13. WEED CONTROL

So long as an improved pasture has been established and managed with an appropriate SR (Stocking RATE). Weeds should not be a major problem. It cannot be too strongly stressed that good management is the key to avoiding the all too colon problem of weed dominated pastures in the South Pacific. Even with the right SR some weeds will grow, these should be controlled using recommended practices. In addition the following practices should become habit on the farm.

1. When cattle are shifted from one grazing area to another, the people who move the cattle should inspect the grazed area and pull any weeds present (woody weeds may be slashed). In this way a few minutes spent will prevent weeds from setting seed - spreading and becoming a major hard to fix problem.

2 In the case of guava (psidium guajava) see Photo 15. plants should be treated with Grazon/Garlon 600. In the case that some areas of the farm are clear of guava and others are not, the manager should avoid allowing cattle to eat fruits in one area and then move to another where there are no plants, as guava seed will be transported in the dung.

3. Some plants, such as mile a minute (Mikania micrantha) can taint milk, that is give it a strange taste. It is important to avoid this as people will reject tainted milk. Large areas, or the weed growing on a fence line may have to be sprayed with butoxone. Generally grazing the paddock about 10 days ahead of the cows with non-milking cattle and ensuring that plenty of water is available Should fix the problem. The farmer Should look for any poisonous or milk tainting plants and remove them from the pasture.

Photo 13. mile- a-minute, Mikania micrantha

  1. Navuia sedge [Kyllinga polyphylla]- see Photo 22. is an extremely serious problem if it becomes dominant on a farm. The manager should try very hard to prevent its establishment on the farm, digging up and burning and regular spraying of any plant seen on or close to the farm,

If navua sedge becomes established on the farm, there is currently no economical way of chemically controlling it, the farmer will have to consider the options of increased fertiliser, pasture development and rotational grazing and may have to decrease the SR.

13.1 Weed Control in Pastures

A weed is often termed a plant in the wrong place.

Some plants that in other circumstances may be considered good are weeds in pastures e.g.

Guava [Psidium guajava] see Photo 15..

Most pasture weeds are exotic [introduced from other countries] and have found a position of competitive advantage in the local ecosystem, it is this success without practical use that make them a weed.

A weed is of no use for cattle food and may even be poisonous to livestock, e.g. Milk weed

[Asclepias cutasavica]- see Photo 19.. Weeds compete with pasture plants for light, water, space and soil nutrients resulting in less food available to cattle.

Some plants which are not pasture, may not be considered weeds either, this 3rd group of plants are useful for providing beneficial effects for people and animals such as fruit, nuts, timber, shade, medicines, improved soil conditions and aesthetic values. These plants Should be maintained as part of a diverse farming system and managed as a whole to provide for the needs of the farmer on an ongoing basis.

13.1.1 Theory of Weed Control

There are many ways of controlling weeds, but in the pastoral situation there are three major factors which if adhered to will radically simplify' the job and cut costs. Good management is the key to weed control, herbicides are most often required because things have gotten out of control.

1 Maintain a vigorous pasture at the appropriate stocking rate, this will allow the pasture to dominate and smother weeds or by giving a thick ground cover, deny weeds the chance to even germinate.

2 Control weeds early. It is much easier to pull out a weed the first time it appears in the pasture [or even close to the pasture] when there are only a few. If they are left to reproduce [i.e. to set seed], there will be thousands and the farmer will have a big job to control them.

3 Follow up. Often the first effort to control a weed invasion will be a large task, after this work is done ensure that improved pasture is established and institute management practices that will minimise weed re-invasion, this should include regularly checking the area for new weeds to be destroyed and a programmed 'follow up' campaign to kill any weed which did not die after the first application.

If weeds are allowed to dominate a pasture, income will suffer as pasture and consequently beef production falls.

