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Effects of grazing management on Bothriochloa macra (red grass) in a native pasture

J.M Virgona and S. Hildebrand

School of Agricultural and Wine Sciences, Charles Sturt University. Email jvirgona@csu.edu.au
Future Farm Industries CRC

Abstract

Red grass is an indigenous species that is commonly found in native pastures in the higher rainfall zone of New South Wales. A grazing experiment was conducted near Cookardinia in southern NSW to determine if simple grazing management strategies could be imposed to maintain and/or increase this species in native pastures dominated by exotic annual species. From September 2003 to April 2006, five treatments were imposed: 1) continuous grazing (year round grazing at 5 DSE/ha); 2) grazing exclusion; 3) high spring utilisation (as with set stocking but increasing to 23 DSE/ha for 6 weeks in spring); 4) high spring utilisation with strategic lock-up over summer, and 5) strategic lock-up over summer. Strategic summer lock-ups were designed to encourage seedling recruitment and were imposed in response to rainfall after seed fall in late summer-early autumn. Treatments were applied in plots (15 x 15 m), replicated four times, communally grazed by Merino wethers. Initially, red grass was a minor component (8% of biomass) of the pasture, which was dominated by broadleaf weeds and annual grasses. By April 2006, the basal cover of red grass was significantly higher in the two treatments that included strategic lock-up over summer (treatment 4 : 8.9% and treatment 5: 10.7%) compared with continuous grazing (3.7%). In contrast, exclusion of grazing over the experimental period had no significant effect compared with continuous grazing. In conclusion, grazing management, in this case summer-autumn rest, may be an effective low-cost tool to increase the proportion and basal cover of Bothriochloa macra.

Keywords

Strategic grazing, red grass, perenniality

Introduction

“Unimproved” pastures on the slopes and tablelands of southern NSW and northern Victoria vary greatly in botanical composition from those dominated by native species with minor exotic content, to those in which exotic (usually annual) species dominate with native grass species relatively sparse. As a simplification, it is reasonable to refer to all such grassland communities as ‘native pastures’ where the major perennial species is a native (Crosthwaite and Malcolm 2001). The relative abundance of native grasses within these pastures is critical for functioning with respect to natural resource management in particular, reducing deep drainage and maintaining ground cover (Virgona et al. 2003). In these landscapes, it is either uneconomical and/or not feasible to introduce new perennial species. Therefore manipulation of existing native pastures has more potential for increasing the “perenniality” of the landscape than introducing either exotic species or domesticated native species (Virgona et al. 2003).

The results of a recent survey of native pastures from central NSW to north eastern Victoria showed that the most common native perennial grass species in southern NSW were Bothriochloa macra, Austrodanthonia spp., Austrostipa spp. and Microlaena stipoides (Virgona, Li, Mitchell, Ridley, Dowling, King, unpublished data). Of these, Bothriochloa macra (hereafter – Bothriochloa) was the most common, but usually at very low frequencies. It is possible that a range of management strategies could be used to increase the relative abundance of this species but, given the low productivity of these pastures, expensive options such as use of herbicides, increased use of fertiliser or subdivision would have very low rates of on-farm adoption by graziers. In contrast, grazing management, a relatively cheap technology, could be used to cost-effectively increase Bothriochloa content in these pastures and be more readily adopted by graziers. The experiment reported in this paper describes the effects of simple grazing management treatments on the abundance of Bothriochloa and other components of the pasture.

Methods

A native pasture near Cookardinia in southern NSW (annual average rainfall 650 mm) with soil P (Olsen) of 11.9 and pH (CaCl2) of 4.7 and typically low levels of Bothriochloa (7.6% of above ground biomass; hereafter, biomass) was selected for the study. From September 2003 to April 2006, five treatments were imposed:

1) Continuous grazing (year round grazing by wethers at 5 sheep/ha),

2) Exclusion – no grazing,

3) High Spring Utilisation (HSU) – as for Continuous + increase to 23 sheep/ha for 6 weeks in spring,

4) HSU + Strategic exclusion late spring to mid autumn + pulse grazing over summer (at 23–31 sheep/ha) after major rainfall events),

5) Strategic (as for (4), but without HSU).

All treatments were grazed at the same stocking rate as the continuously grazed treatment, except when otherwise specified. Each treatment was split for an application of herbicide (MCPA) to be imposed in winter to control broadleaf weeds, but only results from the unsprayed treatments will be considered here. The aim of the HSU treatment was to reduce biomass to <1000 kg/ha. Strategic summer lock-ups were designed to encourage seed set and seed fall and seedling recruitment. Grazing was excluded from these treatments from mid November to mid April–May each year, except for a brief period (normally 7 days of high intensity grazing at 23-31 sheep/ha), following a major summer-autumn rainfall event. Treatments were applied in plots (15 x 15 m), replicated four times, and communally grazed by Merino wethers at 5.3 wethers/ha.

