Management of Pasture Sage

Pasture Sage (Artemisia frigida) is a native species of the Prairies and Parklands. It can be distinguished by its silvery-grey, finely divided, hairy leaves and its distinct sage aroma. It grows low to the ground from a woody base. Stalks may be upright, 30-40 cm (12-16 inches) tall and may bear small yellowish flowers towards the top. It is an increaser species for range condition rating and so population size increases as range condition decreases. It is drought tolerant, increases under heavy grazing and is used as an indicator of overgrazing.

Tillage is Not Applicable

Pasture sage problems cannot be managed satisfactorily by cultivation and reseeding because of the large persistent seed bank, the large number of 'safe sites' created for seedling establishment, the woody nature of the crown and root of pasture sage plants and the susceptibility of pasture soils to erosion.

Pasture sage produces large amounts of tiny seed, over nine million seeds per Kg (four million seeds per pound) (Cooperrider and Bailey, 1984). These seeds are dropped in late summer and fall and, due to their small size, are often buried in the soil. Because pasture sage seeds require light to germinate they remain viable in the soil continually increasing the pasture sage seed bank. These seeds germinate if exposed to light by tillage or other disturbance when there is sufficient moisture. Once grass competition is removed through tillage, pasture sage will proliferate.

Pasture sage plants have weedy crowns with fibrous and sometimes woody roots. Plants uprooted during tillage can survive and establish new roots.

The soils of pasture with a pasture sage problem are often poor and light or the topography is too hilly for cultivation. If tilled, these areas become very susceptible to wind and water erosion.

Integrated Weed Management

Pasture sage is not simple to control. It should be managed as part of a whole pasture management system. A systematic approach is useful when combined with an intuitive knowledge of the pasture. The systematic steps are:

1. Investigate past management practices to determine the cause of the problem (Pasture sage is an indicator of overgrazing).
2. Determine the extent and severity of the problem.
3. Consider the cost/benefit of management options.
4. Implement chosen practices.
5. Maintain healthy grass and productive pastures through grazing management.
6. Monitor pasture sage for reinvasion.

The Problem

To say that 'high density pasture sage is not the problem' is a curious statement until it is qualified. A large pasture sage population is the symptom of the problem rather than the problem itself. The basic problem leading to such symptoms is improper management of the pasture, particularly overgrazing.

Pasture sage survives and increases in conditions in which populations of other species decline. The roots of pasture sage vary, depending on its environment, in order to be most competitive for the available moisture. Pasture sage is therefore drought tolerant. Pasture sage is unpalatable to most livestock and nearly all other forages are preferentially grazed before pasture sage. The low growing nature of pasture sage makes it less likely to be accidentally grazed. In these ways pasture sage is more competitive in drought or overgrazing situations.

Table 1 shows the effect of exclusion from grazing on the competitiveness of crested wheatgrass and therefore on a pasture sage population. The drastic difference in number of pasture sage plants between ungrazed and grazed areas after five years shows the effect of protecting the crested wheatgrass from grazing.

Continuous overgrazing is the most common cause of excessive pasture sage. One must evaluate the grazing practices in place when the problem arose. The grass must be left healthy and able to out-compete this weedy species.

Table 1. Pasture sage plants per square metre in ungrazed and grazed areas of a crested wheatgrass pasture.[1]

Table 2. Pasture sage plants per square metre before, 3 and 5 years after herbicide application.[1]

The check treatment in Table 2 shows the natural mortality of pasture sage. This was due to the increased competitiveness of the grass as it was protected from grazing. By year five the population in the untreated check was reduced to two plants per square metre. For this reason, the best evaluation of forage yield increase due to pasture sage control using a herbicide is the three year accumulated yield.

