19 July 2023

5 minute read

By: Penn State University

North America

The summer slump is when perennial cool-season grasses slow in growth or essentially go dormant. Lots of factors contribute to the summer slump of perennial cool-season grasses. High soil and air temperature in summer is the main factor. The optimum temperature for perennial cool-season grasses is between 60°F to 80°F; anything above this, the growth of perennial cool-season grasses dramatically declines. The second factor is the regrowth of perennial cool-season grasses after spring peak growth, mainly vegetative growth made up of leaves, according to experts at Penn State University. 

The vegetative canopy in summer is shorter than the reproductive canopy in spring; therefore, not as efficient for light interception and photosynthesis. The third factor is the limited soil moisture content. Low soil moisture content limits soil nutrient movement, and plant stomata may close, limiting the absorption of carbon dioxide and loss of plant turgor.

The summer slump of perennial cool-season grasses is significant because perennial cool-season grasses are the predominant species in Pennsylvania pastures. Seasonal distribution of forage production is critical in grazing systems. Animals are alive and need a consistent supply of forage. It is essential to avoid grazing too low to meet animals' needs during the summer months and sacrifice pasture long-term productivity and longevity. Maintaining at least three to four inches of standing pasture is the general rule of thumb for grazing practices. Still, it is especially important during the summer slump for various reasons.

First, what you see above ground is reflected belowground. This is where the graze half–leave half rule comes into play – research has shown that if grasses are grazed very short repeatedly, the root system dies back significantly, and the stand declines. This is critical during summer as grasses try to scavenge for limited water sources. This could cause thin stands and allow for weed encroachment. Most problematic pasture weeds have taproots, which makes them excellent water scavengers because that taproot grows deeper than the fibrous root system of grasses. Research shows, that grazing too short, as could occur in a continuous grazing system, during the summer slump, results in increased weed populations during that time. Grazing too aggressively could also negatively impact the grass's ability to regrow as temperatures cool in the fall.

Second, just like tall, thick forage stands can help to protect crowns and tillers from freezing, they also shade them, providing protection from high temperatures, especially soil temperature for root growth and functioning. Root growth is more sensitive to soil temperature fluctuations and extremes than shoot growth to air and canopy temperature. If grazed too low, base tillers in the crown area could be exposed to temperatures as high as 95°F compared to temperatures in the mid-70s °F under a well-developed canopy. The reproductive tillers in the current growing season will be dead after they produce seeds or short after grazing and cutting for hay. It is the newly developed tillers from the plant crown area in the current growing season that make the plant perennial. Therefore, those high temperatures in the crown area due to limited shading from the short stubble could cause tillers to die and result in thin stands.

Third, many grasses store carbohydrates as energy sources and proteins as nitrogen sources in the lower portion of their stem, i.e., perennial cool-season grasses need a saving account as they prepare for harsh times. They need that 3-4 inches of stem left behind so that they can use the stored energy and nitrogen to grow new leaves. Once enough leaf area acts as solar panels, grasses can photosynthesize more efficiently and begin replenishing energy and nitrogen before the next grazing cycle. That energy is also used to maintain a hardy root system that is able to scavenge for water and nutrients. If the grasses are grazed too short, they use up all their stored nutrients to grow new leaves and don't have enough energy left over to maintain a dense root system so the roots will die off. Eventually, they will regrow new roots once enough leaves are present to photosynthesize and begin storing excess nutrients again.

Implementing a rotational grazing system is one of the best ways to maintain sufficient grass height. Separate a large pasture into smaller sections with permanent or temporary fencing and rotate animals out when grasses reach that 3 to 4-inch height. They can then be rotated back into an already grazed paddock when forage height reaches at least 6-8 inches. As temperatures continue to climb and rainfall becomes less prevalent, there may come a time when grazing animals should be taken off cool season pasture to help maintain its quality and resiliency. To supply summer forage, it is ideal to have a portion of the farm in warm-season annuals and/or perennials. These forages are adapted to high temperatures and dry soils. Examples are millets, sudangrass and sorghum-sudan hybrids, teff, sunn hemp, cowpea, forage soybean, switchgrass, big bluestem, and indiangrass. 

If you only have cool season pastures, bringing animals into a dry lot, feeding reserved forages for a few weeks, and allowing the grasses to regrow will help increase stand longevity and reduce the need to reseed a stand that has become thin from overgrazing. If a dry lot isn't an option, consider using a section of pasture that is in poor condition and needs some Tender Love and Care. The extra nutrients provided from animal urine and dung and imported hay material can supply nutrients and the pasture can be reseeded in the fall, if necessary, once the animals are removed. Be sure to use an area with adequate fencing, whether permanent or temporary, as animals will be prone to push the fence and try to get to "greener" pasture.

Check out Extending the Grazing Season with Plant Diversity, for alternative forage options during the summer slump or Harvesting and Feeding Warm-Season Annuals for Forage, to find more details on utilizing warm-season annuals.

Headline image courtesy of L. Duppstadt, Penn State Extension