Every 5–7 weeks from September 2003 to May 2006, biomass and botanical composition were measured on each plot. Biomass was measured in 15 quadrats evenly spaced along a fixed transect using a falling plate meter (e.g. Li et al. 1998) up to April 2004 and thereafter a visual assessment technique (Haydock and Shaw 1975). Botanical composition was assessed using the dry-weight rank technique as modified by Jones and Hargreaves (1979) on the same quadrats used for measurement of biomass. At the commencement of the experiment, frequency and apparent density of Bothrichloa were measured in each plot in 20 quadrats (0.25m2) per plot. These measurements were also made at the conclusion of the experiment, as well as a calibrated visual estimate of basal cover (McIvor 2001). For the latter, visual estimates on undisturbed samples were calibrated against basal cover by cutting samples to a height of 25 mm and placing a gridded quadrat [measuring 300 x 300 mm (with 25 x 25 mm mesh – giving 144 cells)] over it. Each cell was scored for cover – which was positive if at least half of the cell was covered by the base of Bothriochloa plants. The relationship between the visual cover score (y) and actual cover (x) was y = 14.0x with r2 = 0.95 (P < 0.001). Botanical composition, density, frequency and basal cover data were subjected to analysis of variance. Effects of treatments on biomass and Bothriochloa as a proportion of total biomass through time were analysed using linear mixed models (Orchard et al. 2000).

Results

Rainfall over the term of the experiment was markedly lower than the median as obtained from a patch-point data set for the site (http://www.longpaddock.qld.gov.au/silo/). Annual rainfall from 2003–2006 was 529, 529, 667 and 216 mm, respectively, compared with the long term median of 642 mm. It is worth noting that conditions were especially dry throughout each autumn with none of the autumn months in 2004–2006 having median rainfall.

Treatments had significant effects on predicted biomass throughout the experiment (Fig. 1). Predictably, biomass of the Exclusion treatment was significantly higher than the Continuous treatment from December 2004 onward and was 8800 kg/ha by April 2006 (data not shown in Fig.1). Predicted biomass of the HSU

Figure 1. Predicted biomass (from linear mixed modelling) as a proportion of total biomass (backtransformed from analysed natural log transformation) over time under four management regimes. For statistical differences among treatments, refer to text.

treatment was significantly lower than the Continuous treatment in spring 2004 and 2005 and consistently less than the HSU+Strategic and Strategic treatments from spring 2004 onward. Despite the increased grazing pressure in spring, the HSU+Strategic treatment had lower predicted biomass than the Strategic treatment in spring 2004 only.

Initial botanical composition (September 2003) varied markedly across the plots, with a median values (range in brackets) of Bothriochloa 7.1% (1.6–30.1), broadleaf weeds (mainly Carthamus lanatus, Arctotheca calendula and Hypocheris radicata) 76.5% (34.6–93.4), annual grasses (mostly Vulpia spp. Bromus mollis and Hordeum spp.) 2.1% (0.6–11) and annual legume (primarily Trifolium subterraneum, T. glomeratum and T. campestre) 5.5% (0–33.3). Apparent density and frequency of Bothriochloa were not significantly affected by any of the treatments imposed (Table 1).

Table 1. Initial and final apparent density, frequency and final basal cover of Bothriochloa.

 

Initial (2003)

  

Final (2006)

Treatment

Frequency

Density

  

Frequency

Density

Basal cover

 

(%)

(m2)

  

(%)

(m2)

(%)

Continuous

91.2

10.4

  

66.2

6.8

1.55

Exclusion

86.2

10.0

  

58.8

5.5

2.18

High Spring Utilisation (HSU)

95.0

11.1

  

66.2

9.2

2.55

HSU + Strategic

87.5

9.1

  

83.8

10.4

3.24

Strategic

91.2

9.8

  

80.0

9.7

3.75

LSD (P=0.05)

ns

ns

  

ns

ns

1.23

There was a significant effect of management on basal cover of Bothriochloa, with both treatments that included the Strategic regime having significantly higher Bothriochloa basal cover than the Continuous treatment (Table 1). Consistent with the effect on basal cover, Bothriochloa was a significantly higher proportion of biomass in the treatments that included the strategic regime (Fig. 2). For clarity the Exclusion treatment was not included in Figure 2, but did not differ significantly from either the Continuous or HSU treatments. Finally, while there was a significant effect of grazing on the proportion of legume in the pasture, as a result of the Exclusion treatment (2.4%) being much lower than all other treatments (20–25%).

