North Dakota State University
Central Grasslands Research Extension Center

Seasonal Changes in Forage Quality
By B. Patton, J. Caton and P. Nyren

Forage quality of plant species change throughout the year and can be different at the same time of year in different years. These changes are due partly to the phenological, or growth stage of the plant and partly to the climate in which the plant has been growing. Available water and soil temperatures are the major factors that affect plant growth and they may also affect forage quality.

To better understand these changes and their effect on livestock gains we began collecting samples of the seven most abundant plant species on the grazing intensity trial beginning with the 1995 growing season. Samples were collected weekly throughout the grazing season and once every two weeks throughout the dormant season, provided the ground was not covered with snow. The samples are analyzed for crude protein and in vitro dry matter digestibility. Weather data which includes daily temperature and precipitation is also collected.

Table 1 shows the plant species which dominate by weight in 1998 on the overflow and silty range sites on the area of the Center where these samples were taken. Dominance changes somewhat each year. In 1993, Stipa curtiseta was more than 5% of the composition by weight and is now less than 1%. Agropyron repens and Solidago rigida have increased from less than 4% in 1993 to 6% and 5%, respectively, in 1998. Although five species dominate on each site, 35% to 41% of the vegetation consists of plants with individual abundance of less than 4% of the plant community. However, some of these may be abundant enough in spots on the range to make up a significant part of the animals' diet at certain times of the year.

Figure 1 shows the crude protein content and figure 2 shows the in vitro dry matter digestibility from late April of 1995 through August of 1998. Although most species have their best quality in the spring, none show the same pattern and the species which had the most crude protein or highest digestibility changed through the season. One of the advantages of native rangeland is its diversity of plant species which allows livestock to select the plants which are of the highest quality at any point in time. The dominant plants, Kentucky bluegrass and smooth brome, are both cool-season grasses that green up early in the spring and can also have a significant amount of regrowth in the fall. During these periods of active growth their forage quality can be quite high. Buckbrush forage quality can be extremely high when it first begins rapid growth in the spring. We will be performing correlation analysis to try and determine what climatic factors are responsible for yearly differences in forage quality.

Table 1.
Dominant plant species by weight on the overflow and silty range sites in 1998.
Overflow % Silty %
Poa pratensis-
Kentucky bluegrass

Poa pratensis-
Kentucky bluegrass


Bromus inermis-smooth brome

Agropyron smithii-
western wheatgrass

Symphoricarpos occidentalis-buckbrush

Carex heliophila- sun sedge


Agropyron repens-quackgrass


Stipa viridula-green needlegrass

Solidago rigida-stiff goldenrod 5 Bouteloua gracilis-blue grama 3
All others (160 species) less than 4% each
All others (153 species) less than 3% each

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Bob Patton
Assistant Range Scientist
North Dakota State University
Central Grasslands Research Center
4824 48th Ave. SE
Streeter, ND 58483
Phone: 701-424-3606


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