**2. Grazing impacts to forage utilization and Artemisia absinthium**

## **2.1. Experimental site**

all weedy species in U.S. grazing lands is greater than all other pests combined [1], and has been estimated at 1 billion dollars for forage loss and 5 billion dollars for control costs [2]. Weed infestations cause a variety of problems in grazing lands. Weeds can reduce forage vegetative quality and quantity; displace native plants and animals; reduce animal fertility, weight gains, or be toxic, resulting in fatalities; reduce meat and/or hide quality; increase management costs; and reduce land values [3, 4]. Tactics for weed management in pastures and grazing lands vary with the type of weed, livestock species, and applicability of other

Livestock can help manage weeds by grazing or trampling and can improve pasture condition and competitiveness of desirable plants by increasing soil nutrients through manure and urine deposition [3]. Weed species and stage of growth; livestock species; and stocking rate and duration influence grazing effectiveness on weeds [3, 7]. Unfortunately, cattle (*Bos taurus*), the grazing livestock of choice in the Northern Great Plains (NGP), selectively consume forage in dung-free areas [8, 9], and avoid weeds for a wide variety of reasons [10]. Cattle herds are not managed specifically for weed control for several reasons. First, cattle are expensive to raise and replace and, even with premium prices, the economic margin is narrow [7]. Weeds may not be as palatable as grasses, and lower consumption may reduce weight gains [7], or, if

Rotational grazing often uses a 'take half'- 'leave half' forage philosophy to maintain healthy, vigorous plant communities [12, 13]. Mob grazing has been promoted to mimic the world's historic grassland ecosystems [14] with herds of large animals intensively grazing areas and moving often. The definition of mob grazing is subjective, but typically includes using extremely high stocking rates (100 head or more per ha) for short periods of time (moving every 12 or 24 h) [15] followed by recovery periods of 6–12 months. The goal of mob grazing is to have every plant within the enclosure eaten [16] or trampled [17], limiting selectivity or avoidance of specific species [9], and providing a more homogeneous grazing treatment. Barnes et al. [16] reported that grazing homogeneity correlated with paddock size, with pastures ranging from 1 to 8 ha in size

grazed nearly uniformly, even if the same stocking rate per ha are used on larger areas.

Grazing impact for weed management is maximized when the target weed is most palatable, is the only forage option, or is made more palatable to livestock in some way (e.g., salt or sugar treatment) [7], and the desired vegetation is at its least vulnerable phenotypic stage [1]. High animal densities maximize trampling, which incorporates plant litter, manure, and urine into soil, increasing organic carbon and soil nutrients [17]. The combination of eating, trampling, and long rest periods is expected to increase productivity of more desirable forage [3, 18].

Mob grazing has been adopted by ranchers in Texas, SE Colorado, central Nebraska, Missouri, and other areas [19] where vegetative regrowth can occur quickly due to warm conditions, and high rainfall or irrigation capabilities. Under dryland conditions of the NGP, timing mob grazing to fit within the vegetative and environmental constraints of the area is difficult as growing seasons are short, and pastures often experience summer drought. McCartney and Bittman [20] reported on a mob grazing study that used 7–14 heifers ha−1 (dependent on seasonal timing) on about 0.3 ha paddocks at different intensities (light, grazed twice a year; to intense, grazed five times a year) in northeastern Saskatchewan. They observed positive

high in alkaloids, problems with reproduction and/or toxicity can occur [11].

methods (e.g., mowing, biocontrol, herbicide treatment) [5, 6].

54 Forage Groups

The effects of rotational and mob grazing stocking densities on Artemisia absinthium and surrounding forage utilization were compared in an eastern South Dakota rangeland location in the tall grass prairie habitat near Hayti (44.66°N, −99.22°W) in 2013 and 2014 [23]. The dominant soil series of the rotationally grazed pasture were the: Poinsett-Waubay silty clay loams (Calcic Hapludolls/Pachic Hapludolls); Buse-Poinsett complex (Typic Calciudolls/Calcic Hapludolls); and Poinsett-Buse-Waubay complex (Calcic Hapludolls/Typic Calciudolls/ Pachic Hapludolls) [https://soilseries.sc.egov.usda.gov/osdname.aspx]. Mob grazing pasture soils were similar to the rotational pasture with the addition Barnes-Buse loam complex (Calcic Hapludolls/Typic Calciudolls). The plant communities in these pastures were a mix of cool season native and invasive grasses, warm season grasses, and broadleaf species (**Table 1**).

