May- June precipitation provides early indicator of pasture yield and animal performance
By – Keith Harmoney, rangeland scientist, and John Jaeger, beef scientist
Available soil water from precipitation is the main limiting factor to total forage production in most regions of Kansas. Other factors, such as prior grazing history (stocking rate), time of year in which grazing took place, and duration of grazing can also affect forage production in future growing seasons.
During drought periods, it would be beneficial to know the amount of pasture production that could be expected from decreasing amounts of precipitation so that producers can make informed stocking decisions. Shortgrass rangelands at the KSU Ag Research Center – Hays , have been used for grazing research since the 1940’s. For studies with similar stocking rates, rangeland production was compared to annual precipitation and specific monthly combinations of precipitation for 36 years of data to find the best relationships between the times of year precipitation is received and end of the growing season forage production.
The time period of precipitation with the greatest relationship to end of growing season forage production was precipitation from October of the previous year (OctPY) through September of the current year. Late fall precipitation of the prior year and winter precipitation promote early cool-season grass growth, namely western wheatgrass and annual bromes. The OctPY through September time period also includes precipitation that would
fall during the main growing period of the dominant forage in the shortgrass rangeland system, namely warm-season grasses.
The two month period that had the greatest relationship with end of season forage production was May and June precipitation. This two month period represents the most rapid growth period of warm-season grasses in western Kansas, and therefore precipitation during this time period can reasonably be a predictor of end of season forage production.
Conversely winter precipitation, OctPY to April of the current year precipitation (late fall, winter, and early spring precipitation) had almost no relationship to end of the year forage production and was a poor predictor of yield. The lack of precipitation during the winter does not indicate that a lack of forage production will occur, since precipitation in May and June can still produce favorable forage growth. Precipitation the prior year and prior two years to the current growing season also had no relationship whatsoever with current year forage production.
For drought planning, it appears that stocking at a recommended moderate stocking rate for the rangelands being utilized, and then adjusting that stocking rate based on condition and vigor of the vegetation entering the winter dormant season, should be the baseline for spring stocking rates since winter precipitation had almost no relationship
to end of growing season forage production. Further refinements of the stocking rate could be based on May precipitation and May and June precipitation combined since over half of the variation in end of growing season forage production can be explained by precipitation that occurs during May and June. However, sequential years of drought may place rangelands into a state of lower vigor and lower plant density, and therefore beginning season stocking rates may need to be reduced with further adjustments occurring during the spring growing season.
Producers are also concerned about animal gain during drought seasons. Animal gains during these 36 years showed that as precipitation increased, total animal gain decreased. Periods of drought place plants into moisture stress, so plants do not have the available water to develop and mature as quickly. Therefore, plants in a drought remain in a less mature stage of development for a greater length of time through the growing season and would also have greater forage quality for a longer period of time. The most recent thirteen years of stocker studies show, when animals are managed with consistent vaccination, growth implant, stocking rate, and supplement strategies, that total animal gain during the whole grazing season had an evident negative relationship with precipitation from OctPY to September of the current year . As precipitation increases, individual animal gains decrease over the same time period. The best individual gains tended to occur during drought years.
As long as animals have adequate current year forage available to meet daily dry matter intake needs during drought, the forage consumed should be of greater quality. Animals that do not perform well during drought periods may be limited by heat stress, poor water quality, old residual forage from the prior year, or lack of total available forage.
Generally, grazing animals are likely not limited by current year forage quality of native rangelands during dry years. Early growing season precipitation can be an early indicator of how well animals will perform on an individual basis, but early precipitation in May and June give an even greater indicator of how much forage growth can be expected at the end of the season to enable producers to adjust stocking and to make animal management decisions.