3.1. Avena fatua
Analysis of variance revealed a significant interaction between herbicide treatment and their application timing for seedling survival (
Figure 1), seedling biomass (
Figure 2), and seed production (
Figure 3) of
A. fatua. Grazing alone (control) resulted in 100% survival of seedlings and produced up to 50 g pot
−1 of biomass (
Figure 2) and 600 to 680 seeds pot
−1 (
Figure 3). Clethodim at the high rate (90 g a.i.·ha
−1) and haloxyfop at both rates (52 and 78 g a.i.·ha
−1) resulted in 100% mortality of seedlings (and therefore no biomass and seed production), irrespective of their application timings. Clethodim at the low rate (60 g a.i.·ha
−1) and pinoxaden at both rates (20 and 30 g a.i.·ha
−1) resulted in 28 to 64% of seedling survival, 20 to 40 g pot
−1 of biomass, and 210 to 590 seeds pot
−1 when applied 1 d after grazing. However, delayed application of these herbicides to 5 or 12 d after grazing resulted in 100% mortality. Glyphosate application, irrespective of rates, resulted in >70% of seedling survival, 33 to 46 g pot
−1 of biomass, and 430 to 600 seeds pot
−1 when applied 1 or 5 d after grazing. However, delayed application (12 d after grazing) of glyphosate at the high rate (740 g a.e.·ha
−1) achieved complete control of
A. fatua.
In general, the efficacy of herbicides was better when applied at 5 (i.e., 7 cm tall plants) or 12 d after grazing (i.e., 20 cm tall plants) compared with the application at 1 d after grazing (i.e., 3 cm tall grazed-plants). Immediately after grazing (i.e., 1 d), the plants may not have enough leaf material to absorb a sufficient amount of herbicides. All the three ACCase-inhibiting herbicide groups (dim, fop, and den) were highly effective on
A. fatua when applied 5 or 12 d after grazing, suggesting that the population used in this study was highly susceptible to these herbicides. These results are consistent with the results reported in a recent study [
9], in which clethodim 120 g a.i.·ha
−1, haloxyfop 78 g a.i.·ha
−1, and pinoxaden 20 g a.i.·ha
−1 provided complete control of
A. fatua when applied at the 3–4 or 6–7 leaf stage. In a similar study, clethodim (280 g a.i.·ha
−1) application 7 or 14 d after mowing resulted in 1 to 4 seed heads m
−2 of Vaseygrass (
Paspalum urvillei Steud.) [
27]. The nontreated control treatment produced 11 to 18 seedheads m
−2.
ACCase inhibitors are commonly used to control
A. fatua in a range of crops throughout the world. However, these herbicides are prone to developing resistance and their continued applications have resulted in the evolution of ACCase-inhibiting herbicide-resistant
A. fatua populations [
8,
28,
29,
30]. A survey in the western Australian grain cropping system reported that 50% of populations of
Avena species were resistant to ACCase-inhibiting herbicides [
30]. These observations suggest the need to rotate herbicides between ACCase-inhibiting subgroups and other modes of action.
Glyphosate was the least effective in controlling
A. fatua plants after grazing. The recommended glyphosate dose to control
A. fatua in Australia is 182 to 540 g a.i.·ha
−1. In the current study, glyphosate 370 g a.i.·ha
−1 did not provide effective control of grazed plants of
A. fatua. The herbicide dose was increased above the maximum recommended dose to achieve complete control of
A. fatua plants grazed 12 d before herbicide treatment. These results suggest that glyphosate may not be an effective option to control
A. fatua plants after grazing in fallow conditions. The glyphosate resistance level in the population used in this study is not known but glyphosate-resistant populations of
A. fatua are present in Australia [
9]. In a previous study, mowing to a height of 2 to 5 cm followed by an application of glyphosate (3330 g a.e.·ha
−1) provided effective control of perennial pepperweed (
Lepidium latifolium L.) [
31]. In that study, mowing or glyphosate alone were not effective. In a recent study, sequential application of slashing (to a height of 10 cm) and glyphosate 1830 g a.e.·ha
−1 (14 d after slashing) provided >70% reduction of node production and viable stolon of drought grass (
Ischaemum muticum L.) [
32]. In the USA, glyphosate use was recommended as a tool to increase livestock consumption of medusahead [
Taeniatherum caput-medusae (L.) Nevski] [
20]. However, in that study, glyphosate application was followed by grazing of the weed species.