Figure 2. Predicted Bothriochloa (from linear mixed modelling) as a proportion of total biomass (backtransformed from analysed square root transformation) over time under four management regimes. The HSU+Strategic and Strategic treatments were significant greater than the Continuous treatment from June 2004 and March 2005, respectively.

Discussion

In a relatively short period, the proportion of Bothriochloa in the pasture and its basal cover could be increased by applying a grazing management regime that reduced grazing over the summer-autumn period with or without prior high spring utilisation. As the greater basal cover of Bothriochloa in these treatments was not associated with greater frequency and/or apparent density, the differences in basal cover probably resulted from larger individual Bothriochloa plants. Interestingly, the basal cover of the HSU treatment was not significantly different from the Continuous treatment, despite the consistently lower predicted biomass of the HSU plots (Fig. 1) – probably reflecting preferential grazing of the HSU treatment throughout the year.

The increase in basal cover of Bothriochloa over that of the continuously grazed treatment was not consistent with results of grazing studies conducted by Lodge et al. (2003a, b) on the northern slopes of NSW under more favourable conditions. Lodge et al. (2003a) found rotational grazing and higher stocking rate (with T. subterraneum oversown) improved basal cover over continuous grazing at low stocking rates (4–6 sheep/ha). In contrast, at another site where similar treatments were applied, no effects on the cover of Bothriochloa were found (Lodge et al. 2003b). The inconsistencies between the results reported here and those found by Lodge et al. (2003a, b) may arise from a number of factors. For instance, Bothriochloa may be more responsive to longer rest periods over summer-autumn in southern NSW, where rainfall over these months is less frequent than in the summer rainfall areas of northern NSW. In addition, the levels of Bothriochloa (and other native species) in this study were much lower than those reported by Lodge et al. (2003a, b), and there may have been fewer perennial competitors.

Conclusion

These findings provide some evidence that the cover and proportion of Bothriochloa macra in native pastures can be increased by reduced grazing over the summer-autumn period. These results can be applied by farmers who wish to increase the levels of perennial species in similar pasture communities using strategic grazing management. However, on-farm the opportunity to increase or decrease stocking rates on targeted pastures will vary depending largely on the availability of livestock, and availability of feed in alternative pastures.

References

Crosthwaite J and Malcolm B (2001). Farm scale economics of native grasses. Proceedings of the Second National Conference of the Native Grasses Association http://www.regional.org.au/au/stipa/papers/stipa2001-01.htm#TopOfPage. Accessed 3 August 2010.

Jones RM and Hargreaves JNG (1979). Improvements to the dry-weight-rank method for measuring botanical composition. Grass and Forage Science 34, 181–189.

Li GD, Helyar KR, Castleman LJ, Norton G and Fisher RP (1998). The implementation and limitations of using a falling plate meter to estimate pasture yield. Proceedings of the 9th Australian Agronomy Conference, Wagga Wagga. Australian Society of Agronomy. http://www.regional.org.au/au/asa/1998/3/046li.htm. Accessed 3 August 2010.

Haydock KP and Shaw NH (1975). The comparative yield method for estimating dry matter yield of pastures. Australian Journal of Experimental Agriculture and Animal Husbandry 15, 663–670.

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Lodge GM, Murphy SR and Harden S (2003b). Effects of grazing and management on herbage mass, persistence, animal production and soil water content of native pastures. 2. A mixed native pasture, Manilla, North-West Slopes, New South Wales. Australian Journal of Experimental Agriculture 43, 891–905.

McIvor JG (2001). Pasture management in semi-arid tropical woodlands: regeneration of degraded pastures protected from grazing. Australian Journal of Experimental Agriculture 41, 487–496.

Orchard BR, Cullis BR, Coombes NE, Virgona JM and Klein T (2000). Grazing management studies in the Temperate Pasture Sustainability Key Program; experimental design and statistical analysis. Australian Journal of Experimental Agriculture 40, 143–154.

Virgona J, Mitchell M and Ridley A (2003). Native pastures: Research and development directions with respect to the mitigation of dryland salinity. Proceedings of the 2002 Fenner Conference on the Environment. Eds BP Wilson and A Curtis. pp. 223–234, Johnstone Centre, Charles Sturt University.

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