Seeding rates theory holds water for repellent soils
Author: Natalie Lee | Date: 01 May 2015
Western Australian trial work has confirmed that increasing seeding rates can be an effective way of improving wheat plant establishment and subsequent yields on water repellent soils.
“But in drier environments and soils with poor water holding capacity, higher plant numbers did not always translate into higher yields, and were sometimes a disadvantage,” Department of Agriculture and Food (DAFWA) researcher Stephen Davies said.
“This demonstrates the need for growers to factor in yield potential, as well as the degree of soil water repellence, when determining seeding rates.”
The Grains Research and Development Corporation (GRDC) supported research also highlighted the importance of delayed sowing on water repellent soils to provide growers with an opportunity to control weeds.
Seeding rates had a bigger impact on wheat establishment than soil wetting agents in the trials.
The research was presented earlier this year to Perth’s Agribusiness Crop Updates, supported by the GRDC and DAFWA.
The research, led by Dr Davies and fellow DAFWA researcher Christine Zaicou-Kunesch, was conducted as part of the GRDC funded projects ‘Delivering agronomic strategies for water repellent soils in WA’ and ‘Wheat agronomy systems profitability’.
Water repellence in WA
Dr Davies said furrow sowing with knife points on water repellent sandplain soils in WA was often ineffective due to the flow of dry, repellent soil into the base of the furrow.
This resulted in poor crop establishment which could reduce yield potential and crop competition.
“The problem is exacerbated in severely repellent soils, when dry sowing and in seasons with a dry start,” he said.
“In general, growers have increased seeding rates to deal with this problem, with the aim of increasing plant establishment.
“Another option they are trying is using banded wetting agents in an attempt to help water infiltrate more consistently along the crop row.”
Dr Davies said four trials were established in 2014 in the WA grainbelt to take a systematic approach to assessing agronomic options for managing wheat on water repellent soils.
“The trials aimed to assess the impact of sowing time, seeding rate and wetting agents on wheat establishment and productivity on water repellent soil, and to determine if there was a positive interaction between these options,” he said.
The trials were conducted with Mace wheat and were located at Binnu (deep yellow sand), Warradarge (pale deep sand), Yealering (loamy gravel) and Cranbrook (duplex sandy gravel).
Two seeding rates of 60 and 120kg/ha and two times of sowing were tested at each site.
“The intention of these experiments was to have one early time of sowing into dry soil and one later time of sowing into wet soil,” Dr Davies said.
Sowing dates were April 24 (dry) and May 7 (wet) at Binnu; May 2 (variable moisture) and May 19 (wet) at Warradarge; April 27 (wet) and May 31 (wet) at Yealering; and April 25 (dry) and May 31 (variable moisture) at Cranbrook.
Soil wetter treatments included an untreated control and seven soil wetters comprising six commercially available products and one product under development.
All soil wetters were banded behind the press wheels at an application rate of 2L/ha and a water rate of 100L/ha.
Topsoil water repellence was measured at all sites using the Molarity of Ethanol Droplet (MED) test.
These tests revealed that the sites ranged from moderate water repellence (Yealering), to severe (Binnu and Warradarge) and very severe (Cranbrook).
Dr Davies said good break-of-season rainfall occurred in late April at the Binnu, Warradarge and Yealering sites.
“As a result, the only dry sowing occurred at Binnu and Cranbrook, although Binnu received more than 42mm of rain immediately after seeding,” he said.
“The severe water repellence at the Warradarge site meant pockets of dry soil were still common at the first time of sowing, despite the significant rainfall in late April.
“Seeding was into wet soil for the second time of sowing at each of the sites apart from Cranbrook, which still had numerous pockets of dry soil.
“Following the good opening rains in April and May, all the sites had low rainfall in June.
“However, rainfall amounts increased again in July at all sites except Binnu where little rain fell throughout June to August.
“The deep repellent sands at Binnu and Warradarge have poor water holding capacity so struggle to sustain crops over long, dry periods.”