#### **2.2. Weather**

Growing degree days (GDD) were calculated to provide a reference for plant development between sampling dates and years. The GDD calculation [GDD = ∑ (maximum daily temperature + minimum daily temperature)/2 − base temperature] used the base temperature of 0°C, due to majority of cool season species with GDD accumulations starting on January 1 of each year.


**Table 1.** Plant species in the mob-grazed and rotational grazed sites at Hayti, SD in 2013 and 2014.

Precipitation (from January 1) was also determined. The rotational pre-graze samples in 2013 were taken on June 13, with 641 GDD and 243 mm precipitation (www.noaa.gov) with values similar to the 30-year (1980–2010) average. Post-grazing samples were taken July 22, with GDD of 1540 and precipitation totaled 343 mm. In 2014, samples were taken May 13 with GDD at the spring assessment (which was taken after the early spring grazing) 262 and 65.5 mm of precipitation. The fall assessment was taken September 16 with GDD of 2603, and total rainfall of 370 mm fall. Rotational grazing was done much earlier in 2014 because the rancher was concerned about low amounts of precipitation (nearly 60% below average) during the 2013 fall and winter.

GDD accumulations for mob-grazed areas in 2013 were 1801 (August 6) and 1855 (August 9) for pre- and post-graze samples, respectively. Precipitation totaled 376 mm before and after mob grazing. In 2014, GDDs were 1693 pre-graze (July 29) and 1817 post-graze (August 4) and precipitation for pre-graze and post-graze totaled 230 and 270 mm, respectively.

**Pre-graze**

> Year

Grazing system

Stocking

Grazing

Sampling

Forage biomass1

Sampling

Standing

Trampled2

Forage use

Forage utilization4

efficiency3

date

duration

date

**kg ha−1**

**kg ha−1**

**%**

**%**

density

**kg ha−1**

223,250

12 h

6-Aug

2910a5

9-Aug

570b

530b

62

80

2013 Mob Rotation Rotation/spray

2014 Mob Rotation/

1560

15 days

Summer

ungrazed

1560 53,580

24 h

29-Jul

4640a *~*1700*6*

13-May

870

16-Sep

20907

cm) \* 79 to estimate kilograms per ha.

(estimated)

1Vegetation biomass was estimated using the cool season mixed pasture grazing stick method, (vegetation height cm – 7.6

2Trampled vegetation was any plant with a stem less than a 450 angle from the soil surface.

3Forage efficiency (consumption only) was calculated by: [(before grazing vegetation –[standing + trampled])/(before grazing vegetation)]\*100.

4Forage utilization (consumption + trampling) was calculated by: [(before grazing vegetation – standing vegetation after grazing)/before grazing veg]\*100.

5Values with different letters within the same row for the pre graze vegetative biomass compared with post-graze standing or trampled (mob) or total vegetative biomass

(rotational) differed at P

 < 0.0001. 7Forage in autumn following the grazing treatment in the spring.

**Table 2.**

6Samples were not taken pre-graze in this treatment but estimated from the leave half/take half grazing system.

Mob and rotational grazing stocking density, grazing duration, sampling dates, forage biomass pre- and post-graze, and forage efficiency and utilization by year.

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4-Aug

1170b

1890b

34

75

20 days

13-Jun

4530a

22-Jul

2528b

0

57

57

1560

20 days

13-Jun

2600a

22-Jul

1190b

0

45

45

**Post-graze**

Biomass1

#### **2.3. Grazing treatments**

Stocking treatments (rotation vs. mob) were repeated, although cattle densities and time of grazing differed between the 2 years due to feeding needs and differences in forage growth due to low rainfall in 2014 (**Table 2**). Rotational grazing was conducted in 8 ha pastures with 25 cow-calf pairs (1560 kg ha−1). In 2013, in one paddock, the cow-calf pairs were allowed to graze for 14 days starting June 13 (referred to as 'rotation'). In a separate paddock, generic 2,4-D ester at 1.1 kg ha−1 [24] was applied 1 day before the start of grazing on June 13 with a grazing duration of 14 days (referred to as 'spray/rotation'). In 2014, a different pasture was grazed by 25 cow/calf pairs for 15 days, starting April 27 and ending May 11 (referred to as 'early spring grazed/summer rest').

