Nassella Seedling Control

Press, Volume CII, Issue 30158, 15 June 1963, Page 7

Nassella Seedling Control

[Written by

W. F. LEONARD,

>, Senior Scientific Officer, Department of Agriculture, Christchurch)

A single nassella tussock plant can produce more than 100.000 seeds

in one year. Spaced 12in | apart these seeds would produce a row of nassella tussock almost 20 miles in length—all from a single plant in one year. Unlike the seed of some ; grasses which remain viable |in the soil for only a year !or two, nassella seed has a 1 relatively long life. Just how long the seed retains I its viability is not known, j but buried seed has survived I at least nine years and could possibly last a good deal longer.

Land which has been heavily infested for several years must therefore have built up a tremendous population of nassella tussock seed. Some idea of seed numbers can be gained from small cores of soil taken from hiU country near Waipara. The soil cores were an inch in diameter. The seed present ' was extracted and counted at the seed-testing station at Palmerston North. The number of seeds in each core varied widely, but in the top two inches of soil several cores each contained \ more than 100 fully formed nassella seeds. Such a denI sity ever one acre would i yield enough seed to plant ! a row at 12in spacing which I would stretch more than six times round the world.

Obviously then the killing of existing nassella tussock is only the first step in control, though an important step of course. Before we can gain anything from studying the longevity of nassella seed the formation of new seed must be stopped. But the accumulation of seed in the soil represents a constant threat of reinfestation.

The provision of a dense ground cover of desirable plants is a form of weed suppression practised on almost every New Zealand farm. On nassella-infested land. too. vigorous pasture growth is encouraged with the dual object of providing grazing for stock and suppressing nassella seedlings.

Unfortunately the dry climate under which nassella tussock thrives limits the extent to which pasture can

remain competitive. Judicious grazing, of course, helps the retention of ground cover. But droughts do come along and open the way for nassella seed to strike and become established. Reinfestation of arable land has, in fact, been so serious that the life of a pasture is often limited to two or three years. The frequent cultivation dictated by such a short rotation greatly increases the cost of farming these properties and allows little scope for the restoration of sod structure. The problem is bad enough on arable land, but on hill country reinfestation is an even greater problem. Tussock on arable land can at least be killed by cultivation; but aerial spraying of herbicide on hill country is very expensive and subject to various hazards. Pasture can be established by oversowing but, just as on the dry arable land, nassella reinfestation occurs. The prospect of having to repeat these expensive operations every few years is a dismal one indeed. Research For this reason we have been devoting close attention over the last two or three years to research into the selective control of nassella tussock seedlings in pasture. When we consider that mature nassella will tolerate up to five times the dosage of herbicide needed to kill such grasses as perennial ryegrass and cocksfoot, the possibility of selectively removing nassella from pasture seems remote.

Nevertheless, research to date indicates that by the application of low rates of grass-killing herbicides at a particular time this may be possible. The explanation, I believe, lies in the fact that a one-year-old pasture is relatively mature when compared with a nassella plant growing in association with it. Young nassella seedlings are sensitive to competition and provided the new pasture is reasonably vigorous the tussock plants are still very small 10 to 12 months after sowing.

Furthermore, nassella tussock is most readily killed in the summer and early autumn period, and this probably aids its removal from a pasture sown the previous autumn. Whatever the reason, research so far has shown that, on an experimental scale at least, seedling nassella can be selectively controlled in pasture. The low rates of application employed <e.g.. 1 to 21b of 2.2-DPA to the acre) should mean that the treatment would be a relatively inexpensive one. I must point out however, that the technique is still very much under trial. In order to facilitate assessment of results, work so far has been restricted to the use of ground spraying equipment on arable paddocks. Most of the data so far accumulated have been from plot-scale trials only. Several paddocks were boom sprayed this year, but results from these will not be available for several months. Nor do we yet know whether results from aerial application to hill country will equal those of ground spraying. Apart from the broken nature of hill country, satisfactory spray coverage of a plant such as a nassella seedling by aerial application is not easy. Trials with

aircraft were begun recently, and results are awaited with interest Should selective control of seedlings be possible, it would have a profound influence on the longterm control of nassella tussock. First, though, we must know that it works; second, whether it is applicable to hill country as well as arable land; and, third, that results are reasonably consistent from one year to another.