The one thing that remains constant in society is change. And so it is with pastures. Their composition and thus their productivity vary with the seasons and with time. Over the long term, productivity declines. It is recognition of this long term compositional and productivity decline that this talk is all about. To put the subject into perspective, I have dealt briefly with pasture degradation and the life - span of a useful pasture before rejuvenation is considered to be required. And it should always be remembered that herbicide manipulation is only one of a number of rejuvenation options that can be chosen. I have then looked broadly at the principles of spray topping and spray grazing, and the selective removal of grass and broadleaf weeds from lucerne and perennial grass/clover pastures, and finally I have provided a summary table of chemicals, rates and their costs. However, for recommendations specific to your situation you should consult your local District Agronomist and the relevant Department of Agriculture’s recommendations on pasture renovation with herbicides.

Degradation

Pastures are a highly complex arrangement of herbaceous species. They range from a collection of plants which directly or indirectly result in lowered animal productivity through to balanced mixtures of sown grass and legume, and monocultures of lucerne. I think most pastures would bear a closer resemblance to the former rather than the latter categories. The composition of the first pasture type, however, was probably closer to the second pasture type initially. Over a period of 5 - 20 years the composition has changed. Why? To appreciate why, it is necessary to appreciate the difficulties with which an individual sown plant must cope. Not only must it fight for its food and location with its sown neighbours, but also tolerate bugs chewing and sucking at its roots and leaves, and disease organisms just waiting their chance. And, finally, it has to tolerate the excretive products, and treading and defoliation of the grazing animal. So, in addition to surviving and producing within the extremes of the environment in which it is growing, it must tolerate the attentions of invertebrate and vertebrate grazers. Little wonder that many of the sown seedlings succumb to such pressure. The space created is quickly filled by aggressive annual and biennial species which continue the relentless pressure on the sown species. Given that all pasture environments are in this situation, it seems that we must inevitably accept pasture degradation as a fact of life. What we do not have to accept, however, is the rate at which this process occurs. Provided that the pasture species are adapted to the area, that seasons are not adverse, that nutrition is adequate and that the level of management is reasonable, then the rate of degradation can be slowed.

Table 1. varying complexity of pasture manipulation based on whether seed is sown, degree of soil disturbance, management expertise, cost, and rate of change

Acceptable Productivity Levels

How far should productivity levels be allowed to decline before rejuvenation procedures are introduced? This will be determined by the value of the productivity sacrificed, balanced against the cost of the rejuvenation measure being considered and the accessibility of funds to carry out such measures, and availability of livestock to utilise the increased production resulting from such measures. Clearly, the further the pasture productivity level is allowed to decline below the maximum, then the more expensive is likely to be the rejuvenation measure (Table I). It is also likely to require greater managerial expertise.

The options where seeding is not involved assumes, of course, that there are sufficient desirable species of sufficient vigour present in the sward that will respond to the manipulation choice selected. There is no point in removing undesirable annual grasses in a subclover/ryegrass pasture if the spaces created cannot be utilised by subclover or ryegrass. The decision whether to seed or not to seed will depend on a range of factors but the minimum ground cover for non - seeding manipulation measures would be expected to be 40 - 50%. Associated with all measures that depend on the response of the resident species to fill the gaps (including surface seeding), is grazing management, which ensures that these plants are spelled over spring/summer enabling them to set seed.

Herbicide Manipulation

Annual grasses in perennial pasture

A major cause of pasture degradation is the ingress of annual grasses. The problem is not over autumn and winter, when they contribute a significant proportion of the grazing animal’s diet, but during spring and summer when they quickly become unpalatable and cause animal discomfort and wool contamination.

Spraytopping. Spraytopping, where a relatively low rate of knockdown herbicide is applied in spring when seedheads are in various stages of development, hasprovento be an effective method of reducing grass regeneration in the following year. The system is based on the assumption that there is no carry over of seed from year to year, i.e. there is no dormancy and all seeds formed during the previous season will germinate during the current season. Problems with the technique are in choosing the correct development stage to apply the herbicide, deciding when to apply herbicide in a mixed sward containing Vulpia, Hordeum, Lolium and Bromus sp., because of their different flowering times, and the incidence of long moist springs which can result in late tillering and further seeds being produced. Some typical reductions in seedling numbers resulting from spraytopping are shown in Table 2. Reductions of 95% look most impressive, hut if the number of grass seeds set in the untreated pasture was 20,000/in² (depending on locality, this can probably vary from 10,000 - 40,000 seedlings/in² in a degraded pasture) then 5% x 20,000 = 1000 seeds will germinate on the topped pasture in the following autumn.

Table 2. Effect of spray topping in the previous spring on percentage reduction of seedling numbers in the following autumn compared to an untreated control. Values are for most successful treatment.

