Chapter 2: Breeding Corn for Silage

How Corn Hybrids are Developed

Lana Reid
Agriculture and Agri-Food Canada Research Scientist, Ottawa Research and Development Centre

Corn, a tropical plant, was first domesticated approximately 8,000 years ago in Central America. Many different types of corn evolved with the help of indigenous people who were the first corn breeders.

Today, the ultimate goal of corn breeding is to improve the adaptation of corn to temperate and early season environments. Improved adaptation means higher yield and better quality. The development of a new corn hybrid is a slow and costly process. New hybrids must possess improved yield, standability, pest resistance and tolerance to various stresses. This means that the expertise of breeders, entomologists, pathologists, physiologists and many other specialists are required. Corn grain yields have increased in North America from approximately 1.3 t/ha (0.6 T/ac) in 1930 to 8.7 t/ha (3.9 T/ac) in 1994 or approximately 0.08-0.1 t/ha per year. This steady increase is due to a combination of improved hybrids, increased use of fertilizers, better weed control and higher plant densities.

There are five major steps in the development of a commercial corn hybrid:

1) selection and development of appropriate source germplasm
2) development of superior inbreds
3) testing of inbreds in experimental hybrid combinations
4) identification of a superior hybrid combination
5) multi-location testing of the pre-commercial hybrid
Finally, extensive seed production and marketing of all new hybrids is required.

To understand how a new hybrid is developed, a basic knowledge of corn pollination and breeding processes is required. The corn plant has separate male and female flowering parts (Fig. 1). The tassel is the male flower and produces pollen; the ear is the female flower. A typical hybrid corn ear consists of several hundred kernels attached to the cob or rachis and surrounded by a group of modified leaves called the husk. Each kernel starts as an ovule and has its own silk which grows out of the husk at the top of the ear.

When the tassel is fully emerged from the upper leaf sheath, pollen-shed will begin, usually from the middle of the central spike of the tassel and then spreading out over the whole tassel. Pollen grains are produced in anthers which open up under appropriate weather conditions. Pollen, which is only viable for 18-24 hours, is very light and can be carried considerable distances by the wind. Pollen shed from the tassel usually begins 2-3 days before silk emergence and can continue for several days thereafter, but will stop when the tassel is too wet or too dry.

The silks are covered with fine, sticky hairs that catch and anchor pollen grains. Within minutes after landing on the silks, the pollen grain germinates and a pollen tube grows down the silk to fertilize the ovule or potential kernel. This usually takes 12 to 28 hours. Under good conditions, all silks will emerge and be ready for pollination within 3 to 5 days. Unfavourable environmental conditions during pollination can have a great impact on grain yield. Since there is usually more than enough pollen (a given tassel can produce up to 5 million pollen grains), problems generally occur when there is poor synchronization between silk emergence and pollen shedding.

Corn with its separate male and female flowering parts is a naturally cross-pollinating plant. This means that ovules can be pollinated by pollen from neighbouring plants. Therefore, care must be taken in a breeding program to ensure that pollen from the appropriate tassel fertilizes ovules on the appropriate ear. This is usually achieved by hand-pollinating. As soon as ear shoots are visible in the leaf axils of a plant, a small paper ‘shoot-bag’ is placed over the shoots; this allows the ear to continue growing and the silks to emerge but prevents any pollen from falling on the silks (Fig.2).

When pollen shed begins, a paper bag is placed over the tassel and stapled at the base of the tassel to trap the pollen. The next day the tassel bag containing pollen is removed and quickly placed over the silks of a covered ear after removing the protective shoot-bag (Fig. 3). The tassel bag is pulled around the stalk, stapled and shaken so that the pollen grains fall on the silks (Fig. 4). A plant is ‘selffertilized’ (also referred to as selfing or inbreeding) when the pollen from a tassel is placed on the silks of the ear of the same plant (Fig. 5). A plant is ‘crossfertilized’ or ‘crossed’ when the pollen from a tassel is placed on the silks of a different plant. Of the millions of hand pollinations made by corn breeders, only a handful result in a superior inbred that will be used in a commercial hybrid.

