Thin Grass Stands: Causes and Remedies

Chris Teutsch, UK Research and Education Center at Princeton
Ray Smith, UK Plant and Soil Sciences Department, Lexington
Jimmy Henning, UK Plant and Soil Sciences Department, Lexington

Our forage extension team has had a number of calls on hay stands and pastures that appear to be thinner than normal this spring. There are a number of reasons that stands are thin. In this short article we will discuss potential causes and management strategies for thin stands.

Causes of Thin Stands

Low carbohydrate reserves going into fall. What we did last summer and fall can sometimes come back to haunt us the following spring. Close, frequent, and late fall cutting or grazing can result in low energy reserves in the plant. This can cause stands to regrow slower and become thinner overtime. When you combine this with a long and cold winter, grass stands can struggle to get started in the spring.

Poor soil fertility. Hayfield and pastures need adequate soil fertility to remain strong and vigorous. Soil fertility programs need to be based on a current soil test and ALL required nutrients including lime need to be applied in a timely manner. In hayfields, potassium is often deficient. This is due to the removal of relatively large quantities of potassium compared to phosphorus (Table 1). Potassium is involved water regulation in the plant, translocation of sugars produced during photosynthesis, disease tolerance, and winter survival. So poor potassium fertility combined with a hard winter can weaken forage stands.

Table 2. Approximate nutrient removal in pounds per acre for several commonly grown hay types at specified yield levels.

Cool and late spring. In some years, we just can't seem to warmup in the spring. Cold springs can limit early vegetative growth. Since reproductive growth in cool-season grasses is a function of both day length and temperature, the result is that grass plants will tend to produce a seedhead about the same time each year. In cold springs, this results in a higher seedhead to leaf ratio and ultimately lower yields that have more stem and less leaf.

Managing for Thin Stands in the Spring

Soil test and apply needed nutrients. Applications of lime and fertilizer should be based on a recent soil test. Maintaining adequate soil fertility at all times allows for the development of strong and vigorous sods. It is important to remember that fertility programs need to be balanced according to soil test results and end use. So if you are making hay, you will need to add back more phosphorus and potassium because it is being removed in the forage tissue. In contrast, nutrient removal from pastures that are being grazed is minimal.

Clip or harvest stands at the early heading. It is very tempting to delay harvest and allow stands to "thicken up" before the first harvest. The presence of the seedhead can actually delay the development of vegetative tillers at the base of the plant by acting as sync for sugars made during photosynthesis and shading vegetative tillers. This can actually slow vegetative regrowth in pastures. In addition, the presence of the seedhead and stem also decreases forage quality. By clipping or harvesting the seedhead and stem, regrowth from the base of the plant can be stimulated and forage quality can be increased.

Apply nitrogen after clipping or harvesting seedheads. Combined with a timely first harvest, application of 40-60 lb N/A immediately after harvest can stimulate regrowth of pastures resulting a leafy second cutting. It also can help to thicken stands and exclude summer weed pressure.

Rest hayfields and pastures going into summer. After the second hay cutting or as we get into June in our pastures, rest cool-season grass stands and allow them to go into summer with about 6 inches of regrowth. This will allow pasture plants to accumulate stored carbohydrates that will be used to adapt to the hot and often dry conditions of summer and at the same time buffer the temperatures that plant crowns are exposed to through shading. The best way to rest cool-season pastures during the summer months is to incorporate warm-season grasses into your grazing system. This will provide grazing during summer the months when cool-season pasture growth is limited by high temperatures.

For a more information on hay and grazing management, contact your local extension office or visit the UK Forage Extension webpage.