Often weeds are not a big problem for cattle farmers: Those who have good improved pastures [such as batiki, signal, koronivia or splenda setaria with hetero, Glenn jointvetch, centro, siratro or greenleaf desmodium] which are not overgrazed are able to spend time controlling the few weeds they have before they get out of control. A grass with a strongly creeping growth habit tends to be most competitive against weeds in a grazed situation.

- Use the correct STOCKING RATE

+ A STRONG PASTURE THAT CAN COMPETE WITH WEEDS

= THIS IS A SIMPLE EXAMPLE OF A SYSTEM IN BALANCE

When the grazing intensity is too heavy the ground is left bare, this gives the opportunity to weed seeds to germinate [this is an example of a system out of balance]. Other factors may cause the ground to be left bare, in these cases it is recommended to plant pasture cuttings or another desirable crop in the soil to compete with weed growth.

in some cases, once a weed has become established, direct action be it cutting, pulling out or use of a herbicide is the only remedy, this is often the case with woody species. Slashing and pulling weeds is often the cheapest and most effective form of weed control where management alone fails. Where it is necessary to use chemical herbicides to control a weed, as a general mile this is best done by spraying regrowth after slashing as less chemical is required and the regrowing plant is easier to kill.

When slashing or pulling out weeds, it is a good practice to destroy plants which have seeds on, this is best done by homing.

Once an area has been cleared of a weed infestation, it is essential that ground cover be re-established with an appropriate pasture grass and legume mix. Nature abhors a vacuum and if a desired plant is not planted weeds will quickly re-invade the area.

Always wear appropriate safety clothing and equipment when using herbicides long trousers and boots or shoes with socks are the very minimum, i.e. lavalava and jandals are not acceptable. Wash thoroughly after spraying. Always follow the manufacturers instructions on the herbicide pack label.

13.2 Some Common Weeds of Established Pastures and Their Control

Generally these weeds become a problem due to overgrazing and or lack of fertiliser. Some common weeds of pastures are:

Common name Botanical name

-wild peanut Cassia tora

-lantana Lantana camara

-guava Psidium guajava

-blueratstail Stachytarpheta urticifolia

-giant mimosa Mimosa invisa

-devils fig/pico Solanum torvum

-milkweed Asclepias curasavica

-mintweed , knobweed Hyptis capitata

-swordfern Nephrolepsis hirsutula

-navita sedge Kyllinga polyphylla

-honolulu rose Clerodendron fragrans

-rubber tree ficus sp

-broom weed Sida sps

-tobacco weed Pseudoelephantopus spicata

13.2.1 Control of Individual Species

Following are control strategies for a limited range of important weeds found in Pacific Island dairy pastures.

1 Wild peanut [Cassia tora] see Photo 13.

Infestations of this weed tend to have a very large store of viable seed in the ground. It appears that this seed may remain viable for a number of years.

Method of control

1 pull out plants as they appear

2 slash mature plants between flowering and pod set, and

spray regrowth after 3-4 weeks, and small plants with a wetting agent.

Herbicide Rate

-Butoxone 0.8% in water

[2,4-D+dicamba]

-Escort 0.5 g/litre water

 

 

2 Lantana [Lantana camara] see Photo 14.

Generally lantana is not a serious problem in areas where the beetle (fropiata was introduced in the 1970's which has proven an effective biological control agent. Where this bio-control is not in effect lantana is a serious bushy weed sometimes covering large areas of land.

Method of control

Slash and dig out roots, pullout any young plants. Spray actively growing regrowth.

Herbicide Rate

Butoxone 1.0% in water

3 Guava [Psidium guajava]- see Photo 15.

A very serious weed of pastures in many areas, sometimes forming impenetrable thickets of many hectares. Spreads quickly as cattle eat the fruit and distribute the seeds in the dung. It appears that seeds are also distributed by birds.

