Developing Grazing Prescriptions for Spotted Knapweed
By Rodney Kott¹, Lisa Surber¹, Brent Roeder¹, Tracy Brewer¹, and John Walker²
¹Montana State University, Bozeman, Bozeman, MT 59717
²Texas Agrilife research & extension center, SAN ANGELO, TX 76901
Background
Spotted knapweed is an aggressive invader, replacing native perennial grasses on many foothill range and pasture lands in the Northern Rocky Mountain region. Although knapweed can be controlled with chemicals, widespread landscape level use of herbicides is generally unfeasible. In recent years, many land managers now recommend an integrated approach to weed management that includes grazing when and where appropriate.
The resurgence of the use of prescribed or targeted grazing with small ruminants in integrated weed management programs has resulted in the need to develop more precise and predictable grazing prescriptions. This has led to the need to more specifically characterize differences in diet preferences throughout the grazing period and between individual sheep under typical grazing conditions. In order to maximize the potential for improving livestock as a rangeland improvement tool, a rapid method of determining botanical composition of the diet is necessary. This diet data can then be utilized to fine tune grazing prescriptions.
Purpose Statement
The objective of this study was to characterize the intake of spotted knapweed of a herded band of ewes grazing an infested landscape during a summer grazing period. Specifically we wanted to:
- determine the amount of intake variability within a band of herded ewes; and
- detect weekly variation in intake.
Summary of Findings
NIRS: Fecal near-infrared reflectance spectroscopy (NIRS) was utilized to predict chemical and botanical composition of small ruminants consuming spotted knapweed diets. Precision and accuracy of internal and independent fecal NIRS calibrations for predicting percent spotted knapweed in diets of sheep were R² = .99 and .93 and SEP = 2.06 and 2.49 respectively.
Weekly Variation Estimates: Spotted knapweed intakes by week of ewes are reported in Table 1. Five ewes were randomly selected out of band of 900 ewes and ear tagged. Fecal samples were collected weekly for the five from June 22 through August 17. Samples were analyzed using NIRS to detect weekly variation by ewe. Results indicated that in general ewes consumed a higher percent spotted knapweed in their diet compared to the amount available in the landscape especially later in the grazing season. With the exception of the July 20 percentage knapweed in the diets were higher (P<.01) in July (July 6, 13 and 27 sampling) than in June (June 22 and 29 sampling). Similarly, with the exception of the last sampling date in July compared to the first sampling day in August (July 27 compared to August 3) intakes in August (August 3,10 and 17) were higher (P<.01) than in July (July 6, 13, 20).
| Table 1. Weekly variation in intake of 5 ewes. | ||||
| Week | N | %Knapweed Available in Pastureg | %Knapweed | |
|---|---|---|---|---|
| Diet | SE | |||
| (22-June) | 5 | 0.1 | 2.7ab | 2.25 |
| (29-June) | 5 | 0.7 | 0.7a | |
| (6-Jul) | (13-Jul) | 5 | 7.4 | 9.5c |
| (13-Jul) | 5 | 0.1 | 5.3abcd | |
| (20-Jul) | 5 | 0.1 | 5.0abc | |
| (27-Jul) | 5 | 3.0 | 16.5a | |
| (3-Aug) | 5 | 0.7 | 17.2ef | |
| (10-Aug) | 5 | 5.5 | 25.8f | |
| (17-Aug) | 5 | 7.9 | 22.0f | |
| abcdefColumn values with different superscripts differ P<.01. gRepresents an average of the composition of the pastures sheep were grazing the previous three days. | ||||
This general trend is more evident in Figures 1 and 2. It also appears that percent intake is related to amount available in June and early July. Although the percent available knapweed by pasture represents an accurate characterization of the composition of the pasture, its use in the interpretation of the data should be guarded as by design the sheep were herded to areas of the pasture containing the higher knapweed infestations.
Herded Sheep Variation Estimates: Approximately 150 fecal samples were collected and analyzed for diet knapweed concentration using NIRS. Half of the samples were collected in mid-July and half in mid-August. Estimated intake of spotted knapweed of a random 10% of the ewes in the flock in mid-July and mid-August are presented in Table 2.
| Table 2. Average intake of herded ewes within month | |||
| N | %Knapweed | SE | |
|---|---|---|---|
| July 13 | 89 | 5.04a | .573 |
| August 15 | 88 | 20.70b | .576 |
| abColumn means with different superscripts differ P<.01 | |||
Ewes were generally consuming a higher proportion of their diets in spotted knapweed in mid- August than in mid-July (P<.01). The estimated percentage spotted knapweed in the diet (5.04 and 20.70% for mid-July and mid-August, respectively) were similar to those observed in the weekly sampling trials (trial I) during the same time period (5.34 verses 25.84 and 21.98 on July 13 verses August 10 and August 17, respectively).
Figure 3 graphically reports the percent of ewes consuming varying concentrations of spotted knapweed by sampling period. This figure clearly shows that a higher proportion of ewes were consuming higher proportions of their diets (> 15%) in August. This supports the suggestion that in controlled grazing prescriptions for spotted knapweed time optimal time of grazing is August.
Conclusions
The increased consumption of knapweed in August verses July suggests that either the knapweed was preferred in august or that the ewes were becoming adapted to it. Since this same flock or band of sheep has grazed on these same pastures the previous summers the increased consumption can probably be attributed to an increased preference as the season progresses.
APPLICATIONS
Out data allows fine tuning of targeted grazing prescriptions for Spotted knapweed. Based on this data, appropriate timing for grazing would be late in the season but prior to viable seed production.
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