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This article was published in 1950
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The Mint Weed Menace

F.L. CLARK. B.V.Sc., Inspector of Stock, Warialda

INTRODUCTION

Mint weed, Salvia reflexa, is a native of the North American Continent. It is a free-seeding, summer growing annual plant. It was first observed in Australia growing in the Pittsworth district of Queensland early in this century and it has been suggested that it probably was introduced into this area as a contaminant of fodder during the 1902 drought.

In America, mint weed occurs in Mexico and U.S.A. Although it apparently has caused some trouble in America it is not regarded as a serious weed. Ideal rainfall and temperature conditions are found in certain parts of Australia for the rapid spread of this weed. In Australia the suitable growing season is longer than in America and more germination of seed are seen in the average season.

In Australia a large percentage of properties has been damaged by consistently overstocking with sheep and/or rabbits. The vitality of the pastures in these areas has been lowered and in some cases the better grasses have been replaced by poorer species. This overstocking with sheep, rabbits, and in most cases both, has reduced pasture cover and has resulted in ideal conditions existing for the spread of Mint weed and other weeds.

PRESENT DISTRIBUTION OF MINT WEED IN AUSTRALIA

Generally the distribution of Mint weed in Queensland is on the Darling Downs and in the Clermont-Springsure districts with localised areas of infestation in other areas in south-western Queensland.

From Queensland this weed has extended into the north-western slopes and adjacent plain country of New South Wales. Mint Weed also has been observed as far south as Victoria.

In the Warialda Pastures Protection District, where Mint weed first came into prominence in New South Wales, it appears that it was introduced first into the Graman, North Star and Coolatai areas at least fifteen years ago. The weed apparently was introduced as a contaminant of fodder introduced from Queensland. The spread of Mint weed was at first not rapid but an extensive invasion occurred during the 1946 drought, and three years later it was estimated to have infested two million acres of first class grazing and farming country in the Warialda, Inverell and Tenterfield Pastures Protection Districts. Serious infestations of mint weed also occur in the Coonabarabran and Coolah district. Mint weed also has been found in the Tamworth, Narrabri, Moree, Merriwa and Denman districts.

Mint weed has shown a decided preference for the self-mulching brown to black soils. It has been observed growing also on some areas of red loams. Mint weed does not seem to grow to any extent on granite, poor quality, light coloured shales or sandy soils. Thus it can be seen that this weed is a menace of some of the best soils of New South Wales; generally of basaltic origin.

METHOD OF SPREAD

Mint weed appeared to have spread mainly from Queensland into New South Wales as a contaminant of summer cereal crop seed and grains and hay for hand-feeding of stock.

A survey of summer-crop seed from Queensland was undertaken in 1947; when a considerable percentage of samples was found to be contaminated with Mint weed seed. In one case 4,800 Mint weed seeds per pound of summer-crop seed were found. As Queensland has been the main source of supply for seed for summer cereals such as Japanese millet, Hungarian millet, Sudan grass and grain sorghum, the rapid dissemination of Mint weed by this means is easily understood.

Observations by graziers in the Warialda Pastures Protection District have shown that where stock were fed hay and grain from Queensland, Mint weed was noticed in several instances to grow on the ground where the feed had been thrown.

Travelling stock which had come from or passed through infested areas in Queensland probably also helped distribute Mint weed seed into New South Wales, but it is thought that distribution by this means would not be nearly so great as the spread by contaminated crop seed and fodder. Travelling stock spread the weed by carrying the seed in mudballs on their hooves; and in the case of sheep in the wool as well.

Other methods by which Mint weed can be spread, especially once it has become established in a district, are by rabbits, water wash, and by vehicles and road making plant. Rabbits can carry Mint weed in their fur and on their feet. Mint weed is carried by water wash along creeks and water courses and from higher infested areas into lower paddocks. In infested areas the plant can be seen following roads and car tracks. Shires in these areas help to cultivate it by grading off the surface grass cover at the sides of roads.

The danger of Mint weed entering pastures is especially great in areas where the surface cover of grasses has been denuded; such as around watering places, tracks, sheep camps, etc.

Extensive infestation of Travelling Stock Routes occurs along with the spread of Mint weed over pasture lands. Areas where T.S.R s. are liable to be heavily stocked, such as around Public Watering Places and corners suitable for stock camps, are especially prone to infestation. At present the T.S.Rs. most heavily infested are those between Warialda and Coolatai and Warialda and Inverell. Isolated heavy patches of Mint weed are present in T.S.Rs. in the Croppa Creek area. Infestations of Mint weed are seen following the edges of formed roads in the black soil areas where the surface cover has been destroyed. It seems only a matter of time before the infestation along roads gradually will extend to involve T.S.Rs. and holdings which these roads intersect.

In infested areas cultivation for winter crops such as wheat has resulted in infestation of cultivation paddocks which are left bare in the summer.

ECONOMIC IMPORTANCE

There is no doubt that Mint weed is of serious economic importance in infested areas; the salient features being:-

1. Mint weed is no value to stock as feed as they do not normally eat it. By replacing natural grasses it seriously reduces summer grazing. Many paddocks can be seen where large parts are almost one hundted per cent. Mint weed. As infestation is in most cases a symptom of poor land management, with overgrazing, once infestation occurs on a property there is a tendency in many cases to an increase of overstocking owing to the reduced amounts of summer feed available for stock. This leads to a rapid increase of pasture damage and Mint weed spread.

