By Jessica A. Williamson
Grazing system management should be geared toward changes in forage production in the late summer and early fall
Cool-season perennial forages dominate pastures and hay fields in the mid-Atlantic and northeastern United States. The summer slump is a time when cool-season grasses decline in growth due to high temperatures. The optimum temperature for a popular cool-season grass like orchardgrass is between 60°F and 80°F. Temperatures above this causes a dramatic reduction in cool-season perennial forage growth, commonly referred to as the “summer slump”.
It is critical to avoid grazing too low during the late summer and early fall when temperatures during the day can exceed the optimal temperature for cool-season perennial forage growth. Grazing shorter than 3-4 inches leads to higher surface soil temperature exposing tillers to temperatures as high as 95°F compared to temperatures in the mid-70s under a well-developed canopy. The high temperatures may cause new tillers to die. Further, reserves for regrowth in many cool-season grasses is stored in the stem just above the soil surface, and by grazing too low, these reserves will not be available, causing slower regrowth. If the stubble height is lower than ideal, less leaf area will be available for photosynthesis, causing a reduction in that plant’s ability to regrow for subsequent grazing events.
There is an integrated feedback loop between pasture health and soil health. If a pasture is healthy, the soil will be healthy, but if a pasture is in poor condition, the soil will also be in poor condition. Therefore, often interrill and rill erosion occurs, as well as ephemeral and classic gully erosion in overgrazed pastures.
Overgrazing leads to a smaller, greatly weakened root system, causing greater susceptibility to drought during the late summer, when precipitation can lessen. During the fall, when temperatures cool to ideal growing conditions for cool-season perennials, these forages are experiencing the development of new shoots - which allows for the accumulated forage to graze - as well as root regeneration. During the period of root regeneration, carbohydrates are being stored as an essential part of the root rebuilding process, which provides the necessary stores for proper over-wintering. These carbohydrates are stored within the crown and roots of the plant, which is generally in the lower 3-4 inches of the plant in cool-season perennial forages, so it is critical that pastures are never grazed below a 3-4 inch stubble height at any point in the season, but especially during the fall. It is often recommended to leave a higher stubble height - often 4-5" - in the fall to give pastures a chance to store those carbohydrates that will give them a "jump start" the following spring. If forages are grazed below their growing point, nutrient stores will be depleted and the "protection" from stress will be dramatically reduced. Overgrazing during the fall inhibits regeneration of the root system and the development of new shoots for the next season's growth. Implementing a rotational or strip grazing system can help to manage grazing height by reducing paddock size and increasing the ability to monitor plant residue height.
Early fall is a great time to apply nutrients such as lime, potassium, and phosphorus, as this aids in root regeneration and regrowth. Soil tests should be completed, and if pH is below the recommended level for the targeted forage species within that pasture, liming at the recommended rate to improve soil neutrality will help with forage growth and competitiveness with weeds. If moisture is available, pastures will respond to a fall nitrogen application and lower rates of fall-applied nitrogen will not negatively affect legume population within pastures. However, pasture plants' response to nitrogen is directly correlated with the amount of moisture available, fertilizer application date, and rate of application. It is generally recommended that for cool-season mixed species pastures, no more than 40 lb of nitrogen per acre should be applied in the fall of the year. High rates of nitrogen application could lead to winter kill. If a fall application of fertilizer is desired, no later than a mid-September application date is generally recommended.