Fig. 3a) indicate adequate to surplus moisture conditions for most of the state. Exceptions are parts of the Panhandle and southwestern Nebraska, where there are pockets of soil moisture below the 30^th percentile. This would indicate soils are deficient on moisture. Even areas of western in the near-normal range (gray) are likely a bit short on soil moisture. VegDRI (Fig. 3b), which combines satellite reflectance and meteorological data, is likely a better reflection of stressed pastures in western Nebraska.

Figure 3a. Soil moisture percentiles for the top three feet of the soil profile from the NASA SPoRT LIS model. The percentiles are a reflection of whether a location has less, more, or around average levels of soil moisture for the time of year and are not a literal measure of soil moisture.  

Figure 3b. Latest VegDRI map for rangelands

July and summer/early fall temperature and precipitation outlooks are trending warm and dry for most of the state. The July precipitation outlook for the eastern part of the state has an equal chance of being normal, dry or wet while the western part of the state has increased chances of being dry (Figure 4). The seasonal outlook shows increased chances of being dry and warm across the state (Figure 5).

Figure 4. Map from NOAA Climate Prediction Center showing the probability of normal temperature and precipitation for the United States for the month of July 2024. Issued June 30, 2024.

Figure 5. Map from NOAA Climate Prediction Center showing the probability of temperature and precipitation being normal for the United States May through September 2024. Issued June 30, 2024. 

The seasonal outlook from the European Centre for Medium-Range Weather Forecasts (ECMWF) model shows warmer and drier than average conditions for the August-October period. A closer inspection of monthly data from the model shows near-average precipitation across the entire state in August but a dry September and October. Temperatures are expected to be above average in all three months, with September expected to be the most above average. If this forecast verifies, drought in the Panhandle is not likely to improve in the coming months and further drought development in western Nebraska would be possible.

Figure 6. European Centre for Medium-Range Weather Forecasts precipitation model for the continental U.S. for August-October. Brown colors depict lower than normal precipitation, units are in inches of precipitation.

Figure 7. European Centre for Medium-Range Weather Forecasts temperature model for the continental U.S. for August-October. Warm colors depict warmer than average temperatures, units are in degrees Celsius.

Figure 8. U.S. Drought Monitor for Nebraska issued on July 2, 2024. Showing continued dry conditions in the southeast part of the state and developing drought in the panhandle.

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. At this point in the season the forecast predicts that below normal precipitation conditions through the rest of the season will not have a negative effect on forage production for most of the state.

Figure 9. 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 June 25, 2024.

Figure 10. 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 June 25, 2024.

Figure 11. 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 June 25, 2024.

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 diver 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 12. 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.

• 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)