As a rule, few introduced forage species have any ecological value. In most cases the “improvement” of rangeland by planting tame species such as crested wheatgrass or Russian wildrye reduces biodiversity and may result in the reduction of biodiversity and deterioration in ecosystem stability. However, biodiversity does not pay the bills.
The decision to use tame species is usually done from an economic point of view with the expectation of increasing forage production, carrying capacity and ultimately profits.
There have been a number of studies suggesting the use of tame (agronomic) species will dramatically increase Above Ground Net Primary Production or ANPP (forage production) of pastures. Unfortunately, much of this research has resulted in conflicting results. Part of this uncertainty is due to poor research design, creating a large amount of bias in the interpretation of the results.
In 1993 Dr. Walter Willms and his colleagues initiated a research project that was designed to eliminate many of these problems and test a crucial hypothesis. The researchers wanted to determine if forage production of native plant communities was the same as that of introduced plant communities and the production stability of native plant communities was greater than that of introduced plant communities.
Two sites were chosen for this project; a Stipa-Bouteloua (Needle and thread-blue grama) plant community on the Dry Mixed grass prairie, at Onefour Alberta, and a Stipa-Agropyron-Bouteloua (needle and thread-wheatgrassblue grama) plant community on the moist mixed grass prairie, near Lethbridge, Alberta.
The study was initiated in 1993 at the Lethbridge site and in 1994 at the Onefour site and then concluded in 2006. The four treatments reviewed in this research project were: two cultivated species (crested wheatgrass var. Parkway and Russian wildrye var. Cabree), native grassland that was harvested and a control (native grassland not harvested). Forage production for each of these treatments was determined by harvesting each plant community at a time when forage production was at its peak. On each site, a fence was constructed to prevent wildlife, livestock and small rodents from grazing the research plots.
At both sites, the forage production of the harvested native plant community and the Russian wildrye plant community declined over the entire study. The crested wheatgrass plant community was the most productive for the first four years of the trial and then declined sharply after the first four years at both study sites. This decline in forage production was more pronounced on the Lethbridge site than on the Onefour site.
Once the initial peak in forage production had faded on the Lethbridge site, forage production of the crested wheatgrass plant community was very similar to that of the control plant community and the harvested native grass plant community. Forage production of Russian wildrye plant community was the lowest of all the treatments throughout the entire study period.
After the initial peak in production of the crested wheatgrass had faded on the Onefour site, forage production of the crested wheatgrass plant community was similar to the control plant community but greater than the forage production of the harvested native plant community. Forage production of the Russian wildrye plant community was less than either the control or the western wheatgrass plant communities but similar to the forage production of the harvested native grass plant community.
The peak in production of western wheatgrass and a lesser extent Russian wild rye was the result of a number of factors. The most important factor was the interaction between the plant species and their environment. Establishing the seedbed resulted in the release of nutrients and conservation of water. The ability of crested wheatgrass to become established earlier and rapidly establish leaves allowed the plant to quickly respond to the increase in availability of nutrients and surplus soil moisture.
On the other hand, Russian wildrye was slower to develop. This slower rate of development could be attributed to the plant investing more of its energy resources into the development of crowns and roots and the potential competition between seedlings. The decline in forage production of crested wheatgrass and Russian wildrye was due each of these plant communities reaching equilibrium with its environment as the availability of soil moisture and nutrients declined.
The forage production of the St ipa-Agropyron-Bouteloua (needle and thread-wheatgrassblue grama) plant community was greater than the forage production of the Stipa-Bouteloua (Needle and thread-blue grama) plant community. The reasons offered by Dr Willms and his colleagues were: 1) the precipitation was greater at Stipa-Agropyron-Bouteloua site than at the at the Stipa-Bouteloua site, 2) the amount of soil nitrogen may have been greater on the Stipa-Agropyron-Bouteloua site than on the Stipa-Bouteloua site and 3) the relative amounts of western wheatgrass and needle and thread. Western wheatgrass was more abundant on the Stipa-Agropyron-Bouteloua site than on the Stipa-Bouteloua site. Western wheatgrass is rhizomatous grass where as needle and thread is a bunchgrass. Being a rhizomatous grass, western wheatgrass may be more responsive to soil moisture than needleandthread.
Although the forage production of crested wheatgrass and Russian wildrye declined after the first four years, the forage production of these plant communities remained as stable as the forage production of the native plant communities. However, this stability in forage production should not be confused with the stability of ecosystem structure and function. Replacing a diverse ecosystem with a monoculture may actually reduce the ecological stability of that range site through increased rates of soil erosion and the loss of biodiversity.
This study suggests the forage production of crested wheatgrass plant communities may be greater than the forage production of a Stipa-Bouteloua plant community associated with the dry mixed grass prairie (such as at Onefour). However, the forage production of a crested wheatgrass community is very similar to the forage production of a Stipa-Agropyron-Bouteloua plant community associated with the moist mixed grass prairie (such as at Lethbridge).
Despite this, Dr. Willms and his colleagues felt this study does not unconditionally support the assumption improved pastures will always out perform native pasture, especially if sustainable management practices are used on native pastures. In addition, Dr. Willms and his colleagues also recommend producers examine all of the economic and ecological costs before breaking native grassland and then seeding it back to introduced grass species.
Agronomic (tame) species such as crested wheatgrass and Russian wildrye are often used as means of increasing forage production on Western Canadian rangelands. However, the increase in forage production is often short lived and may require additional inputs and costly management strategies to maintain the desired level of forage production. On the other hand, if native grass communities are in a managed in a sustainable manner their productivity will remain stable over long periods of time with relatively few inputs.
Hyland Armstrong is a retired rancher from the Cypress Hills Alberta. He can be reached at [email protected]or 403 528 4798.