**4. Discussion**

**3.2. Selby**

28 Forage Groups

Forage at Selby averaged about 1940 kg ha−<sup>1</sup>

graze and about 790 kg ha−<sup>1</sup>

had about 470 and 2920 kg ha−<sup>1</sup>

the first to the second sampling.

volume < 6500 cm3

**3.3. Initial WS plant volume and grazing impact**

the 24-hr grazing event, forage remaining was 1110 kg ha−<sup>1</sup>

Forage consumption and utilization were estimated at 15 and 42%, respectively, in 2013. In 2014, efficiency and utilization were estimated at 48 and 88%, respectively. The three-fold increase in forage consumption (efficiency) in 2014 compared to 2013 may have been due to timing of the grazing. Forage was likely more mature and less palatable for cattle in late September (2013) compared to late July (2014). The increase in both consumption and utilization may have also been due to the slightly higher stocking density in 2014 compared to 2013 (**Table 2**). In the 2013 rotation-grazed pasture, the forage biomass averaged 2690 kg ha−<sup>1</sup>

as very little newly trampled biomass was present. In 2014, the ungrazed comparison pasture

Volume data from WS plants were combined for the 2013 and 2014 mob grazing treatment, with 66% (±8%) of the tagged plants decreasing in volume by 46% after grazing. In the rotational-grazed area, pre- and post-sampling volumes were similar and averaged 15,000 cm3

However, 43% of these sampled plants had a 45% reduction in volume, but the remaining plants increased in volume by about 90%. Basal stem counts (data not shown) indicated that WS plants in mob-grazed areas had fewer stems (*P* = 0.001) after grazing, whereas no difference in stem number was observed in rotational-grazed plots. In the 2014 ungrazed pasture,

Initial WS plant volume impacted final volume after mob grazing. Mob grazing data, com-

In the early spring rotationally-grazed paddock at Chamberlain (2014), initial volume did not impact final size (*P* = 0.46). About 66% of all plants increased in size an average of 168% (±81%). In the late-season rotation treatment at Selby in 2013, about 50% (±13%) of the small plants were reduced in volume by about 52% (**Figure 2B**), with the remaining plants increasing in volume an average of 150%. About 44% of the large plants were reduced in volume by about 38%, with the remaining plants increasing an average of 37%. While the plant size reduction in the less intensively grazed rotational treatment was similar between the large and small plant classes (*P* = 0.46), the increase in size of the small plants was greater than the size increase of the large plants (*P* = 0.02). This may be due to smaller plants being trampled and stems spread apart thereby increasing the final volume (i.e. plants lost vertical height but both horizontal

lengths increased), whereas larger plants may have been more difficult to trample.

grazed areas tended to be more damaged than smaller plants.

These data indicate that WS plants were more impacted by mob grazing compared with plants in paddocks rotationally-grazed early or later in the season. Larger plants in mob-

, 73% (±7%) of these plants had a 42% reduction in volume. However, about

74% of the tagged plants increased in volume by an average of 5000 cm3

bined by location, indicated that the median plant size was about 6500 cm3

87% (±5%) of the larger plants were reduced in volume by about 62%.

each year prior to mob grazing (**Table 3**). After

post-graze, with an estimated 70% consumption and utilization,

at the first and second sampling, respectively.

in 2013 and 240 kg ha−<sup>1</sup>

in 2014.

pre-

.

, a 3000% increase from

. When initial plant

Cattle in NGP mob-grazed settings were more competitive for available forage, and were less selective in consumption, eating vegetation that would normally be avoided in a less intense grazing. The high stocking densities also resulted in more trampling and greater animal impact (e.g. dung deposition, data not shown) per unit area [29]. Other studies have reported similar results in other intensive-grazing systems although terminology [e.g. ultra-high stocking density [23]; intensive stocking [34]; cell-grazing [35]; high intensity, low frequency grazing [36], stocking rates, grazing duration, and seasonal timing often differ. High stocking densities have been shown to maintain animal performance if carefully managed [36]. Lush regrowth during the rest period following an intense grazing event increased forage crude protein (from 8.9 to 10.2%) and digestibility (from 44.6 to 54.7%) compared with more mature forage in lessintensively grazed areas [36]. Timing of grazing events, both within and among seasons on the same parcels, must be carefully controlled as repeated grazing when grass is at a vulnerable growth stage can result in rangeland degradation [37, 38].

Other studies have reported that cattle graze less palatable, weedy species when grazing intensity is high. For example, cattle have browsed prickly pear (*Opuntia macrorhiza*) [39], absinth wormwood [29], and thistles [40], species that are typically avoided in low-intensity grazing. The least desirable species at Australian sites, purple wiregrass (*Aristida ramose*) and gray tussock-grass (*Poa sieberiana*), decreased 45% in basal diameter in a cell-grazing treatment with a stocking rate of about 35,000 kg ha−<sup>1</sup> and moved every 1–3 days compared with <5% decreases observed in continuously grazed sites [35]. These results suggest that during mob-grazing events, animals will browse less desirable species. In addition, mob-grazing, or similar high stocking-density, low frequency grazing management, has been suggested to maximize forage use [21], aid in maintaining a balance of desirable and undesirable vegetation [41] and may enhance nutrient cycling in the paddock with minimal to no risk to animal gains if properly managed [42, 43]. However, mob-grazing should be strictly managed with recovery periods for forage regrowth to ensure adequate feed. Returns to management can be low for mob-grazing [45] if high stocking densities for long periods reduce average daily gain per animal [46] and may degrade range resources and resilience.

Size of WS plants influenced the efficacy of mob-grazing for weed management. In contrast to absinth wormwood (*Artemisia absinthium*) (AW) where small patches and plants were most affected by mob-grazing [44], larger WS plants were most impacted. Larger WS plants may have leaves closer to the cattle's face, which may facilitate browsing strictly due to convenience, even though the stems are woody. Smaller AW plants, which have herbaceous rather than stiff woody stems, may be more easily trampled and/or consumed.