Mob grazing was conducted for 12 h in a 0.65-ha paddock on August 8, using 125 cow-calf pairs (stocking rate of 223,250 kg ha−1 day−1) (**Figure 1**). In 2014, a different 1.3-ha area was mob grazed on July 30 for 24 h with 125 cow-calf pairs (stocking rate of 53,580 kg ha−1 day−1).


Precipitation (from January 1) was also determined. The rotational pre-graze samples in 2013 were taken on June 13, with 641 GDD and 243 mm precipitation (www.noaa.gov) with values similar to the 30-year (1980–2010) average. Post-grazing samples were taken July 22, with GDD of 1540 and precipitation totaled 343 mm. In 2014, samples were taken May 13 with GDD at the spring assessment (which was taken after the early spring grazing) 262 and 65.5 mm of precipitation. The fall assessment was taken September 16 with GDD of 2603, and total rainfall of 370 mm fall. Rotational grazing was done much earlier in 2014 because the rancher was concerned about low amounts of precipitation (nearly 60% below average) during the 2013 fall and winter.

**Table 1.** Plant species in the mob-grazed and rotational grazed sites at Hayti, SD in 2013 and 2014.

**Mob-grazed sites Rotational sites**

Common name Scientific name Common name Scientific name Big bluestem *Andropogon gerardii* Western wheatgrass *Pascopyrum smithii* Sweet clover *Melilotus officinalis* Absinth wormwood *Artemisia absinthium* Alfalfa *Medicago sativa* Smooth brome *Bromus inermis* Red clover *Trifolium pratense* Kentucky bluegrass *Poa pratensis*

GDD accumulations for mob-grazed areas in 2013 were 1801 (August 6) and 1855 (August 9) for pre- and post-graze samples, respectively. Precipitation totaled 376 mm before and after mob grazing. In 2014, GDDs were 1693 pre-graze (July 29) and 1817 post-graze (August 4) and

Stocking treatments (rotation vs. mob) were repeated, although cattle densities and time of grazing differed between the 2 years due to feeding needs and differences in forage growth due to low rainfall in 2014 (**Table 2**). Rotational grazing was conducted in 8 ha pastures with 25 cow-calf pairs (1560 kg ha−1). In 2013, in one paddock, the cow-calf pairs were allowed to graze for 14 days starting June 13 (referred to as 'rotation'). In a separate paddock, generic 2,4-D ester at 1.1 kg ha−1 [24] was applied 1 day before the start of grazing on June 13 with a grazing duration of 14 days (referred to as 'spray/rotation'). In 2014, a different pasture was grazed by 25 cow/calf pairs for 15 days, starting April 27 and ending May 11 (referred to as 'early spring grazed/summer rest'). Mob grazing was conducted for 12 h in a 0.65-ha paddock on August 8, using 125 cow-calf pairs (stocking rate of 223,250 kg ha−1 day−1) (**Figure 1**). In 2014, a different 1.3-ha area was mob grazed on July 30 for 24 h with 125 cow-calf pairs (stocking rate of 53,580 kg ha−1 day−1).

precipitation for pre-graze and post-graze totaled 230 and 270 mm, respectively.

**2.3. Grazing treatments**

Kentucky bluegrass *Poa pratensis* Dandelion *Taraxacum officinale* Absinth wormwood *Artemisia absinthium* Western wheatgrass *Pascopyrum smithii* Smooth brome *Bromus inermis*

56 Forage Groups

**Table 2.** Mob and rotational grazing stocking density, grazing duration, sampling dates, forage biomass pre- and post-graze, and forage efficiency and utilization by year.

**2.5.** *Artemisia absinthium* **measurements**

Estimated biomass (kg ha−1

reported at a significance level of *P* ≤ 0.10.

no = same or greater volume) using the equation:

**2.6. Statistical analysis**

method.

