Herbicide resistance - what is it and how has it affected me? I will share my experience with you, firstly how I first became concerned, how the problem was diagnosed and finally then how I am tackling the problem.

I farm 1200 ha on two separate farms in the Grenfell area, running 4000 sheep and 50 breeding cows. I crop about 700 ha.

In 1984 I first noticed that ryegrass sprayed with Hoegrass was not dying the way it should. That year coincided with a change in boomspray nozzles to reduce my water volume to 45 L/ha. I therefore blamed this change for my poor results. The next year I went back to water volumes of 80 L/ha, but the result surprisingly was again poor. I suspected something was wrong but again blamed weather conditions and myself.

Not until 1986 did I realise that it was not me. In the meantime I had wasted 3 years and the ryegrass resistance had spread to the majority of my cropping country. In the spring of 1986 Hoechst technical representative from Dubbo, Sam Howard, took samples of ryegrass heads. The results were very slow in coming back and I did not receive them until after the 1987 crop had been sown. Fortunately I had suspected herbicide resistance and had sown one-third of my cropping area to pasture. Results of chemicals applied are given in Table 1.

Table 1. The effectiveness of ryegrass control by different herbicides

Overview of my problem

My biggest problem was my ignorance and the apathy of the chemical company towards the problem. After complaining to agronomists and chemical companies, I was repeatedly told it must be weather conditions or application problems. I was told more than once that if it was resistance, I was the only reported case. This did not do a lot for my confidence, but I had to keep trying. When the results of the ryegrass head samples came back it gave me something to work on. Instead of working with the unknown the problem was pinpointed.

Tackling the resistance

Firstly, I had to consider the options, which were:

1. using herbicides

2. cultivation

3. fire

4. alternate crops

5. controlled stocking rate

I decided to use alternate crops in conjunction with herbicides and fire.

Four years’ experience

Alternate crops seem to have been the correct decision. I had been growing rape and lupins for many years but weed control became a lot easier when Fusilade was released. I had not been using Treflan for many years but decided to go back to it. I found by using Treflan at 2 L/ha with proper incorporation and following with either Fusilade or Sertin-Verdict mixture gave best results.

My country is undulating and soil erosion is a major consideration. It worries me if a paddock is cultivated more than three times, so I have avoided excessive cultivation.

Fire is a useful tool but to do any good this should take place before the weather cools down too much, probably before the end of March.

Livestock can be very useful, not so much on the cropping area but in the pastures. If ryegrass is not allowed to set seed in spring this will reduce seed population.

Herbicide application to pastures has to be carefully considered. The use of Verdict or Fusilade in pasture can add to the resistance problem, as these chemicals will only take out the easy-to-kill weeds. Roundup, on the other hand, is very good in the year prior to cropping as it will completely stop seed-set in that year.

Preferred Attack Program

Paddocks should be sown down to pasture as soon as resistance is suspected. Sow plenty of clover seed, as this will help to smother out ryegrass. Keep the pasture grazed well in spring. If this is not possible, cut for hay prior to ryegrass seedset. Then after 3-4 years of legume-based pasture I like to use Roundup in mid-spring as this puts the paddock in good condition for the cropping phase.

The first year of crop on my property has to be either wheat or oats as wild radish is a problem also, thus eliminating the use of canola. After two years of wheat I like to sow either peas or lupins. An early burn is followed by one cultivation, then Treflan with a backup of 400 Sertin-400 Verdict.

Where herbicide resistance occurs the option of growing different crops and using alternate chemicals without involving pasture and livestock is the wrong approach, to my mind. I have found this method is too risky because there is such a high seed count of ryegrass that it puts both the crop and the chemicals under too much pressure. A partial failure can easily result.

Hindsight - 20:20 vision

In looking back over the past 15 years it is easy to see where I went wrong. In the mid-to-late 1970s continuous cropping of wheat was a very viable enterprise. With reduced tillage and stubble farming, soil structure was well looked after. With the introduction of Hoegrass, I, along with a lot of other learned people, thought we were made.We progressed nicely for six years with good weed control and increased wheat yields, together with improved soil structure, before the penny dropped.

With hindsight we tried to create a resistant problem, as weed resistance is really a selection process. In any paddock there are some weeds that cannot be killed. The better kill of the remaining weeds and the more years it is done, the faster the resistance will develop. Ryegrass resistance is not a death sentence.With a good rotation in the first place the problem should not occur.