Establishment, biomass and weeds
Dr Davies said increasing seeding rates did increase plant establishment significantly on the water repellent sands in the trials.
For the first time of sowing, a 100 per cent increase in seeding rate (from 60 to 120kg/ha) increased plant establishment by 73 per cent at Binnu, 56 per cent at Warradarge and 77 per cent at Yealering.
“For the Cranbrook site, there were no plant counts but emergence ratings indicated that the emergence was better with the higher seeding rate,” Dr Davies said.
At Binnu and Warradarge, time of sowing also affected plant numbers.
“At Warradarge, plant numbers for the second time of sowing were lower, while at Binnu there was an increase in plant numbers at the lower seed rate for the second time of sowing, but a decrease in numbers at the higher seed rate,” Dr Davies said.
Dr Davies said soil wetting agents did not affect crop plant establishment at any of the sites except for the first time of sowing at Yealering.
“At Yealering, the use of soil wetters increased the plant numbers over the control by at least 18 per cent at the 60kg/ha seed rate and 22 per cent at the 120klg/ha seed rate,” he said.
“Vigour ratings undertaken on June 27 indicated that Yealering plots treated with a wetting agent were more vigorous in the early growth stages than the untreated control plots.
“However, the difference became less evident over time.
“Visual impacts for some of the wetting agents were also evident at Binnu during vegetative growth, with the plants in some of the soil wetter treatments being noticeably and consistently darker green than the plants in the other plots.
“The observation was stronger for the first time of sowing than the second, but again this difference became less evident over time.”
Dr Davies said that at the higher rainfall sites of Warradarge, Yealering and Cranbrook, the weed burden was large and difficult to control for the first time of sowing compared with the second, when there was an opportunity for an effective knockdown herbicide application prior to seeding.
Grain yield and quality
The delayed time of sowing treatment produced higher yields than the early sowing treatments at Binnu and Cranbrook.
“The first sowing time at both sites was into dry conditions and crops emerged following the rains in late April,” Dr Davies said.
“The second sowing time at the Binnu and Cranbrook sites was into wet or variable soil moisture.”
Seeding rate had no influence on grain yields at the low-yielding sites of Binnu and Warradarge at the first time of sowing.
“However, at Cranbrook there was a significant increase in yield from 2.48 to 2.78t/ha with an increase in seeding rate from 60 to 120kg/ha at the first sowing time,” Dr Davies said.
Dr Davies said Binnu was the only site where a wetting agent had a significant effect on grain production, and this soil wetter produced much higher grain yields (across all sowing times and seeding rates) than the other site treatments.
“It is possible that this wetting agent enabled the crop at this site to more effectively use the small rainfall events received during the dry period, by increasing water penetration,” he said.
Sowing time, but not seeding rates, influenced grain screenings at Binnu and Warradarge.
“At Binnu, screenings and grain weight were lower in the first sowing time treatments, but at Warradarge, screenings were lower in the second sowing time treatments,” Dr Davies said.
Dr Davies said increasing seeding rates was the most effective method for improving wheat plant establishment in the trials.
“However, while high seeding rates could be beneficial in high rainfall environments, they were observed to reduce yields at later sowing times in low rainfall locations and on poorer soils with very low water holding capacity,” he said.
“Growers should factor in yield potential as well as the degree of soil water repellence when considering seeding rates.”
Dr Davies said weeds were a risk with early sowing in any environment, but were a particularly big risk if establishment was likely to be variable and staggered on water repellent sands.
“Delayed sowing provided an opportunity in the trials to get a knockdown of weeds,” he said.
“This is important on water repellent soils where staggered weed germination and reduced herbicide activity reduces the efficacy of in-crop and pre-emergent applications.”
For more information about managing soil water repellence, view the GRDC Wheat GrowNote.
Stephen Davies, DAFWA
08 9956 8555
Christine Zaicou-Kunesch, DAFWA
08 9956 8549
Natalie Lee, Cox Inall Communications
08 9864 2034, 0427 189 827
GRDC Project Code DAW00204, DAW00218