Outsmarting the weed seed bank

Introduction

Ask any person who has been on a diet about what the hard bit is. Losing the weight or keeping the weight off? Chances are that most of them will tell you that losing weight is very difficult, but once they have lost the weight it is much easier to keep it off if they are disciplined.

And then there are plenty of people who start a diet, don’t see anything change for a few weeks, and give up. At the end of the day it is all about attitude and how motivated the person is.

Weed seed banks are just the same. Eroding a very high seed bank to a low, near-zero seed bank takes considerable effort and cost. But once the seed bank is low, keeping it low is easy peasy. And of course, there are plenty of growers who make a start and then give up after a couple of years because they can’t see anything happening.

Once again, it is all about the attitude. Growers who put their mind to it will have a win. Growers who either give up or settle for mediocrity will not.

Focus paddocks

For the past 15 years I have monitored about 30 focus paddocks in the northern cropping zone of Western Australia (WA). These focus paddocks were originally part of a series of GRDC-funded projects delivered by me at the Department of Agriculture and Food WA (DAFWA).

I now monitor them as part of my Australian Herbicide Resistance Initiative (AHRI) (GRDC-funded) work. Every year I turn up to the same part of the same paddock in spring and throw some quadrats which is the frame used to pick units of ground for study purposes. There are never, ever, any surprises. 

Every year I see the growers who are taking an aggressive approach to managing their ryegrass seed bank. They have near zero ryegrass. I walk around in these crops for 15 minutes and don’t see a single weed. Last year I found one ryegrass in 240 quadrats (0.5m2) in these paddocks. They use harvest weed seed control (HSWC), especially narrow windrow burning, about every second year on average. They also mix and rotate herbicides, crop top lupins, and in general they never let weeds -  especially ryegrass - set seed. These growers eroded their ryegrass seed bank to near zero seeds/m2 from 2001 to 2008 and have kept it to those levels ever since.

And then there are the weedy paddocks. Every year there are five to 10 ryegrass setting seed in these crops. These growers rarely use HSWC (about one in 10 years), and are generally not too stressed about letting a few ryegrass set seed. They are happy that they can continuous crop with relatively low weed numbers. Don’t get me wrong, they are good growers and they have successfully eroded their ryegrass seed banks from very high levels in the 1990s, but they have done this with a herbicide focus, and they just don’t have the same ‘take no prisoners’ attitude that the low seed bank (plus HWSC) growers have.

Figure 1 shows the difference between these two groups of growers. The graph is split into plus and minus HWSC. But in reality, it could be plus and minus a ‘take no prisoners’ attitude.

Both groups are at about 90 per cent crop. There are 14 paddocks in the minus HWSC group and 12 paddocks in the plus HWSC group.

Figure 1: Focus paddocks and surviving ryegrass numbers in spring.

What is the big deal?

Ten ryegrass plants per square metre is not exactly a blowout. One could well argue that all of the growers involved in this project are having a win. So what is the big deal?

The big deal is seed set. We do not measure ryegrass seed set in this project, in the graph below we have estimated it. How? Dr Michael Walsh from AHRI has travelled the entire cropping zone of Australia comparing HWSC tools at 24 sites in wheat crops. For the record, narrow windrow burning, chaff cart and the tow-behind Harrington Seed Destructor all performed equally, reducing the ryegrass germination by 56 per cent the following autumn. In this research, Michael measured ryegrass seed set. On average, across all of the 24 sites there were 10 ryegrass plants per square metre in crop, and they set an average of 210 seeds per plant.

In Figure 2 I have estimated ryegrass seed set by multiplying ryegrass density in crop by 210. Now you can see why ten ryegrass/m2 is a big deal. In 2016, the minus HWSC growers face a seed bank of about 2000 ryegrass/m2. Roughly half of these ryegrass will decay (rubbery figure) leaving the grower with about 1000/m2 to kill with herbicides. This is significant selection pressure, and this high selection pressure has been happening for more than 15 years in these weedy paddocks.