Method of control

Management- the farmer should attempt to stop spread of the weed by not allowing cattle access to unaffected areas if they have grazed areas where fruit are available, Interventions- slashing to ground level will promote better pasture growth and will slow the spread of the weed, however it will not kill the plant and must be done repeatedly. Some people report that cutting guava to ground level and then covering with soil is effective. There are several effective means of herbicidal control using triclopyr [as the butoxyethyl ester] this is marketed by Dow as Garlon and Grazon and has 600 g/litre ai.

l basal bark application, apply diesel mixture to the bottom 30cm [from ground level] of all stems of the plant, ensuring that an even coat around the tull circumference has been achieved. This is very quick and effective, requiring limited labour and as the plants are treated in-situ, access to the area is not hampered by slash on the ground. Application can be with a knapsack sprayer or paintbrush.

2 cut stump application, apply diesel mixture directly to the remaining stump immediately [within 15 seconds] after cutting the plant down close to the ground. Whilst requiring more labour for cutting the plant down, this is about twice as economical in its use of herbicide. Application can be with a knapsack sprayer, paintbrush or pump-spray bottle.

2 stem injection, after making a cut into the stem of the plant about 50cm above ground a measured 2.5m1 of diesel mixture is poured onto the blade of the bush knife before it is withdrawn from the cut. Iii this way the chemical is taken directly into the plants system, via the fresh cut. Whilst successful results have been obtained from this method, more work needs to be done to fine tune recommendations. It appears a method with great promise for dealing with larger plants and should prove very economical.

Herbicide Rate

arlon/Grazon 5% in diesel

600g/l triclopyr

4 Blue ratstail [Stachytarpheta urticifolia] see Photo 16.

Method of control

Pull plants out where possible, if there are many plants spraying may be necessary. Young plants of less than 0.5m can be sprayed directly whilst larger plants should be slashed and the actively growing regrowth sprayed [after about 3-4 weeks]

Herbicide Rate

Butoxone 0.8%

5; Giant mimosa [Mimosa in visa] see Photo 17.

In a pasture situation, slashing will give adequate control in some countries whilst in others a herbicide is required. In Samoa appears that grazing and physical damage by cattle keep this weed in check, also the author has noticed loss of vigour in some stands of Mimosa invisa accompanied by many small [@I nim] black lesions on the stem which suggests that some biological control agent is at work.

If spraying is necessary, the following are recommended:

Herbicide Rate

Butoxone 0.8% in water

Escort O,25g/l in water

 

 

6 Devils fig/pico [Solanum torvum]

This is found most often in seasonally dry areas, where it can form quite dense thickets and

radically reduce available pasta re.

Method of control

Pull out plants where possible. Slash to 0.75m and spray actively growing regrowth.

Herbicide Rate

Butoxone I .0% in water

7 Honolulu rose [Chlerodendron fragrans] see Photo 18.

This is a very serious weed in pastures throughout many parts of the Pacific. It is remarkable in its ability to spread vegetatively as it bears no seed. Once established by an accidentally introduced root cutting or by spreading from neighbouring land honolulu rose spreads rapidly by stolons. It rapidly forms a dense thicket of approx. 5m high smothering all plants of lesser height.

Method of control

A Where there are only a few plants, pull them out and plant the area with grass such as batiki or signal. This is best done before the weed actually spreads in the pasture area.

B If the area is to large to hand pull, herbicide may be needed. Slash plants to 15cm

2 Wait until it grows back to 0.4m -0.5m and leaf'.

3 Spray with herbicide.[as below]

4 3 - 4 days post spraying plant a pasture grass such as batiki or signal [and legumes] on

the sprayed area. This is most important when controlling weeds, using any technique you

must replace the weed with a useful plant otherwise the same or another weed will grow back.

Follow up is essential to ensure that all plants in the treated area are killed.

