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Minimising aflatoxin risks

There is a risk of high aflatoxin levels in peanuts in dry and hot seasons. Research trials show early harvesting (or cutting) can significantly reduce aflatoxin levels and increase gross returns.

Gross returns are affected by a combination of the effects of aflatoxin levels, kernel grades and pod yields. Growers need to decide when to cut the crop and they need a way to determine 'how early is early?'.

It is important for growers to know how to assess and quantify a fully mature crop so they can make informed decisions about whether or not to harvest early.

On this page:

Assessing peanut maturity

There are four major methods to assess crop maturity in peanuts:

  1. Internal 'shell-out' method: pods are shelled and sorted into mature and immature categories based on the extent of blackening of the inner side of the shell wall. Consider the crop fully mature when the proportion of mature pods is above 95%. The shell-out method is the recommended method for dryland peanut growers to use.
  2. Weeks after planting: this method is dependent on variety and location (e.g. around 21 weeks for the Streeton variety in the Burnett area). It is not an accurate method because temperature differences and variations in patterns of drought stress during different years can lead to differences in maturity for the same variety of up to three weeks.
  3. Hull-scrape method: scratch the external surface of pods using a knife or high-pressure hose and sort them into categories based on colour differences (white, yellow, orange, brown/black). Once the proportion of brown/black coloured pods reaches 60-65% for Virginia types, and 65-75% for runner types, the crop is considered to be fully mature.
  4. Thermal-time method: this method is based on the theory that a variety requires an accumulation of 'heat units' above a 'base' temperature of around 10ºC (e.g. Streeton requires around 1950 heat units, also known as 'growing degree days'). This method is experimental and is better suited for maturity prediction in irrigated crops.

Field and research experience shows that the hull-scrape and thermal-time methods of maturity assessment are the most reliable in irrigated situations where the pods tend to mature, or 'set', at the same time.

In dryland situations, crop water stress at different stages of crop growth can result in a range of pod setting events, spreading crop maturity over time. The internal shell-out method is a more reliable method to assess optimal maturity in dryland crops.

A number of factors influence the decision on when to cut a peanut crop, including:

  • maturity of the crop
  • peg strength and general crop health; if the crop is highly diseased the grower is advised to cut early to reduce harvest losses
  • weather outlook; hasten or delay cutting if wet weather is imminent
  • risk attitude of grower
  • soil type differences may cause variations in maturity within a block
  • machinery logistics; if a small acreage can be cut or harvested this will spread harvesting over a longer period.

Using the shell-out maturity index

  • Pull at least six bushes from a wide cross-section of the block.
  • Remove all 'thrashable' pods from the bush and avoid the temptation to pick only the mature-looking pods.
  • Mix pods from all bushes in a bucket and take out a sub-sample of at least 100 pods.
  • Shell the 100 pods and separate them as mature or immature. Mature pods have some orange, brown or black colouring inside the shell, while immature pods are white inside the shell.
  • Use the shell-out maturity index photo gallery to check your sorting. There are photographs for three varieties: Streeton, VB97 and NC7.
  • The number of mature pods is the percentage, otherwise known as the 'maturity index'.

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Graph showing relationship between time and rate of crop maturity
Close relationship between time and rate of increase in crop maturity.

Using the maturity index to determine cutting time

Although it is best to leave the crop to fully mature, it is often necessary to make the critical decision on the most appropriate cutting time, particularly during seasons with high aflatoxin risk.

Trials over three seasons showed that pulling the crop early in seasons with high aflatoxin risk increases gross returns. Growers can use the following information and the maturity reference photographs to help them make the difficult decision about when to pull the crop in high aflatoxin risk situations.

The relationship between maturity index and weeks after planting was determined for typical Streeton and NC-7 variety crops grown in the Burnett region during 1999-00 season. The graph at right shows the close relationship between time and the rate of increase in crop maturity.

This data can be further summarised into the following table, which indicates that early cutting of between one to two weeks equates to a 'maturity index' measured by the 'shell out' method of 70 to 90%. It is important to note that these figures are only averages and there will be large variations around these figures depending on crop, variety and seasonal conditions.

Cutting time % of mature pods
Normal time Above 95%
One week earlier Approximately 80 - 90%
Two weeks earlier Approximately 70 - 75%

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Graph showing hypothetical illustration of relationship between yield and kernal grades and crop maturity index
Hypothetical illustration of relationship between yield and kernel grades, and crop maturity index, in a fully irrigated crop and a dryland crop experiencing end-of-season drought

Minimising aflatoxin risk and maximising gross returns

The Department of Employment, Economic Development and Innovation (DEEDI) has developed a web-based, in-season aflatoxin decision support package, AFLOMAN, to allow more accurate prediction of the cutting time that minimises aflatoxin risk and maximises gross returns on an individual paddock basis.

When to begin cutting a peanut crop may be one of the most difficult decisions facing growers during the season. It is the decision that has the greatest effect on crop profitability, especially if it influences aflatoxin levels.

The graph at right provides a hypothetical illustration of the relationship between yield and kernel grades, and crop maturity index, in a fully irrigated crop and a dryland peanut crop experiencing end-of-season drought.

Under fully irrigated (or adequate rainfall) conditions, crop yield and kernel grades increase linearly as the maturity index approaches 100%. Obviously, these well-watered crops should be left in the ground until they achieve the full yield and quality potential (i.e. near to a maturity index of 100%).

In the case of dryland peanuts facing prolonged end-of-season drought, there is little opportunity to increase yield or kernel weight. If the environment is conducive for aflatoxin, then any delay in harvest will also lead to increased aflatoxin contamination, severely affecting gross returns.

DEEDI trials conducted over the trial period of three seasons consistently showed that in years, or situations, where there is a high-aflatoxin risk, early crop cutting can help to minimise aflatoxin levels and increase gross returns.

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Graph showing in high-risk alfotoxin situations, early cutting optimises gross returns.
In high-risk aflatoxin situations, early cutting optimises gross returns.

High-risk aflatoxin situations

The normal cutting time referred to in the graph at right for a high-aflatoxin risk situation refers to the time when the farmer decided to cut his crop surrounding the trial site.

The early cutting time was approximately two weeks before the normal time.

The key points highlighted in this figure include:

  • Aflatoxin levels increased mainly at the sites that had the lowest rainfall and highest temperatures during the four to six weeks before harvest.
  • Seed grades were poor with early cutting but they were not as poor as most growers feared. Across nine sites with different varieties, the average reduction in price was around $40/t.
  • Yields were similar or slightly less with early cutting, suggesting that crop stress prevents any increase in kernel weight.
  • Average increase in gross returns with early cutting was $46/ha, excluding the one site that had higher rainfall. In a year when general aflatoxin levels were low to moderate, early cutting provided a slight increase in gross returns. More importantly, at the higher-risk sites where loads went Seg 2 or 3 at the normal cutting time, the average increase in gross returns by cutting early was $103/ha.

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In low-risk aflatoxin situations, cutting at the normal time optimises gross returns
In low-risk aflatoxin situations, cutting at the normal time optimises gross returns

Low-risk aflatoxin situations

In low-risk situations, where end-of-season rainfall is adequate and/or soil temperatures are mild (less than 25°C), DEEDI experiments show that early cutting has no benefit, and can lead to significant reductions in gross returns, owing to reduced yield and kernel grades.

As shown in the graph at right, cutting crops early on sites that had good rainfall distribution significantly reduces crop yield and quality. This confirms that growers should cut well-watered crops at the normal time.

Further information

Last updated 03 February 2011