The plus HWSC growers, on the other hand are facing near zero seed banks, and they have faced near zero for the past eight years. Which group would you rather be in? Once the seed bank is very low, keeping it there is a walk in the park. These growers can plant whichever crop suits them, whenever they want (no need to wait for a germination) and are generally optimistic about the future of continuous cropping. 

Sure, it is not all beer and skittles, they still have some weedy patches to deal with and their herbicide bill is still very high. But it is a whole lot easier than facing up to a weedy farm every year.

Early/dry sowing is a big profit driver in the northern wheatbelt of WA. Growers who have invested in a low seed bank are reaping their rewards by optimising sowing times.

Figure 2. Estimated ryegrass seed set across 26 focus paddocks.

So what?

Herbicide resistance evolution is the so what! Herbicide resistance is Darwinian theory to a tee. Darwin taught us that evolution equals genetic diversity times population size. Ryegrass is genetically diverse and  we can’t do anything about that. But, we can do something about the population size.

Keeping numbers low is easier than fixing a blowout

As with most things in life, an ounce of prevention is worth a pound of cure. Many growers comment that once the seed bank is low it is easier to keep it low compared with fixing a blowout.  This data from Dr Michael Walsh from AHRI is testimony to this. This data is from two of his trials comparing HWSC techniques. It shows that HWSC is more effective on low ryegrass seed banks than high seed banks. HWSC reduced the ryegrass germination by 84 per cent on a low seed bank site and only 33 per cent for a high seed bank site. Why? Probably largely due to the germination of two- to three-year-old ryegrass seed at the high seed bank site.

Table 1: The effect of various harvest weed seed control treatments on ryegrass emergence in the following autumn for a low ryegrass seed bank site compared to a high seed bank site. - Dr Michael Walsh, AHRI.

Growers

Ultimately it is up to you to decide if you want to be in the very low/zero seed bank group. It is all about attitude. Only you can light the fire within. Once the fire is lit we have plenty of tools at your disposal and we are here to help. The trick is to surround yourself with the right people - agronomists, researchers, other growers, etc.

And don’t give me that “it might work in WA, but it won’t work here” story. Sure, there are different weed species, and each region has its own challenges. What matters is attitude. If you want to have a win, you can. Make a plan, including non-herbicide tools (e.g. HWSC) and stick to it - for at least six years. That is how long it takes. Don’t give up in year three or four - you are nearly there.

First step, subscribe to AHRI insight for simple explanation of the science at AHRI website.

Second step, visit the Weedsmart website for practical info at http://www.weedsmart.org.au.

Third step, follow me on Twitter @peterdnewman. Twitter is brilliant for growers, from solving machinery problems, to connecting with scientists, to showing off big fish. Give it a go.

Agronomists

There are plenty of agronomists who are adept at writing herbicide recommendations, and I take my hat off to you. It is no easy task (I was once one of these). And there are a few agronomists, with more experience, who can take a grower from point A in their farming business to point B. These agronomists are prepared to work with their clients and have difficult discussions about adopting practices other than herbicides. When you are ready to take the next step in your agronomy career, have a go at the next level and have the difficult conversation.

While I am on the topic of career development, a shameless plug for our on-line learning courses from AHRI via Weedsmart. Sign up for our first course, ‘Diversity Era - Herbicide Resistance 101’, at Weedsmart website delivered by Prof. Steve Powles. It includes nine short videos of seven to 15 minutes each that explain the science of herbicide resistance.  

Don’t get me wrong, I realise how hard it is to sell these concepts to growers. My advice is to sell them the dream of the very low seed bank and how nice it would be, and how good it would be for their business. Then sit down with them and work out how to get there. Together! Light the fire within.

Acknowledgement

Funding for this work was provided through the GRDC supported projects Australian Herbicide Resistance Initiative and WeedSmart and their support is gratefully acknowledged.

Contact details

Peter Newman
Australian Herbicide Resistance Initiative
petern@planfarm.com.au