Another three 50-m transects were established in each pasture with vegetative height measured pre- and post-graze every 2.5 m along the transects. *Artemisia absinthium* patches (individual plants if small or a patch if large) were selected and tagged near the base of the plant/ patch every 5 m along these transect lines in each treatment (in 2013 rotation; spray/rotation; and mob graze; and 2014 rotation/summer rest and mob graze). Pre- and post-grazing grass height and *Artemisia absinthium* patch volume (height and two perpendicular widths) were measured at the same time as forage measurements in 2013. In late May of 2014, *Artemisia absinthium* patches measured in 2013 experimental pastures were inspected for recovery and shoot regrowth. The rotation/summer rest had *Artemisia absinthium* measurements taken in

May, 2014 just after grazing, and again in September (as summer rest measurement).

method. The grazing stick equation, based on plant height, was:

Data analyses were performed using JMP®, Version 5.0.1, (SAS Institute Inc.). Forage amounts pre-graze were based on clipped biomass measurements and compared with the grazing stick

This estimated biomass for a cool season mixed grass pasture [26, 27]. The 7.6 cm value accounts for basal stems and leaves that would not be eaten by grazing animals. Two-tail, two-sample homoscedastic t-tests were used to compare forage biomass with the grazing stick estimates. Grazing stick estimates were found to be statistically similar to the clipping

Forage biomass and *Artemisia absinthium* volume were compared pre- and post-grazing and forage utilization (consumption plus trampling) was determined by examining new litter and remaining biomass at each transect point. These data were analyzed using one-tailed (post-graze < pre-graze) matched pair t-tests. Due to timing and treatment differences among rotational treatments, data were analyzed by treatment and year. Treatment differences are

Binomial analysis of *Artemisia absinthium* patch volume data (yes = less volume post grazing;

[p ± t(0.1) sqrt (p (1 − p)/n)] (2)

was used to examine the influence of each treatment on *Artemisia absinthium* patches [28]. In the mob grazing treatments, *Artemisia absinthium* data were combined across years. To better understand the relationship between weed patch size and grazing system impact, *Artemisia* 

Myer [23], four volume classes originally were designated, but were combined into the two

*absinthium* patches were separated into two volume classes (<19,000 cm3

volumes due to similarity of results within smaller and larger size classes.

) = [plant height (cm) − 7.6 cm] × 79 (1)

Mob vs. Rotational Grazing: Impact on Forage Use and *Artemisia absinthium*

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and >19,000 cm3

). In

**Figure 1.** A mob grazing herd waiting for the next pasture.

#### **2.4. Forage amounts and utilization**

Eight 50-m long transects were established in each paddock for vegetative production evaluation. Sampling points were placed every 5 m along each transect, with GPS coordinates (Garmin etrex 20, Garmin, LTD, Schaffhausen, Switzerland) recorded so that resampling occurred at the same points pregrazing and post-grazing. At the sampling points, pre-graze measurements (in 2013, rotational graze and spray/rotational graze—13 June; mob graze—6 August; 2014, mob graze—29 July) included vegetation height using a grazing stick [25], and ocular estimates of basal cover of living vegetation, litter cover, and bare ground (0–100%) in a 1 m2 area around the point. In 2013, vegetation in a 0.25 m2 area was clipped to within 1 cm of the soil surface, and bagged (n = 30). Litter under the vegetation also was collected. Samples were weighed, dried at 38 C to constant weight, and dry weight of vegetative biomass and litter per unit area were calculated. The biomass values and grazing stick estimates were compared at each sampled point.

A few days after grazing (in 2013, rotational graze and spray rotational/graze—22 July; mob graze—9 August; in 2014, mob graze—4 August), the same transects and sampling points were reestablished for post-grazing measurements. Vegetation height was measured using the grazing stick, and percent trampled vegetation (e.g., new litter; defined as living vegetation oriented less than 45° from the soil surface) was estimated in the same areas as pre-graze sampling.

In 2014 due to the producer's needs, cattle grazed the designated rotational pasture in April and then this pasture was untouched for the remainder of the season (summer rest). Unfortunately, due to the early timing of the grazing in the second year, no pre-grazing measurements were taken for this pasture. Measurements occurred on 13 May, after the early season grazing was completed, and then resampled on 16 September (designated as regrowth after early spring grazed/summer rest). In addition, the transects which were sampled in 2013 were reestablished and vegetative height was quantified in May 2014 to examine recovery after grazing.

#### **2.5.** *Artemisia absinthium* **measurements**

Another three 50-m transects were established in each pasture with vegetative height measured pre- and post-graze every 2.5 m along the transects. *Artemisia absinthium* patches (individual plants if small or a patch if large) were selected and tagged near the base of the plant/ patch every 5 m along these transect lines in each treatment (in 2013 rotation; spray/rotation; and mob graze; and 2014 rotation/summer rest and mob graze). Pre- and post-grazing grass height and *Artemisia absinthium* patch volume (height and two perpendicular widths) were measured at the same time as forage measurements in 2013. In late May of 2014, *Artemisia absinthium* patches measured in 2013 experimental pastures were inspected for recovery and shoot regrowth. The rotation/summer rest had *Artemisia absinthium* measurements taken in May, 2014 just after grazing, and again in September (as summer rest measurement).

