Undersowing maize crops with grass to keep valuable nitrates in the soil can help growers reduce losses caused by late-season harvesting and compacted ground, but getting it established correctly is crucial to prevent leaching after rainfall.

Dorset dairy farmer Brian Chandler and head tractor driver Tom Mears have been employing this technique for the past 20 years.

During that time, the pair have trialled a range of different methods and machines to get grass established under the maize crop’s canopy.

See also: Driver’s view: Vaderstad Tempo maize drill

The sandy soil makeup at Hurst Farm has required this kind of soil management to protect it against even small amounts of rainfall that would otherwise lead to nitrate leaching in the sensitive Poole Harbour catchment area the farm sits within.

Wessex Water has now set up a number of trails at Hurst Farm to assess the merits of different establishment tools and techniques – one of a number of trials taking place around the country (see “Other trials in the UK”).

First attempts

“The bare soil between the rows of maize both before and after harvest presents a big run-off risk on our gravelly, sandy loams. With any amount of rain, it will just wash straight out the gateway,” explains Mr Chandler.

“Years ago, I realised we needed to have a catch-crop to avoid that run-off, but it was a bit of a trial-and-error process to work out how best to do it.”

Initially, grass seed was spun on with a broadcaster immediately behind the forager, but establishing it so late meant that the seedlings were unable to develop a decent root structure to hold back the soil prior to the high-risk winter period.

Having a green cover established in the growing maize crop was considered essential, so a different approach was adopted.

“Rather than putting fertiliser down the spout at drilling, we’ve always tended to top dress the crop with urea once it’s up and away.

“That presented an opportunity to sow some grass.

“We started mixing seed with the fertiliser and spinning it on at the same time.

“It certainly worked and there was a carpet of fine, green leaves once the crop was cut, but it was incredibly wasteful as half the seed would end up trapped in the maize plants.”

The next step was to develop a machine capable of accurately distributing grass under the crop canopy.

For this purpose, the folding toolbar from an old set of Parmiter zig-zag harrows was dragged out of the nettles and, with a bit of grinding and welding, Tom Mears fashioned a frame to accommodate an APV air-seeder.

Its eight fan outlets were then evenly spaced along the folding beam and Euro 8 loader brackets grafted on in place of the original headstock.

“We wanted to front-mount the machine to maximise visibility but didn’t want the expense of a front-linkage – it fitted perfectly on the loader,” explains Mr Mears.

“That arrangement means we can easily alter the height dependent on crop growth stage.

“The only modification required was to tilt the fan outlets down a touch to limit the throw of seed between the rows.”

Establishment issues

This setup immediately proved itself in the first year, with seed rates reduced accordingly to allow for the reduced losses.

However, germination rates weren’t deemed to be as good as they could have been due to a lack of seed-to-soil contact.

Mr Mears went back to the workshop once again and decided the best course of action was to fit grass-harrow-type tines on the toolbar – three to run between each row of maize plants.

While this arrangement improved things, it wasn’t aggressive enough and establishment wasn’t particularly uniform – grass generally came up more reliably where it had been rolled in by the tractor wheels.

So last year saw a further modification to the system with the addition of a 6m Einbock grass harrow carried on the rear linkage.

Tines on the implement’s frame were removed to avoid damage to the growing maize crop, leaving eight rows to scratch up a tilth and cover the seed behind the tractor.

“The Einbock worked a treat in loosening the soil surface and covering the seed but it didn’t provide any consolidation, which is critical on our gravelly soil types.

“Seeing how well the seed used to come up when it was run in by the tractor, I’d like to try to add some tyres to run between each row to get decent seed-to-soil contact.

“Whether the Einbock frame is up to it, I’m not sure. I think it’ll take some fairly major modification to do that.”

Trials

Alongside these on-farm developments, Hurst Farm has also played host to some more formal trials with Wessex Water for the past three seasons.

Keen to limit nitrogen leaching in the Poole Harbour catchment area, the utility company has been working with farmers for a while to help develop techniques to hold back nutrients in the field.

Wanting to draw on Mr Chandler and Mr Mears’ experiences in 2017, the firm set up a field-scale experiment to measure the effectiveness of various techniques of establishing catch crops in maize at the farm.

