WEEDS

New Zealand Journal of Agriculture, Volume 67, Issue 1, 15 July 1943, Page 15

WEEDS

Much Farm Land Has Deteriorated Through Lack of Control

-By-

S. H. SAXBY,

Instructor in Agriculture, Dunedin.

THE term “weed” has many definitions, which quite naturally vary with, the outlook of the persons responsible for their, definitions. One fact that all definitions imply, even if they do not express it, is that, to a farmer a plant is a weed only when its presence interferes with farm production to such an extent that production is lowered. In other words, a plant becomes a weed only when its presence results in financial loss. This definition is necessarily very broad, because, until one knows the circumstances in which a plant is growing, it is impossible to say whether it is a useful plant or a weed. For example, gorse in a well-trimmed hedge may nearly always be regarded as a useful plant. On the other hand, where it has virtually taken charge of much of our second class hill country, gorse is undoubtedly a very bad weed. Similarly, some of the thistles are useful plants when growing on poor country,. but are weeds, on firstclass country. Four Major Groups Weeds may be divided into four major groups, these, groups indicating the manner in which the various weed plants are responsible for loss in farm production. The first and most important is that large group of plants which are weeds because they compete. with, and tend to dominate more desirable plants. Into this group fall most weeds of arable crops and pastures. The competition exerted by these weeds varies considerably according to the type of crop, the climate, the nature of the soil, and the kind of weed. On the whole, however, perennial weeds are more serious in perennial crops, the most important of which is pasture. Such perennial weeds are rushes, manuka, blackberry and fern. Simi-

larly, annual weeds are most harmful in annual crops and young pastures. Plants of this type are fat hen, spurry and fumitory. There is, however, a group of perennial weeds which is equally harmful in both annual and perennial crops. This is the group known as “twitch” or “couch.” Although these terms usually refer to some half a dozen or so rhizomatous grasses, they could well be enlarged to include all those plants which have creeping underground or overground roots or stems, portions of which readily form new plants when broken off from the parent plant. Such plants are Californian thistle, sheep’s sorrel, yarrow, creeping fog and onion twitch. The second group of weed plants are parasitesthat is, they derive their nourishment entirely from other plants and not from the soil and air, as do most plants. Fortunately, only two such plants are at all common in New Zealand. These are dodder and broom rape. Of these, broom rape is the more common, but appears to do comparatively little harm. On the other hand, considerable damage is periodically done by dodder. The third, group of weeds consists of plants of which various parts cause direct damage to and reduction in value of livestock products. The best known example of this group is bidi bidi, which apart from taking up valuable ground in the pasture . is; responsible for lower wool values. Several other “burr” plants have a similar effect. In

addition, the seeds of some plants, notably spear grass and barley grass, actually penetrate the skin of sheep, reducing the value of both the skin and the carcass.

Yet another type of plant which reduces the value of livestock is that which taints milk. Many plants, varying from turnips to white clover are blamed from time to time as being responsible . for tainted milk. Frequently, when tainted milk is produced a search is made and the most conspicuous . and likely looking plant. is blamed, often quite wrongly. There are, however, a number of plants which unquestionably produce tainted milk. The most , glaring example is wild garlic.

The fourth group of weed plants is that which embraces those poisonous to livestock. Exact information on this subject is hard to obtain, and a large number of cahes of reported poisoning are a matter of opinion rather than of certainty. Many plants which are known to contain certain poisonous substances are often of common occurrence in pastures, and yet stock thrive on those pastures. In farm animals certain species, and individuals within a species, show marked differences in susceptibility to plant poisons is the toxic dose of certain poisons varies considerably from animal to animal. In addition, the chances of animals obtaining a toxic dose of plant poisons also vary.