#### **2.6. Statistical analysis**

**Figure 1.** A mob grazing herd waiting for the next pasture.

area around the point. In 2013, vegetation in a 0.25 m2

Eight 50-m long transects were established in each paddock for vegetative production evaluation. Sampling points were placed every 5 m along each transect, with GPS coordinates (Garmin etrex 20, Garmin, LTD, Schaffhausen, Switzerland) recorded so that resampling occurred at the same points pregrazing and post-grazing. At the sampling points, pre-graze measurements (in 2013, rotational graze and spray/rotational graze—13 June; mob graze—6 August; 2014, mob graze—29 July) included vegetation height using a grazing stick [25], and ocular estimates of basal cover of living vegetation, litter cover, and bare ground (0–100%) in

of the soil surface, and bagged (n = 30). Litter under the vegetation also was collected. Samples were weighed, dried at 38 C to constant weight, and dry weight of vegetative biomass and litter per unit area were calculated. The biomass values and grazing stick estimates were com-

A few days after grazing (in 2013, rotational graze and spray rotational/graze—22 July; mob graze—9 August; in 2014, mob graze—4 August), the same transects and sampling points were reestablished for post-grazing measurements. Vegetation height was measured using the grazing stick, and percent trampled vegetation (e.g., new litter; defined as living vegetation oriented less than 45° from the soil surface) was estimated in the same areas as pre-graze

In 2014 due to the producer's needs, cattle grazed the designated rotational pasture in April and then this pasture was untouched for the remainder of the season (summer rest). Unfortunately, due to the early timing of the grazing in the second year, no pre-grazing measurements were taken for this pasture. Measurements occurred on 13 May, after the early season grazing was completed, and then resampled on 16 September (designated as regrowth after early spring grazed/summer rest). In addition, the transects which were sampled in 2013 were reestablished and vegetative height was quantified in May 2014 to examine recovery

area was clipped to within 1 cm

**2.4. Forage amounts and utilization**

pared at each sampled point.

a 1 m2

58 Forage Groups

sampling.

after grazing.

Data analyses were performed using JMP®, Version 5.0.1, (SAS Institute Inc.). Forage amounts pre-graze were based on clipped biomass measurements and compared with the grazing stick method. The grazing stick equation, based on plant height, was:

$$\text{Estimated biomass (kg ha}^{-1}\text{)} = \left[\text{plant height (cm)} - 7.6 \text{ cm}\right] \times 79 \tag{1}$$

This estimated biomass for a cool season mixed grass pasture [26, 27]. The 7.6 cm value accounts for basal stems and leaves that would not be eaten by grazing animals. Two-tail, two-sample homoscedastic t-tests were used to compare forage biomass with the grazing stick estimates. Grazing stick estimates were found to be statistically similar to the clipping method.

Forage biomass and *Artemisia absinthium* volume were compared pre- and post-grazing and forage utilization (consumption plus trampling) was determined by examining new litter and remaining biomass at each transect point. These data were analyzed using one-tailed (post-graze < pre-graze) matched pair t-tests. Due to timing and treatment differences among rotational treatments, data were analyzed by treatment and year. Treatment differences are reported at a significance level of *P* ≤ 0.10.

Binomial analysis of *Artemisia absinthium* patch volume data (yes = less volume post grazing; no = same or greater volume) using the equation:

$$\left[\mathbf{p} \neq \mathbf{t}\_{(0,1)} \text{ sąrt } \left(\mathbf{p} \left(1 - \mathbf{p}\right) / \mathbf{n}\right)\right] \tag{2}$$

was used to examine the influence of each treatment on *Artemisia absinthium* patches [28]. In the mob grazing treatments, *Artemisia absinthium* data were combined across years. To better understand the relationship between weed patch size and grazing system impact, *Artemisia absinthium* patches were separated into two volume classes (<19,000 cm3 and >19,000 cm3 ). In Myer [23], four volume classes originally were designated, but were combined into the two volumes due to similarity of results within smaller and larger size classes.
