The value of a break crop in a rotation is widely accepted and disease control is offered as the reason for the success of the strategy, but the beneficial results of a break crop are sometimes seen even when there's little or no cereal disease present. Some interesting experiments at Rutherglen suggest that there might be something other than just disease control involved.

Rutherglen experiments conducted by Glen Scammell between 1992 and 2000 showed surprising long-term effects of a break crop in the rotation. There, a wheat crop sown three years after a break crop of canola out-yielded one sown three years after a wheat crop by 17% but there was no significant difference in crop yields in the intervening two seasons. In other words, the break crop effect didn't show up for three years. What's going on?

John Angus and Megan Ryan of the CSIRO have a speculation that borders on heresy and they have a series of trials in the Temora district to test the hypothesis.

In recent years there's been increasing interest in what goes on in the soil and researchers and farmers alike have begun to take notice of the activities of mycorrhizae. Usually referred to as VAM they're seen as friendly fungi that help to extend the root system, enabling the crop to get access to the immobile nutrients phosphorus and zinc that would otherwise be denied. If that's true, we should be doing all we can to encourage them. Angus and Ryan suggest that this may not always be the case and that in certain circumstances VAM could be parasites.

They base their theory on the sequence of crops in the Rutherglen experiment and the fact that two of them weren't host to mycorrhizae. Canola and lupins figured in the four-year cropping cycle that resulted in the 17% increase in wheat yields and while almost all crop plants play host to mycorrhizae, canola and lupins don't.

To test their theory they set up trials at Temora in 2002 starting with two years of non-VAM hosts canola and lupins compared to two years of VAM hosts linola and faba beans. The 2004 wheat crop following these sequences was Clearfield Janz dry-sown on 21 May without any nitrogen fertiliser but with 20kg of phosphorus per ha.

Their speculation is that where we supply adequate phosphorus and zinc in the form of fertiliser, the fungi aren't needed and may actually be taking carbon from the wheat as well as interacting with other soil microbes.

If they're right there are some obvious implications. We might be looking at a situation where the positioning of break crops in the rotation becomes more important than just having a break crop, and where two is better than one. There may even be benefits in breeding wheat varieties that are not hosts to VAM.

Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994.