Friday, March 29, 2024

Boosting brassicas

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Dr Warwick Scott covers some of the principles governing best-practice establisment of brassicas.  “I reckon the seed is crook,” said the farmer as Matt Strahan and I tried to look knowledgeable about the reasons for the good and bad strips in the paddock of Barkant turnips just out of Hamilton (see photo). It was my first day on the job working for the Forage Brassica Development Group (FBDG). I had been allocated responsibility for the Waikato and Southland as my stamping grounds and over the next three years I gained a wealth of experience working in these two contrasting environments.
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Our experience would suggest that about 20% of brassica crops sown are failures and that failure generally results from what is or isn’t done in the weeks before and immediately after sowing. This article covers some principles governing best-practice establishment of brassicas, especially ground preparation and time of sowing. It does not cover detailed management packages on cultivars, sowing rates, chemicals etc, as there are many company reps out there well versed in these matters. 

Paddock selection and planning

Paddock selection for a brassica crop needs to consider several factors such as crop sequences, maintenance of soil fertility and structure, weed and pest control, plus requirements for feed supply.

Although brassicas are often regarded as a watery crop, stock must have ready access to clean water or intakes may suffer.

In reality one of the “worst” paddocks on the farm is usually chosen because it is up for regrassing. For this reason it may require high inputs of things like herbicide, cultivation, lime and fertiliser. Some of these inputs such as liming and weed control may need to be started several months in advance of sowing the crop. If contractors are to be involved they need to be contacted well in advance.

In the FBDG we were commonly asked to compare conventional cultivation with direct drilling – or zero tillage as it is sometimes called. After many experiments throughout the country our conclusion was that there was little difference between the two methods providing both were well-planned and executed thoroughly and on time. 

Conventional cultivation

Most crops are established following conventional cultivation where the soil undergoes considerable mechanical disturbance but has the prime objective of producing the required seedbed at minimum cost.

For brassicas our ideal seedbed should be fine, firm, warm, moist and free of weeds and pests (see photo). The initial application of herbicide or cultivation may need to be several months in advance where problem weeds are involved, under low fertility conditions, or where a fallow is required to allow time for mineralisation and conserve soil moisture.

Failed brasica crop from broadcasting.

Although the practice of sowing alternate rows has been practised for years using a ridger, it can also be done with a conventional drill (see photo).

Many drills, especially if they are a bit ancient, are not capable of sowing low rates of brassica seed accurately and may require about 5kg/ha of Italian type ryegrass as a carrier. Seed can also be mixed with “drilling”-type fertilisers but mixing with acid fertilisers should be avoided.

The fertiliser must also contain sufficient fine material to prevent the seed dropping to the bottom of the box. If in doubt consult your local fertiliser rep.

Finally the seed can be broadcast from an aircraft, a bulk spreader or even a drill with the coulters removed or lifted under trashy conditions. For reasons described above, broadcasting is usually greatly improved by harrowing and rolling after sowing.

Broadcasting is a risky business, especially in dry conditions. Some of the worst brassica crops I have seen have been those that are broadcast (see photo).

It was suggested that the banding in the Barkant turnip crop shown in the photo was the result of crook seed. Matt Stahan’s reply was: “So are you suggesting that all the good seed was on one side of the drill and all the crook seed was on the other?”

There was an awkward silence.

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