With porous pots installed across the trial field to measure nitrate leaching, strips of crop were treated in several different ways, mostly during the five-to-seven-leaf stage but also following the previous harvest:

• Post harvest – stubble disced, seed broadcast and rolled in
• undersowing – grass sown using Hurst Farm’s home-built, loader mounted seeder toolbar with inter-row tines
• Grass harrow – grass sown with a Stocks broadcaster on a rear-mounted Browns harrow with rows of tines removed to avoid damage to the maize
• Slurry seeder – grass seed mixed with slurry through the induction hopper on the filling pipe and applied with 24m dribble-bar.

Results

The results of the trial were as expected. Undersowing maize with an Italian ryegrass reduced nitrate leaching by an average of 42kg/ha of nitrogen compared with leaving the stubble bare over winter.

This compares favourably with grass sown after harvest, which reduced leaching by 20kg/ha of nitrogen.

The Hurst Farm home-built band-sowing rig produced the greatest establishment rates and, consequently, was the best in holding back nutrient loss into groundwater.

Grass splattered on with slurry came in a close second.

Establishment with the harrows and broadcaster saw similar issues to the farm’s original broadcasting approach, with seed loss into the standing maize crop.

The 2018 and 2019 growing seasons saw a further technique added to the mix.

Developed in conjunction with a number of contractors and catchment protection organisations, the Weaving Inter Row drill employs three disc coulters with individual rubber press wheels running between each crop row placing seed directly into the soil surface.

In a dry year, this was felt to offer big advantages in guaranteeing reliable establishment thanks to the consolidation and seed-to-soil contact it provides.

“You could definitely see the grass had come up quicker and thicker where the drill had been, but we didn’t feel it was significant enough to justify the expense, particularly as it would only have a two-week working window each year,” says Mr Mears.

“We had another bigger concern. It is a heavily built bit of kit, which means you need a fairly hefty tractor to run it on.

“With skinny wheels on recently ploughed ground, we were concerned about the amount of compaction that could cause.”

The Browns harrow and broadcaster suffered the same issues of seed loss into the standing maize as early Hurst Farm attempts and would probably work better if the seeder was changed for one capable of placing the seed in bands under the crop canopy.

Mixing grass seed with slurry and applying it via dribble bar worked well with cow muck, but pig slurry didn’t produce such good results, possibly being too rich in certain nutrients.

In addition, the heavy machinery required was a cause for concern.

Post-harvest grass establishment has been tried both with the Einbock and with the front-mounted seeder toolbar and a set of ring rolls, depending on conditions.

Although this works, the young seedlings weren’t established enough by the winter to develop a decent root system capable of holding back run-off effectively.

For now, Hurst Farm’s home-built seeder and harrow approach at the four-to-six-leaf stage remains the practice of choice.

If an element of consolidation can be added to aid seed-to-soil contact then so much the better.

Lessons learned

• Herbicides – everything hinges around weed control chemicals. Experience with 30 years of continuous maize at Hurst Farm suggests it’s best to use a standard pre-emergence product and follow that two to three weeks later with a selective herbicide. However, it’s important to choose the right one as residual activity can continue right into the autumn and has been found to completely knock out the undersown grass.
• Timing – closely linked to the herbicide regime, Mr Chandler’s advice is to leave undersowing as late as you dare, ideally at the four-to-six-leaf stage. This affords the pre-em products plenty of time to work their magic.
• Crop damage – don’t be overly concerned by damage done to the maize by the seeding rig’s wheelings, especially crossing the headlands. Experience at Hurst Farm suggests it’s surprising how well run-down, dead-looking plants will revive.
• Seed rates – Mr Chandler’s view is that the best bet is to use low seed rates – 15-20kg/ha. Then, if the grass doesn’t establish, you can still afford to have another go in the autumn.
• Grass varieties – Mr Chandler has tried all sorts of cover crops over the years, from vetches and clovers through to ryegrasses and barley. The go-to seed choice is now a Westerwold, as it’s quick to establish but happy to sit dormant and then rocket away after harvest.
• Establishment – don’t be disheartened if the grass doesn’t seem to come to anything. Often there are tiny green wisps that sit dormant until the maize crop is off, and within two to three weeks, there’s generally a lush, green carpet. In dry years, the seed can sit on the soil surface doing nothing until there’s rain, and even then it tends to get up and away once the forager has been through.
• Slurry – if you want to apply a dressing of slurry to the growing crop, use tractors and tankers on wide, low-pressure tyres at the two-leaf stage running at 45deg to the crop rows. The young seedlings aren’t badly damaged by big flotation rubber and will quickly bounce back.