Grading Processes for Sweet corn seed

Hard Dry Shell (HDS) is the term given to the harvested corn grain taken directly from the field. The HDS seed lot may also contain leaf and cob plant material, as well as other field contaminants such as soil, which can potentially produce a positive GM result.

The HDS seed lot is cleaned to remove extraneous material by a series of sized screens and then graded by density, size, shape and colour to produce uniform seed lots, which are then sold to growers for planting. These cleaning and grading processes also remove “off type” maize kernels, including GM maize kernels.

Differentiating between Maize and Sweet Corn

Maize and sweet corn belong to the same plant species Zea mays, and differ from each other only by three genes which are controlled by traditional breeding methods and agronomic practices, such as isolation distances and flowering times.

These genes are:

Normal sugary gene (su),

Sugary enhanced (se) and

Supersweet or shrunken (sh2).

Maize is the dominant phenotype, so if a sweet corn kernel has been pollinated by a maize pollen grain, that kernel contains a dominant copy of that gene, and will be a maize kernel as a result. This is a similar scenario to the scenario of eye colour in humans: Remember that for most genes, you have two copies of each gene that you inherited from your mother and your father. Each copy of the gene could be different, so you can be BB, Bb, or bb—BB and Bb have brown eyes, bb, blue eyes. In this example, brown (B) is dominant over blue (b) so you end up with brown eyes.

For sweet corn, each corn kernel on a cob receives one set of genes from both the male (pollen donator) and female (flower) parents. Both parental sources (pollen and flower) must be recessive for the respective sweet corn gene. In addition, supersweet sweet corn varieties have another gene - sh2 - which must also be homozygous (double) recessive to produce supersweet corn.

The varieties Krispy King and Dominion are supersweet varieties, with the double recessive shrunken gene sh2. The sh2 gene slows the conversion of sugars to starch after harvest, which extends shelf-life. Due to their higher sugar content, these kernels are smaller, lighter in weight, and shrunken compared to maize. It is therefore likely that any unplanned crosses from pollen blown in from maize fields will result in seed which is maize in appearance. These differences are easily seen and maize “off types” can be removed during cleaning and grading of seed lots.

Flat and Rounds, Large and Medium – what is this?

Corn kernels are graded into different categories to assist with the set up of planting equipment and seed handling equipment in the field. These grades are described as Large Flat (LF), Medium Flat (MF), Large Round (LR) and Medium Round (MR), depending on their physical size and shape. Kernels growing at each end of the cob tend to be irregular or round in shape, while those in the middle of a corn cob tend to be rectangular in shape, due to the physical pressures exerted on the kernels by neighbouring kernels. No particular kernel grade is more or less likely to contain GM material, and the grading process is purely to produce a uniform seed lot for planting.

How can we be certain that graded seed lots do not contain GM seeds?

The majority of genetically modified corn crops grown world wide are maize varieties intended for food processing or animal feed markets. Should out-crossing occur in the field, donor pollen is more likely to be of maize origin, and this will be removed from sweet corn grain by the sorting and grading processes described above. There are also a few GM sweet corn varieties which have been commercialized. Should these be the pollen donors, the resulting GM seed will not be removed by the practices described above, and subsequent testing of the finished, graded seed lots will test positive for GM material.

Summary:

Sweetcorn is a natural mutation from maize. The genes which control sugar levels in the kernel also control kernel size. When the kernel on the cob is accidentally crossed with maize seed, the kernels on the cob will be like maize in size (significantly larger). These maize seeds can be graded from sweetcorn seeds after harvest. If the GE contamination came from maize seed, then the GE contamination can also be removed. However, if the GE contamination came from sweetcorn, then the contamination cannot be removed and the seed will test positive.

Page last updated: 8 May 2008