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Seasonal Changes in Forage Quality

By B. Patton, and P. Nyren, NDSU Central Grasslands Research Extension Center
J. Caton, NDSU Animal and Range Science Department


Forage quality of a plant species changes 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 collected samples of seven abundant plant species on the grazing intensity trial at Central Grasslands Research Extension Center from the beginning of the 1995 growing season to the beginning of the 2000 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 were analyzed for crude protein and in vitro dry matter digestibility. Weather data, which includes daily temperature and precipitation, was also collected.

Table 1 shows the plant species that dominate by weight in 2000 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 (needlegrass) and Stipa viridula (green needlegrass) were more than 5% of the composition by weight and now they are about 1 and 2% respectively. Carex heliophila (sun sedge) has decreased from 6% in 1993 to less than 2 % in 2000. Solidago rigida (stiff goldenrod) and Spartina pectinata (prairie cordgrass) have increased from less than 3 and 1% in 1993 to 7 and 5% respectively in 2000. Although only 2 or 3 species dominate on each site, 35% to 37% 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.

 

Table 1. Dominant plant species on the overflow and silty range sites in 2000 (by dry weight).
Overflow Percent Silty %
Poa pratensis- Kentucky bluegrass 25 Poa pratensis - Kentucky bluegrass 49
Bromus inermis - smooth brome 16 Agropyron smithii - western wheatgrass 12
Symphoricarpos occidentalis - buckbrush 10 Bromus inermis -smooth brome 4
Solidago rigida - stiff goldenrod 7 Aster ericoides - heath aster 3
Spartina pectinata - prairie cordgrass 5 Stipa viridula - green needlegrass 2
All others (163 species) less than 4% each 37 All others (153 species) less than 3% each 30

Figure 1 shows the crude protein content and figure 2 shows the in vitro dry matter digestibility from late April of 1995 through early May of 2000. Although most species have their best quality in the spring, none show the same pattern and species which had the most crude protein or highest digestibility changed through the season. For a detailed graph of any of these species indicating the times that they met the nutritional needs of a lactating or dry cow click on one of the links below.

Agropyron smithii - western wheatgrass would have met the nutritional requirements of a lactating cow in any year from late May to late June and it would have met the nutritional needs of a dry cow from late May to early August in any year.

Bromus inermis - smooth brome always had a period in the spring when it had high nutritional quality but when it reached its peak quality varied each year. The time that its nutritional quality first became great enough to meet the requirements of a lactating cow varied from as early as late March in 1999 to as late as late May in 1996. In 1999 it would have only continued to meet her needs until mid May. In 1996 it would have continued to meet her needs until late June. In every year smooth brome had nutritional quality sufficient to meet the needs of a dry cow from late May to early June.

Carex heliophila - sun sedge would have had sufficient quality to meet the needs of a lactating cow every year from late May to early June. It would meet the needs of a dry cow every year from late May to late June and again from mid July to late July. It seems to maintain its quality well while dormant and in the winter of 1998 to 1999 it maintained high enough quality to over winter a dry cow if it had been available in sufficient quantity.

Poa pratensis - Kentucky bluegrass always had sufficient quality in the spring to meet the requirements of a lactating cow beginning as early as late April in 1998 and as late as late May in 1996 but dropping off below lactating requirements by mid May in 1998 to mid June in 1996. However it could be depended on to meet the nutritional requirements of a dry cow from late May to late June in every year.

Stipa curtiseta - needlegrass only had sufficient quality to meet the nutritional needs of a lactating cow in early June of 1999 but not in any other year. It would have met the nutritional needs of a dry cow every year from early June to mid June.

Stipa viridula - green needlegrass never reached sufficient quality to met the nutritional needs of a lactating cow in 1998 but in every other year it could be counted on to meet her needs at least in early June. It had sufficient quality to meet the nutritional needs of a dry cow in every year from early June to early July.

Symphoricarpos occidentalis - buckbrush always had high forage quality at some point in the spring of the year meeting the nutritional requirements of a lactating cow as early as late April in 1998 to as late as early June in 1996. How long it maintained this level of forage quality also varied each year ending as early as late May in 1998 to as late as late July in 1999. It had sufficient forage quality each year to maintain a dry cow from mid May to early June and in early July and late July.

One of the advantages of native rangeland is its diversity of plant species that allows livestock to select the plants that 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. Click on this link for a detailed graph overlaying forage quality of each of the seven species we studied. As you can see from the graph there was always some species that would meet a lactating cows nutritional requirements each year from late May to early July and to meet a dry cows requirements from late April to mid September. They also would have at least come close to meeting the requirements of a dry cow through the winters of 1997-1998 and 1998-1999.

We will be performing correlation analysis to try and determine what climatic factors are responsible for yearly differences in forage quality.


Bob D. Patton
Central Grasslands Research Center 
4824 48th Ave SE 
Streeter, ND 58483
E-mail: bob.patton@ndsu.edu

NDSU Central Grasslands Research Extension Center

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