Poisonous Properties P ■ ... On the whole, therefore, the poisonous properties of many plants vary with both the animals concerned and the conditions under which the plants are eaten. For example, sheep s sorrel is not infrequently referred to as being poisonous to sheep, yet there are districts where sheep depend to a large extent on sheep’s sorrel for their fodder, and do well on it. Similarly, the

native tutu plant has a bad reputation as a poisonous plant, yet many run cattle have access to it all their lives. whether they are poisoned or not depends i ar g e ly O n both the condition they are in and on the extent to w hj c h they are rorced to feed on it through lack of other feed Another example is that of ragwort, the poisonous effect of which is cumulative. For this reason it is most

harmful to animals, the life of which farm practice decrees shall be long. Hence, horses and cattle have greater opportunities of becoming affected than sheep or pigs. Sheep are certainly sometimes seriously affected, but this may be obviated by judicious management. . , In some publications dealing with plant poisons long lists of plants which are reputed to be poisonous are given,

but if they were always as poisonous as stated, there is no doubt that the mortality of stock through plant poisons would be infinitely higher than it is.

Some of our worst weeds are plants which were deliberately introduced into the country from a utility or sentimental point of view. Plants such as blackberry, sweet briar and gorse are introduced plants that have “escaped.” If it had been possible to keep these plants either out of the country or at least within their desired confines they would not have presented the major problem that they do now. On the other hand, the greatest number of unwanted plants were introduced accidentally in the early days of colonisation. Many came as impurities in agricultural seeds, some came in the wool of imported sheep, and yet others arrived in packing and ships’ ballast. In addition to the imported weeds, there is a considerable number. of native plants which, as soon as farming in this country began, immediately became weeds. Of these, the most striking examples are bracken fern and bidi bidi. Further, there are innumerable other native plants which constitute the secondary growth problem in bush areas throughout the country. Means. of Dispersal In considering the subject of weeds the question of means of dispersal is of some moment. Once introduced into a country, many plants will, if allowed,

spread rapidly by a number of means. During the last hundred years or so some 400 species of plants have been introduced and have become fully naturalised in one or more parts of the country where conditions are suitable to them. Of the various ways in which seeds of weeds have been dispersed within the country, that of impurities in agricultural seeds is probably the most common. Although the dressing of agricultural seeds has now reached a high standard, this has not always been the case, and the trading from one part of New Zealand to another of weed-infested seed doubtless spread the weed population far and wide. Even at the present time, the distribution of cheap pasture seed mixtures means the distribution of many weeds. Wind plays an important part in seed distribution, especially with plants of the thistle family, whose seeds may be

carried many miles in * this manner. Animals, particularly sheep, collect a large number of seeds in their wool and hair. When the animals are transported the seed goes with them, to fall to the ground, perhaps many miles away, and to start a new colony of weeds. There is little doubt that much St. John’s wort has spread in this manner. Birds also play an important part in weed distribution, of which the most notable example is that of the blackberry. Agricultural implements such as threshing mills and other harvesting machinery, also spread weed seeds when travelling around the country. Cost of Weed Control The whole question of weed control is centred around cost. Everyone recognises that weeds are harmful and everyone appreciates the’ fact that the

cost of eliminating or controlling a particular weed cannot be more than the value of the increased production effected. For example, it is not profitable to clear gorse, manuka, or secondary growth, if the cost of it is greater than' the return that will be obtained from the extra stock carried. This is a salient feature of the farming of “marginal land,” where cost of clearing and the return from it are almost balanced. If the cost of permanent clearing can be substantially reduced, or if the value of returns are increased, the operation at once becomes profitable.

Similarly, it is well recognised that careful cleaning of mangel, carrot and

turnip crops is profitable because the cost of cleaning is more than balanced by the extra yield obtained. Method of Control Annual weeds may be controlled in several ways. Firstly, there are many weeds, such as spurrey, fat hen and wireweed, which germinate best in the spring, and consequently affect crops sown at this time. If possible, paddocks known to be infested with weeds of this type should be sown in the autumn rather than in the spring. If, however, it is essential to sow in the spring, as is often the case, a considerable measure of control may be effected by completing the main cultivation operations in ample time before sowing.' Rolling and harrowing several times before sowing will do much to germinate and destroy a large proportion of weed seeds in the top soil. Care should be taken to ensure that cultivation at this stage is not deep. Grubbing, for example, will only bring innumerable seeds to the surface, and the previous work will 'be undone. Once established in root crops, weeds may be controlled only by repeated hoeings when the weed plants are

