Integrated Weed Management - Crop Topping in wheat

| Date: 04 Aug 2010

Head

Integrated weed management - Crop topping in wheat 
                                                                                                                  
Patrick Head, Agronomist, Landmark – Cummins Ag Services
 
 
Background
 
In cereal crops where there are few post-emergent selective herbicide options available due to resistance; crop topping provides another tool in managing Annual Ryegrass (ARG) (Lolium rigidum). Crop topping has traditionally been done when the crop has reached physiological maturity or 32% grain moisture. At this point in time, the ARG has often past flowering which is the optimum timing for reduction of seed set.
 
Glyphosate and diquat are the only products registered for crop topping wheat and labels state the crop must be mature and below 28% moisture. Diquat is the only product registered for crop topping in barley.
 
Following work conducted in the Mid North which suggested crop topping can be carried out well before 32% moisture, we undertook this trial to look at the effect earlier crop topping has on wheat yield and quality in our area.
 
How was it done?
 
Wyalkatchem wheat was sown using a trial plot seeder. 1.2L/Ha Roundup PowerMax® was applied to plots at 4 timings between 65% and 35% moisture in a randomised block design.
 

Grain moisture was assessed by collecting 10 heads per plot at random from various heights in the canopy, resulting in a sample of 30 heads (10 x 3 replicates). Samples were weighed then dried down in an oven (65oC for 24hrs), then weighed again and moisture calculated with the following formula:

 
Plots were harvested for grain yield and grain quality was analysed. A germination test was also conducted on wheat samples. There were only low levels of ARG present at the site so no ARG control assessments were made.
 
What happened?
 
In work carried out at the Mid North High Rainfall Zone trial site in 2007, applications of Roundup PowerMax® at 55% moisture provided 80% and 99% control of ryegrass seed set in wheat and barley respectively with no significant yield reduction.
Yield results from the trial (Table 1) show that grain yield was significantly reduced at early timings with a 61% yield loss at 64% moisture and a 26% yield reduction at 55.8% moisture. The next spray timing at 45.9% did not show a statistically significant yield reduction.
 
Table 1. Effect of crop topping with Roundup PowerMax on the yield and quality of Wyalkatchem wheat at Cummins in 2009.
Treatment
Moisture at Application
Yield (kg/Ha)
Yield Loss
PR
TW
SC
GRADE
Wheat Germination %
Untreated Control
-
7061
Nil
10.3
80.9
0.6
ASW
100%
Timing 4 (9/11)
27.8%
6731
5%
10.4
83.2
0.5
ASW
99%
Timing 3 (30/10)
45.9%
6550
7%
10.2
80.4
0.7
ASW
94%
Timing 2 (22/10)
55.8%
5211*
26%
11.1
76.4
1.1
APW
46%
Timing 1 (9/10)
64.0%
2722*
61%
14.8
56.0
14.3
-
21%
LSD0.05 [Trtd vs Ctrl]
610
 
 
 
 
 
 
CV
 
4.4%
 
 
 
 
 
 
* Indicates significant over control.
 
As seen in Table 1, crop topping timing had immense affect on grain quality. Grain from the early (64%) timing would have been undeliverable due to low test weight, with screenings and protein being affected also. While timing 2 (55%) had an effect on yield, it actually had a positive effect on grain protein, but reduced test weight and increased screenings. While screenings were still only 1.1%, a variety with less stable screenings than Wyalkatchem may have been worse affected. Spraying at 45% moisture and less resulted in no affect on grain quality.
 
Wheat germination was severely affected by early crop topping timings; therefore seed should not be kept from crop topped areas.
 
 
Contact details:
Patrick Head,
Landmark – Cummins Ag Services
13 Light Road, Cummins SA 5631,
patrick@cumminsag.com.au,
08 8676 2161