Whatever the state of the degraded pasture, spraytopping in one year only will not necessarily result in an improvement in productivity over the long term. The main advantage with spraytopping is its relative cost

(Appendix I),but the optimum timing of the operation can be difficult to determine. In most pasture situations there will be a mix of annual grasses and these would be expected to mature in the order: barley, brome, Vulpia, ryegrass. Timing of treatment should be for the least desirable species. There seems to be some variation in recommendations on timing, but generally spray topping should occur when the majority of heads have emerged but are still green. Inevitably this will result in a range of development stages. These should be kept between the booting to milky dough stage. Grazing during late winter - early spring may prevent early seed heads emerging and narrow the range of development at spraytopping. The timing is not as critical with Roundup compared to Gramoxone because of its different mode of action. Roundup would be expected, however, to have an effect on subclover also. If in doubt about timing, the literature (mostly work carried out in Western Australia) suggests better results are obtained from earlier rather than later herbicide application.

Selective removal Herbicide applied during the cooler months is probably the most effective method of reducing grass dominance in a mixed pasture but hard decisions on timing still need to be made. Effectiveness of autumn and spring herbicide application on pasture composition is shown in Table 3. When applied early in autumn, rates and carried out during autumn and winter and, as with other procedures, the later the application of herbicide, the higher the rate required and the greater the cost. An important point to be recognised is that many species not normally grazed by sheep are made more palatable by the herbicide. Problems from poisoning can be reduced by not grazing with valuable stock and by not grazing for extended periods.

Assorted weeds in lucerne

The options for weed removal in lucerne are more restricted. Spraytopping in spring would not be expected to be as successful because of greater interception of the chemical by lucerne resulting in reduced effectiveness on the annual grasses and a greater suppression of lucerne growth. Similarly, the spray - grazing technique may result in overgrazing of the lucerne. However, the greater dormancy of lucerne over winter means that selectivity between weeds and lucerne is improved when chemicals are applied at that time. In addition, the tap - root system of lucerne means a greater tolerance to residual herbicides. Note that the chemicals detailed in Appendix I have not been tested on all lucerne varieties.

Conclusions

The decision to change the botanical composition of a pasture depends on the productivity of that pasture. Assuming that the proportion of desired species in the pasture is no less than 40 - 50% and will respond to manipulation, then an appropriate non - seeding method needs to be chosen. If annual grasses are the problem and herbicide manipulation in the option selected, then further choices need to be made between autumn and spring procedures and which herbicide to use, bearing in mind cost, expertise required, winter feed shortages, effectiveness and likely returns. With broadleaves, the choice is between autumn and winter application. For information specific to your location, you should consult your District Agronomist and Weed Specialist. Further reading on the subject is listed below.

Further Reading

1. Anon. (1986). Pasture manipulation pays off. Cheins. in Agric. 1986, pp.9 - 11.

2. Blowes, W.M., Jones, S.M., England, P.J. and Fraser, P.K. (1984). Pasture - topping using Roundup - a review. Proc. 7th Aust. Weeds Conf. 1:351 - 4.

3. Dellow, J.J. and McDonald, W. (1987). Herbicides for weed control in lucerne and pastures 1987. Dept. Agric. NSW. 28pp.

4. England, P. (1986). Pasture topping with Roundup herbicide. Proc. Conf. on Recent Advances in Weed and Crop Residue Management. (Eds. Pratley, J.E. and Cornish, P.S.). Southern Conservation Farming Group. Occasional Publ. No. 2. pp.¹⁰⁴ - ⁵.

5. Gammie, R.L. (1984). Cleaning up pasture for better crops. Aust. Country 27(10) :20-1.

6. Holmes, J.E. (1984). Seed - set control - potential of fluazifop - butyl and Dowco 453. Proc. 7th Aust. Weeds Conf. 1:358 - 62.

7. Hutchings, T. (1986). Grass weed control in pastures in Autumn. Proc. Conf. on Recent Advances in Weed and Crop Residue Management (Eds. Pratley, J.E. and Cornish, P.S.). Southern Conservation Farming Group. Occasional Publ. No. 2, pp.106 - 7.

8. TOT Australia Operations Pty Ltd
- Various Bulletins
- The Direct - Drill Farmer )
- Chemicals in Agriculture) various editions

9. Jones, S.M., Blowes, W.M., England, P. and Fraser, P.K. (1984). Pasture topping using Roundup herbicide. Aust. Weeds 3: 150 - 1.

10. Parker, P.A., Gammie, R.L. and May, M.G. (1983). Chemicals as tools.

11. Herbicides in tillage systems. Proc. Seminars on Conservation Tillage - Crop it for Profit. Agric. Technol. Australasia, pp.42 - 6.

12. Pierce, J. (1985). WA’s Spray - graze experiences. Aust. Country 28(15):53 - 4.

13. Thorn, C.W. and Perry, M.S. (1987). Effect of chemical removal of grasses from pasture leys on pasture and sheep production. Aust. J. Exp. Agric.

14. 27:349 - 57.

15. Venn, N.R. (1984). Control of annual grasses in pasture and legume crops with fluazifop - butyl. Aust. J. Exp. Agric. Anim. Husb. 24:612 - 6.

APPENDIX 1

Possible alternatives for removal of annual grass and broadleaved species from pastures with rates of application and cost.

A. MIXED PASTURE

a pre - emergence

b early post - emergence

B. LUCERNE