Breeding sweet corn - from a Breeders' perspective!
4/17/2013 11:24 AM
Sweet corn breeders have drawn heavily on the work done by field corn breeders, and have found that in many respects the two crops are essentially alike; in others, however, they differ considerably. Sweet corn, like maize is a hybrid crop requiring at least two parents to create the resultant progeny. Sweet corn breeders focus on a greater number of what can be termed subjective traits such as pericarp tenderness, eating quality and overall ‘bite’ – the experience of what it is like to actually bite into an ear of corn. These are quite complicated traits and ones that are hard to measure with any sort of mechanical device. The simplest method of determining these traits is to just take a bite of cob! The similarity with field corn is more in relation to the agronomic traits such as disease resistance, standability and yield.
In the process of breeding new sweet corn hybrids, the breeder must focus on two distinct areas. One is the development of new parents that will ultimately go into a new hybrid, and two is the subsequent evaluations of new hybrids formed from the crossing of these new parents, or the crossing of one of these new parents with an existing parent.
If we focus initially on the process of developing new parents, typically a breeder will start out by intercrossing between finished inbreds, or perhaps the crossing between a finished inbred with a commercial hybrid, or perhaps the crossing between two commercial hybrids. With each cross the breeder has in their mind what the outcome of this cross will ultimately ‘look’ like. There should be a range of agronomic traits that the breeder is looking for that the inbred, when finished, will possess. In the fixing of these agronomic traits, the breeder will also be focusing on ensuring that the inbred has traits that are essential for a sweet corn hybrid. It could be kernel colour, kernel depth, eating quality, husk cover etc. The process then is relatively simple. In year 1 we cross our two sources of sweet corn to create an F1 progeny. In our year 2 we plant this out in the field and we screen for tolerance to diseases, standability, eating quality – whatever traits we are hoping to have expressed in our final product. The plants with traits of interest are then ‘selfed’ – the process of transferring pollen from that plant’s tassel to its ear, without introducing any ‘third’ party pollen. It is by the process of ‘selfing’ that we develop homozygosity in the parents – homozygosity, or fixing, of the parents is essential in the ultimate development of a uniform hybrid – a hybrid that emerges from the ground develops at the same time, flowers at the same time, and is ready for harvest, at the same time – essential traits for a crop that is to be machine harvested. The selfing process is repeated for typically 5 generations. We normally use a winter nursery to speed up the process. This allows two generations per year. With the use of a glasshouse it is possible to achieve 3 generations in one year. At the F5 stage we would cross this nearly fixed inbred with one of our tester inbreds. Our tester inbreds allow us to ‘see’ more clearly what work the new inbred is doing in hybrid combinations. We can also use an elite inbred that we know works well in hybrid combinations to possibly come up with a successful new hybrid more quickly. This method of inbred development is known as pedigree breeding. We will also use techniques such as backcrossing, which is the process of incorporating a single trait into an existing parent. This process is sped up with the recent adoption of the use of marker assisted selections, which takes out some of the guess work in the process.
For Snowy River Seeds, we utilize three locations for sweet corn inbred development. Our primary site is at Orbost, in the East Gippsland of Victoria. We get good pressure from both northern corn leaf blight and common rust. We then have two other sites where we focus our breeding effort. One is in north Queensland at Bowen, where we can grow sweet corn during the winter which is ideal for bulking up new selections and general advancement. The other site is at Forest Hill, in the Lockyer Valley of south east Queensland. This is an ideal site for developing inbreds with very good agronomic traits and especially tolerance to maize dwarf mosaic virus. We can run two consecutive nurseries in the Lockyer – one in spring, one in summer. When we start testing new hybrids, we typically screen them in one location initially. This typically is a single row of each new hybrid with commercial checks at various points for benchmarking. Each year we test between 2,000 and 3,000 brand new hybrids. Hybrids which are successful at this stage move on quickly to testing at multiple locations and times of planting. Currently we have 3 different planting times in Orbost, two in Bathurst, N.S.W., two in the Lockyer Valley, one in Bowen, one in Gisborne New Zealand, one in the south west of France, one in eastern Hungary, a modified trial in the south east of China, and various modified trials from Washington State across to New York, in the U.S.A. The aim of this extensive testing is to ensure that these new hybrids work consistently well over a range of environments and are able to grow successfully under a range of different disease pressures and environmental stresses.
This testing typically takes place over several years. As we move closer to the commercialization stage, larger volumes of these new experimentals are produced which are then placed in farmers’ fields. It is only at this stage that we can adequately assess the yield performance and it is this stage which is one of the most critical. We rely heavily on our customers to help us gather this data.
As this process of testing takes place, our Production team is carrying out research on the performance of each parent in Production trials. These trials are designed to evaluate the suitability of new parents as to whether they can perform as the female parent in the hybrid combination.
It is essential that not only the hybrid performs in the paddock, but it can be produced in a cost effective way that ensures quality seed is delivered to the growers in the field.
We are continually looking at ways of improving the breeding process. We utilize breeding programmes from around the world also as a means of expanding our genetic reach. We also regularly visit our markets and end users to talk directly to our customers.
As a breeder, we know how important it is that we continually gather feedback from the end user to ensure our breeding programmes remain focused and able to deliver hybrids that meet the grower’s needs. We have highly technically skilled distributors in each of our markets that help guide us in our direction. We value highly their input and we value their feedback when they visit us in the trials. It is only through these collaborative efforts that we can remain successful.
With thanks to Damien Courtier, Sweetcorn Breeder, HSR Seeds and Warren Hobson, Lefroy Valley New Zealand