2. Mint weed has been suspected of being poisonous for stock for a long period. Considerable stock losses, some very heavy, have been attributed to it. The poisonous principle has been found to be the high nitrate content of the plant: this nitrate content being sometimes as high as 5% on a dry weight basis.

Losses are seen generally in hungry travelling stock or in stock recently introduced from clean properties on to heavily infested holdings. The plant has a strong smell and usually is not eaten by stock grazed on Mint weed infested country. However, losses have occurred and wherever this weed is present it is a potential killer. Experience in this district has indicated that stock are more likely to eat the weed when it is dying off in the autumn. This is probably due to the reduction in the strong smell of the weed at this time. Travelling stock usually are affected in dry summers when other feed is scarce on T.S.Rs. The symptoms shown by affected stock are typical of nitrite poisoning.

3. Mint weed interferes with production and distribution of summer cereal crops. The growing season for summer cereals coincides with the growing season of mint weed. In the early stages Mint weed is often able to dominate the cereal although eventually the cereal grows above it. Sales of contaminated summer cereal crop seed has been a vital factor in the rapid spread of Mint weed. Sale value of contaminated crops is reduced and the seed is listed under the Agricultural Seeds Act which prohibits the sale of any seed containing Mint weed.

4. Prejudice against stock from infested areas is mounting and is resulting in a lessening in demand for stock, especially sheep, from infested holdings. Buyers are in many cases fearful of introducing the weed on to clean holdings.

METHODS OF CONTROL

Prevention is better than cure. Mint weed will not compete with vigorously growing and carefully grazed pastures. Once Mint weed has become established in a weakened pasture dominance by the weed is assured unless every effort is made to develop and preserve a healthy, vigorous sward of native grasses. This means that stocking must be greatly reduced and rabbits controlled. Unfortunately, once Mint weed has become the dominant species in a pasture effective control is not likely to be achieved by understocking alone. Mint weed is a prolific seeder and owing to the mucilaginous coat over its seed is able to germinate rapidly after light showers of rain; and before the grasses.

Promising results have been achieved by sowing of certain grasses in paddocks heavily infested with Mint weed. The grass with which most work has been done at present is Liverseed grass. This is an annual and is able to germinate early in the Spring and offer considerable competition to Mint weed, especially in wet seasons. Liverseed grass is, however, practically useless for winter feed and leaves the ground bare at the end of winter. The grass being an annual has to compete again with Mint weed the following spring.

Giant Panic, a perennial grass which provides a permanent ground cover and fair winter feed, also has shown indications that it may be useful in controlling Mint weed. This grass, however, is difficult in many cases to establish from seed. Crested Wheat Grass also has been tried and has shown promise.

Hormone-type weedicides are effective against Mint weed. These weedicides, although they kill the weed, do not injure the grasses and so are a considerable improvement on the older types. The use of weedicides usually is governed by economic factors. Mint weed seed in the ground will germinate with every shower of rain during its growing season and the young plants start seeding very early in their life. Thus, effectively to control Mint weed by spraying alone in areas where Mint weed is the dominant pasture plant a whole series of treatments would be needed at a cost of about 10/- an acre per treatment. Thus, spraying alone is unlikely to be the solution except in isolated small patches in otherwise clean country, around camps and watering places and along roads.

A combination of sowing of grasses and spraying the first crop of Mint weed to allow the slower germinating grasses successfully to compete with Mint weed may offer a sounder line of control.

In areas where Mint weed is only scattered and has not started to dominate pastures, understocking and removal of rabbits will allow the grasses more vigorously to compete with the weed and control it. This has been observed on several holdings in the Warialda district.

DISCUSSION

As can be seen from the above the best method to prevent the spread of Mint weed is to maintain a good pasture cover. Control measures are very difficult on heavily infested land which cannot be cultivated and sown down again with grasses. Control of Mint weed on T.S.Rs. involves special difficulties. Boards have not sufficient funds to engage in vigorous control campaigns. T.S.Rs. are generally unsuitable for sowing down with grasses. The rate of stocking on T.S.Rs. cannot be controlled except by closing them altogether to travelling stock, which is generally not practicable, or by fencing off heavily infested areas, which would involve great cost. In dry times, when the maintenance of good grass cover is most essential to beat Mint weed, it would be impossible fully to police these closed areas of T.S.Rs. for trespass. Boards should take control measures in certain instances. Where an isolated patch of infestation occurs on a T.S.R. in an otherwise clean area, efforts should be made to deal with it. Action also should be taken around infested watering places.

It can be seen that the problem is a serious one and urgent control measures are required. The immediate task should be not so much to eradicate Mint weed but to prevent infestations from becoming worse and to prevent spread on to clean areas. Finance should be made available to Shires and Pastures Protection Boards to enable them to control Mint weed on roads and T.S.Rs. It also appears that eventually some assistance must be given to owners of infested holdings. The general opinion it that Mint weed is beyond the control of the individual; and except in instances very little effective work is being done. Authorities agree that Mint weed is a potential menace over a very large area of fertile soils in this State. Urgent co-ordinated effort is needed to deal with this menace.

 


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