small. This operation should, of course, be carried out on a hot dry day, if possible, in order to obtain a good kill. In young pastures, annual weeds such as spurrey or fat hen may be kept within bounds by the use of the mower or with sheep. Many means are adopted to control perennial weeds in pastures, each method depending on the type of weed, the extent of infestation and the value of the land. In some cases, grubbing by hand is an economical proposition, in others it is necessary to burn, plough, or employ heavy machinery. Weed Killers During the last few years much has been accomplished in the use of chemical weed killers. Of these chemicals, sodium chlorate and arsenic pentoxide have given the best results. The use of these chemicals will be referred to when-specific weeds are being discussed. The question of the control of weeds by insects has received a considerable amount of attention during the last few years. In some cases such as with prickly pear in Australia, this method

of control has given spectacular results. This, however, is, exceptional, and undue optimism with regard to weed control by insects in . New Zealand is not warranted. Complete control is unlikely, and any control that is effected must be regarded as supplementary to existing means: Insects have been introduced for the control of weeds such as gorse, bidi bidi and ragwort, and while some success has already been obtained, it is too early to be dogmatic regarding the ultimate effect of any of them. In the control of some weeds, such as blackberry and ragwort, animals play quite an important part. In addition to the . foregoing, there are several other ■ aspects which play an important part in weed control. Of these, drainage, lime and fertilisers are all of considerable importance. Many weeds, such as rushes and buttercups, may be eliminated at least partly by efficient drainage, which also has the effect of strengthening the desirable plants. , Liming and manuring also tend to banish weeds such as sorrel and spurrey, not because lime and manure kill them, but because the soil conditions are so improved that useful plants will thrive and ultimately crowd out the weeds. Suitable Species The question of the use of suitable species is also . closely connected with that of weed control in pastures. On heavy land,' for example, the inclusion of foxtail and timothy in mixtures will do much to arrest such weeds as docks and thistles. On the other hand, the use of browntop and lotus major on bush burns will at least assist in the control of secondary growth.

The various strains of pasture plants play quite an important part in weed control. Some strains are not aggressive and die out rapidly, allowing weeds to gain entry into the sward. Other strains are aggressive and per-

sistent, thus checking the ingress of weeds.

The necessity for sowing seed of a high standard of purity cannot be over-emphasised. It is not only a matter of securing seed with a low percentage of impurities, but also of knowing what these impurities are. For example 1 per cent, of suckling clover in a sample of white clover will do little harm, but a much smaller percentage of dodder in the same sample may cause considerable damage.

One important aspect of weed control which is not given the consideration it warrants is that of the eradication of weeds new to a district. Generally speaking, a farmer should look with suspicion on any unfamiliar plant he finds on his property. _ He should have it identified, and if the plant has proved to be a menace elsewhere, every effort should be made to eradicate it at once. It is surely easier to destroy a weed species occupying a square yard of ground this year than to wait and eradicate it when it has covered an acre or more in several years’ time.

Names of Weeds

All plants have what is known as a botanical (or Latin) name, by which they are known to botanists in every country. “Common names,” however, vary, not only from country to country, but also from district to district. Thus, the plant Plantago lanceolata in some parts is known as plantain, while in others it is called rib-grass, lambs’ tongue, or soldiers. The position is similar with the naming of many of our weeds. Confusion, however, frequently arises when one name is applied by different people to different plants. For example, the name “wart cress” is used commonly in connection with at least four different plants. For these reasons heated arguments often arise regarding the names of weeds. In many cases both parties are quite right in that the name by which they know the plant is the one most commonly used in the district. There does not appear to be any way of obtaining uniformity of “common names” for plants. Old names die hard, and a change of name is distasteful. Botanical names are frowned on by most people, perhaps not without some justification, as many are tongue twisters. There are, nevertheless, many botanical names that are in common use by farmers and gardeners, and include such as convolvulus, paspalwn, phalaris, Lotus major, Eschscholtzia. For the foregoing reasons, it cannot be hoped that the names of every weed described will meet with the approval of everyone. An endeavour, however, has been made to use the commonest and most descriptive name in each case. Where several names are in common use these are all given. (To be continued.)