3.2. Chloris virgata
Seedling survival (
Table 2) and biomass (
Table 3) of
C. virgata were affected by the main effects, i.e., herbicide treatment and time of herbicide application (after grazing). Compared with the grazing-only treatment (control), most herbicide treatments reduced seedling survival of
C. virgata (
Table 2). Glufosinate application after grazing resulted in the greatest mortality (69 to 81%) of
C. virgata. Similarly, glufosinate at 750 and 1500 g a.i.·ha
−1 resulted in 86% and 92% reductions in biomass, respectively, compared with the grazing-only treatment (
Table 3). The next best herbicide treatments were haloxyfop 40 and 80 g a.i.·ha
−1, which reduced biomass by 65 and 80%, respectively, compared with the control treatment.
Across herbicide treatments, a greater number of seedlings survived when herbicides were applied 1 or 3 d after grazing (i.e., 2–4 cm tall plants) (
Table 2). The lowest number of
C. virgata seedlings survived when herbicides were applied 7 d after grazing (i.e., 10 cm tall plants); which was statistically similar to herbicide treatments applied at 10 or 14 d after grazing (i.e., 16–22 cm tall plants). The biomass data revealed that the best time for herbicide applications was 7 d after grazing (
Table 3). The maximum biomass (5.6 g pot
−1) was produced when herbicides were applied 1 d after grazing, which was similar to the biomass produced by herbicide treatments at 3 and 14 d after grazing.
Leaf chlorophyll content, as measured by SPAD units, was significantly affected by herbicide treatments (
Table 4) but it was not affected by application timing. The chlorophyll content was similar among the control, butroxydim, and clethodim treatments. Glufosinate reduced the chlorophyll content by 40 to 60% compared with the nontreated control treatment, suggesting that glufosinate was the best herbicide in causing injury in
C. virgata. The next best herbicide was haloxyfop, which reduced the chlorophyll content by 19% to 37% compared with the nontreated control treatment.
Analysis of variance revealed an interaction between herbicide treatment and application timing for panicle (
Table 5) and seed production (
Table 6) of
C. virgata. The grazing-only treatment had similar numbers of panicles and seeds at different timings.
Chloris virgata produced the maximum number of panicles (
Table 5) and seeds (
Table 6) when herbicides were applied 1 day after grazing; however, the numbers were statistically similar across herbicide application timing for clethodim 90 g a.i.·ha
−1, glufosinate at both rates (750 and 1500 g a.i.·ha
−1), and haloxyfop 80 g a.i.·ha
−1. Glufosinate was the only herbicide that resulted in no seed production (i.e., 750 g a.i.·ha
−1 when applied 10 d or 14 d after grazing). Butroxydim, irrespective of rates, provided the best control of seed production when applied at 7 d after grazing; resulting in 84% to 90% reductions compared with the grazing-only treatment. Clethodim- and haloxyfop-treated
C. virgata plants, irrespective of their rates, produced a similar number of seeds when herbicides were applied 7, 10, or 14 d after grazing (
Table 6), suggesting that these herbicides can be applied at any time between 7 and 14 d after grazing.
The results of this study are similar to results reported in recent studies [
25,
33]. The previous studies suggested that the ‘fop’ herbicides (e.g., haloxyfop) performed better on
C. virgata than ‘dim’ herbicides (e.g., butroxydim and clethodim). Clethodim is recommended to control 5-leaf to fully tillered
C. virgata and haloxyfop to control 2-leaf to early tillering
C. virgata plants. In the current study, these herbicides were applied after grazing (or mowing) and >40% of seedling survival could be due to the large size of the plant. In a recent study, 97% and 100% plant mortality was achieved when clethodim 180 g a.i.·ha
−1 and haloxyfop 40 g a.i.·ha
−1, respectively, were applied at the 24–28 leaf stage of
C. virgata [
25]. Another Australian study reported 96 to 98% control of
C. virgata when haloxyfop 156 g a.i.·ha
−1 was applied to flowering plants [
34]. In the current study, glufosinate provided the best control of
C. virgata after grazing, resulting in 19 to 31% seedling survival, 86 to 92% reductions in biomass, and reduction in seed production by up to 100%. A recent study also reported that glufosinate 1500 g a.i.·ha
−1 provided promising control of large plants (24–28 stage) of
C. virgata, resulting in 27% of seedling survival and the survived plants did not produce any panicles [
25]. The efficacy of glufosinate could be further increased by adding an adjuvant, such as Hasten
® [
25]. As
C. virgata is known to have a high level of natural tolerance to glyphosate [
35] and this species is increasingly evolving resistance to glyphosate [
8,
17], alternate herbicide options need to be used to manage
C. virgata. Glufosinate and some ACCase-inhibiting herbicides provided promising results to control
C. virgata after grazing. Across herbicide treatments, the least number of seedlings survived and the least biomass was produced when herbicides were applied 7 d after grazing. These results suggest that the best control of
C. virgata would be achieved when effective herbicides are applied approximately 7 d after grazing (approximately 10 cm in height).