Another promising variation is to plant the pasture grass at the time of slashing, then when spraying is carried out pasture will establish more quickly. Because of the nature of the herbicides used, legume planting must be delayed until after spraying. It is reported from Fiji that painting stumps with diesel straight after slashing is effective in killing Honolulu rose,

Herbicide Rate

Butoxone 1.0% in water

Escort 0.5 g/l in water

 

8 Milk weed [Asclepias curasavica]-see Photo 19.

This weed is poisonous to cattle although the author has no record of reported stock deaths

from eating the plant in W Samoa.

Method of control

Pull it out.

9a Mintweed [Hyptis capitata]- see Photo 20.

This weak perennial heavy seeding weed spreads very quickly and is unpalatable to cattle. It forms dense thickets to 2. Same height and spreads rapidly high rainfall areas.

Method of control

I Remove any plant as soon as it is seen. Pull them out or dig them up.

2 With larger areas, slash to 15cm and spray the regrowth after about 4 weeks.

3 In an established pasture, slashing as low as possible followed by the application of fertiliser [100kg/ha banana fertiliser has been effective] and withdrawing the area from grazing for 4 - 6 weeks to allow the pasture to smother further mintweed growth.

It is vital that after pulling or spraying mintweed pasture be planted on the bare ground e.g. batiki [+ legumes]

Herbicide Rate

Butoxone 0.8% in water

 

 

 

 

9b Hyptis pectinata also sometimes called mintweed

This is a close relative of H capitata it is it can be easily distinguished by its smaller leaves,

more slender growth habit and different seed head. It is a common weed but more easily controlled than H capitata, generally slashing or pulling [before seed is set] will give good control.

10 Swordfern [Nephrolepsis hirsutula - see Photo 21.

Generally a sign of overgrazing, lack of fertiliser and or low soil pH. Method of control

Slashing and planting batiki grass whilst [at least temporarily] reducing stocking rate is the most effective short term practice. In the longer term use of fertiliser, increasing the soil pH by spreading coral sand and setting an appropriate stocking rate are recommended. No herbicidal treatment is recommended as swordfern is always associated with problems associated with poor management and fertility.

11 Navua sedge [Kyllinga polyphylla] see Photo 22.

This is a very serious and difficult to manage weed in pastures of higher rainfall areas. It spreads quickly by seed and stolons, and is very difficult to eradicate by any method once established.

Method of control

1 Small areas should be dug up and one should ensure that the roots dry and die [or are burnt], plants in common ground, access roads or neighbours land should be destroyed before they spread into the pasture.

2 Rotational grazing with a rotation length of approx. 50 days, fertiliser [100 kg/ha banana fertiliser] and reduced stocking rate will allow a batiki pasture to recover from virtually complete domination by navua sedge in 6 - 9 months.

3 Chemical control is appropriate for small areas, often 2 applications of glyphosate [Sting/Roundup] is necessary. This will also kill the pasture so it Should be replanted. Spraying should not be delayed, but is most effective when 1/3 of plants are flowering and actively growing.

Herbicide Rate

Sting 2.0% in water

4 in Fiji, planting of splenda setaria has successfully smothered navua sedge - producing a highly productive, weed free pasture.

12 Tobacco weed [Pseudoelephantopus spicata see Photo 23.

This plant generally becomes dominant in seasonally dry areas where overgrazing is practised.

Method of control

Reduce stocking rate

and plant improved pasture such as signal, koronivia, Bisset creeping blue grass, or [with legumes]

Should the use of a herbicide be chosen, steps 1 & 2 should be followed afterward.

Herbicide Rate

Butoxone 1.0%- 1.5%

13 Rubber tree [Ficus sp]

This should be treated in the same way as guava: Psidium guajava, although it is not necessary to limit the movement of cattle which have had access to the plant as the plant is not palatable and cattle do not appear to have any role in the spread of seed which is wind born and produced in huge numbers.

14 Broom weed [Sida sps] - see Photo 24.

There are 3-4 similar species of economic importance. They should be treated in the same way as blueratstail: Stachytarpheta urticifolia.