Extent and Severity

The number of pasture sage plants and seedlings should be estimated throughout all pastures. The density (plants per square metre) of pasture sage plants helps to estimate its effect on forage yield. One should remember that pasture sage is a native species and is a common component of rangelands and naturalized pastures. It is unnecessary and impractical to expect to eradicate pasture sage. Evaluating the extent of the problem in a pasture can help reduce costs. If a herbicide is used, the knowledge of the extent of the problem will determine whether full pasture application is necessary or whether spot application would be most cost effective.

Estimate of Costs and Benefits

The costs can be investing in the purchase and application of herbicide or decreasing the stocking rate of a pasture in order to allow the desirable species to compete. Other costs may include extra stock handling time, learning to identify pasture species or monitoring grazing and the health of the grass in your pastures. The estimated direct cost of the chemical treatments and the resulting yield increases over three years are shown in Table 3.

The benefits of decreasing the pasture sage population size include increased vigour and increased yield of desirable forage species. Livestock may show increased weight gain as they have to move less to get sufficient forage. Because pasture sage is reported to be high in nitrogen compared to other forage species (Anisimova and Ojun, 1974) there may be more nitrogen available in the soil for growth of desirable species when pasture sage is controlled.

Table 3 indicates that the most cost effective herbicide for control of pasture sage is a single application of 2,4-D LV ester 700 at 1.0 L/acre. The cost of the herbicide was $13.90/acre (2015 prices). The benefit after three years was a 281 kg/acre (618 lb/acre) increase in the yield of crested wheatgrass. 2,4-D should be applied early in the spring when pasture sage is actively growing.

The density at which controlling pasture sage would lead to an increase in yield depends on the particular conditions of the pasture. In the crested wheatgrass pasture in which these experiments were carried out, herbicide treatment on pasture sage densities of 40 and 55 plants per square metre resulted in forage yield increases. Pasture sage population of nine to 11 plants per square metre did not affect yield.

Table 3. Crested wheatgrass yield increases and 2015 herbicide costs (averaged over two experiments).

ReClaim herbicide is also registered for control of pasture sage and would provide control of established perennial plants and suppress seedling emergence in the following season. Check with your local Range and Pasture herbicide supplier for prices.

Grazing Management

Grazing is the most important tool in the management of pasture sage. Grazing for healthy forage plants enables the desirable forage species to out compete pasture sage, particularly during drought. A pasture is best grazed when the grasses are in a vegetative state (prior to heading of grass) and grazing periods should be timed to accommodate this. The appropriate time to graze a pasture then depends on the species present. Some grasses flower early in summer and therefore should be grazed early whereas others flower later and can be utilized later.

Grazing periods should be short enough to leave sufficient leaves for photosynthesis by desirable forage species for rapid regrowth. Once grazed, a pasture should be allowed sufficient time for regrowth and rebuilding of carbohydrate stores in the grazed plants (New Grazing and Technologies Project, 1991). Fall grazing reduces carbohydrate stores and the plant's ability to trap snow, thereby reducing the plant's potential for spring growth.

Season long grazing does not allow forage plants to recover and rebuild stores of carbohydrates before it is grazed again (Alberta Agriculture). Too much energy is expended in replacing grazed material and the roots are not replaced, thereby decreasing the plant's ability to take in moisture and nutrients from the soil. This eventually leads to decreased forage yield.

Proper grazing management is especially important following the control of pasture sage using herbicides. Grazing must be adjusted to allow desirable forage species to expand their dominance and take over the spaces that pasture sage plants once occupied. Heavy grazing decreases the ability of the desirable forage species to compete with pasture sage seedlings.

Monitor Pasture Sage

Pasture sage populations must be monitored for population increase. Seeds of pasture sage will germinate in cool moist conditions and therefore seedlings can start growth earlier in the spring, than many other species. Seedlings are very tiny but have distinct silver-grey hairy leaves (see page 1). The cotyledons are oblong with round tips but the first leaves are much more distinct, being trilobate and hairy. If seedlings are found in large numbers, the pasture should be closely monitored to determine if these seedlings survive and develop. If the seedlings are surviving then one should change management practices in order to increase competition from desirable forage species.