Herbicides and

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HERBICIDES AND 

M.P. 

In the battle against weeds one can either try the Father Christmas

approach -hoe, hoe, hoe,   or try herbicides However, before beginning on

the herbicide approach it must be stated that with increases- in the price of

herbicides, and awareness of associated problems including weed tolerance,

hoeing and spot-spraying should be a more common practice.

The weed problem is such that one in ten 

 seed crops are

rejected or downgraded because of weeds. This is a massive mortality rate.

Seventy-five percent of all rejections and downgradings are because of weeds;

they are a major 

 in seed production. Wild oats are a major contributor

to the problem, also soft brome (also known as goosegrass) particularly when

over-threshed; bearing this in mind headers should be adjusted to’ keep awns

intact.

Crops may be rejected from certification after field inspection. Of those

that reach Seed Testing, wild oat contamination results in 5.5% of perennial

 crops being rejected and 

 1% of annual 

 crops being

rejected. If ‘undesirable’ weeds, of which thirteen are specified, are found within

a seed crop, financial penalties ensue. The undesirable weeds include barley

grass, wild oat, and winged and nodding thistle.

An analysis of 1800 seed lots using Ministry of Agriculture and Fisheries’

seed testing records indicate that after dressing 77% of all 

 seed crops

contain goosegrass, 40% contain hair grass 

 spp.), ‘40% contain annual

poa, 35% contain chickweed and 29% contain field madder.

The range of herbicides actually registered for use in seed crops is

extremely small. In 

 United States unregistered products cannot be used;

in 

 legislation is loose but becoming tighter. Of the six herbicides

that are registered for use in grass seed crops four are for broad-leaf weeds

and two are for wild oats, 

 there are no chemicals registered for use against

grass weeds, which are the major problem in grass seed 

 Nortron or

 (ethofumesate) is very effective but very expensive   at over $200 

it is not considered to be economic. Preliminary trial results for TCA (one of the

oldest herbicides) or TCA dalapon mixtures (e.g., 

 are promising in terms

of being able to control annual grasses, but the chemicals are unregistered (and

now unavailable). Furthermore, most research work has been done in the North

Island. As environmental conditions do influence the effect of herbicides they

should be applied only in small amounts initially (i.e., on test strips) in order to

avoid the possibility of destroying the crop.

Test strips of the herbicide at

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different rates and times will allow the grower to get a feel of the chemical in his

own environment.

Many broad-leaved weeds are developing a tolerance to common

herbicides such as 2,4-D and MCPB. On grass seed crops most of the

hormone sprays registered for cereals can generally be used safely (but

dicamba cannot be used on 

 seed crops without detrimental effect).

Non-hormone sprays cannot generally be used (e.g., Glean used on 

seed crops can result in severe damage).

Eighty to 90% of 

 seed crops are sown with white clover.

Although this practice may reduce the amount of nitrogen required by the crop,

it creates limitations in terms of what herbicides may be used. Most sprays that

will tackle problem weeds such as nodding thistle and yellow gromwell will be

extremely damaging to the clover.

 order to have a more flexible spray

programme clover should not be sown with 

 seed crops. A further

consideration is that white clover is extremely competitive, and it probably

decreases seed yield from the 

 crop.

There is more flexibility in herbicide use in established crops, i.e., those

more than one year old (e.g., tall fescue, cocksfoot and second year 

crops). Atrazine is used widely in Palmerston North but it is an unpredictable

chemical and results can be spectacularly good or bad   it is very easy to

destroy a crop completely. The. chemical works because the crop is deeper

rooting than the weeds that are being sprayed out. Atrazine, simazile and

diuron are root-absorbed. When a small dose of chemical is applied, shallow

roots absorb more than deep roots, thus tall fescue, which is deep rooting, can

survive. Atrazine is applied in late autumn/early winter when low light intensity

means the plant is less active than otherwise but soil moisture is present to

activate the chemical. Use of these products must be timed with irrigation or

to coincide with rain (they can even be applied during rain) so that the chemical

is washed off the leaf into the soil. If allowed to sit on the, leaf for a long time,

particularly when morning dews are followed by bright, sunny days, crop

damage and even crop failure will occur. Thus the products are difficult to use

effectively and require 

 For instance on light sandy soils atrazine

rates must be reduced considerably. In fact, as atrazine water solubility is 40

ppm and simazine water solubility is only 5 ppm, simazine is safer to use on

light 

 than atrazine. The drawback is that established weeds will not be

killed. The difficulties with atrazine use cannot be over-stressed. In Oregon,

where the product was developed, an uncommon winter drought (which meant

that spray was not washed off leaves) resulted in 70% of the 

 crops

being damaged. Since then the registration for use of Atrazine and Simazine

in 

 crops has been withdrawn in Oregon, This has resulted in problems

with maintaining quality of grass seed, and in increased research on alternative

measures.

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Volunteer 

 is a serious problem in tall fescue and can be in

cocksfoot. 

 or Hoegrass-atrazine mixtures has been effective in

cocksfoot and the mixture has proved more effective than 

 alone.

Atrazine can be effective if the timing is right. Kerb has been used successfully

by some people, 

 is not a recommended chemical. Alloxol or Fusilade can

 used on Chewings fescue as they kill almost all other grasses.

control in tall fescue is very difficult and 100% control is impossible to achieve.

However, the correct use of atrazine can 

 some control.

In all this it should be remembered that planning a crop and herbicide

rotation can alleviate some build-up in a weed problem. Ultimately this forward

planning can reduce expenditure on chemicals and increase profitability.

Fertilisers

Talking about the use of fertilisers in seed crops opens up a 

 as

there is much divergence of opinion. Many of the problems lie in interpreting

research data because in the past researchers have not defined the site being

worked upon. This was particularly the case in early nitrogen (N) trials as the

residual N status of the soil was not known.

In general it is possible to say that autumn N applied to first year 

does not result in increased seed yield but does produce winter grazing for

sheep. No more than 

 of the total N for a seed crop should be applied

in the autumn. Spring N is important for increased seed yield.’ It should be

applied in the 

 to four week period between the period of 

 initiation

and stem elongation. A split application is not necessarily important in terms

of increased seed yield, but does spread the risk in terms of leaching loss (in

the event of high rainfall). Elongation is controlled by photoperiod and, to a

lesser extent, temperature.

This means that calendar date can be used to

determine when N should be applied: For 

 it should 

 applied in 

September. Annual 

 is late flowering and N should be spread later.

 flowers much later and so N can be delayed. Applications of N after

flowering are not generally effective but may increase thousand seed weight.

The estimated total N requirements for a seed crop is 130 kg ha” (equal

to 280 kg urea ha“). The amount applied as fertiliser should be the difference

between soil N and 130. Traditionally, Oregon and Denmark have used more

N and achieved higher seed yields than New Zealand. Over recent years more

nitrogen has been used in New Zealand, too, and more growers are taking soil

tests to give them an indication of residual N.

In general the type of N that should be used is the cheapest. However

it should be remembered that there are limitations-to the use of urea particularly

on very dry soils or if soil temperatures are above 15°C (when volatilisation

losses occur).

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Potassium is removed from seed crops in large quantities and so growers

must be aware of their soil type.

Fifty percent potassic serpentine super

matches losses in seed crops more closely than ordinary super, but in most

cases potassium and phosphorus applied to seed crops do not result in

increased seed yields,

Soil acidity is not usually a problem in seed production except with prairie

grass; the biggest problem is overliming leading to trace element deficiencies,

e.g., in zinc.