     
  Fig. 3 Transferring pollen from tassels of male parent
  to silks of female parent.
  Fig. 4 Maintaining inbred lines.

 

Between 1850 and 1910, North American corn breeders developed higher yielding corn varieties by open-pollination. In this procedure, plants were allowed to shed pollen without covering silks, resulting in a mix of cross and self pollinated kernels on each ear. The best plants would be selected and their ears (usually the largest ones in the field) would be kept to use as seed the next year. The resulting populations were gradually improved for agronomic traits, but were very variable in plant height, ear height, maturity, etc., due to the random cross-pollinations.

In the 1920’s, the concept of hybrid vigour (heterosis) was discovered. If corn plants are self-pollinated for six or more generations, the plants become smaller and less vigorous due to inbreeding depression, but their traits become more uniform. At every generation, selection can be made for specific traits such as pest resistance, plant or ear type, ear size, etc. This repeated inbreeding produces an ‘inbred’ line of corn. We can save breeding time by getting two generations per year using winter nurseries in warmer climates.

An inbred is genetically uniform for all traits and will always breed true to form. Hybrid vigour occurs when we crosspollinate two inbred lines from different unrelated backgrounds (Fig. 6). The offspring of such a cross will have a larger-yielding ear and will be a more robust plant. It is also uniform for most traits. There are many theories to explain hybrid vigour, but this phenomenon is still not well understood. Note that if an ear of hybrid corn is self-pollinated, the resulting progeny will be variable in yield as well as in other traits. This is why farmers must buy their hybrid corn seed each year and should not plant the seed from a field of hybrid.

Development of inbreds takes about 75% of the effort in a corn breeding program. Most of the effort is spent evaluating inbreds by crossing to another inbred, which is called a tester, to see if it will produce a desirable hybrid. The process is called evaluating the combining ability of the inbred. The cross is called a testcross. The field performance of this testcross is extensively evaluated in replicated multi-location trials. Inbreds with superior testcross performance are advanced to the next generation. If we could select at the inbred level, i.e. if the performance of the inbred on its own could predict the performance of the hybrid testcross, we could considerably reduce expenses. In fact, this can be done for some traits such as earliness, plant height and some disease resistance but, unfortunately, not for yield. It is important to note that the seed sold to farmers is produced on small inbred plants. Therefore, besides having good combining ability, an inbred line must be easy to maintain and to cross in order to keep seed costs down.

The inbred lines used for commercial hybrids must be maintained by hand-pollination, a painstaking process (Fig. 4). For production of hybrid seed, inbred seed is planted in fields isolated from other corn by at least 200 m (600 ft). Hybrid seed is produced by planting the ‘female’ and ‘male’ inbred lines together in a field (Fig. 7).

The choice of which inbred to designate female and which to designate male depends on the ear and tassel characteristics of each; usually the female has higher yield and the male has better pollen production. The ratio of female to male rows varies among seed companies. Differential planting dates can be used to ensure synchronization between male and female flowering.

Female rows are detasseled mechanically or by hand shortly after the tassels have emerged from the uppermost leaf sheath and before they begin to shed pollen (Fig 8). This ensures that all pollen is from the male parent. Commercial seed-corn fields are normally harvested by a picker-husker and the husked ears are sorted to remove off-type ears. The ears are dried and shelled and the seed is cleaned and graded by size. Finally, germination is tested and the seed is treated with a fungicide before packaging.

Today, 80% of corn seed grown in North America is single-cross hybrid as described above. The remaining 20% of hybrids are double, three-way and modified (related-line parents) crosses. Three-way cross hybrids have only one inbred parent and are somewhat cheaper to produce.

The Industry Speaks

DEKALB CORN SEED

Genomic and biotechnology research will be the key to better corn performance. Biotechnology and genetic enhancement are powerful tools Monsanto uses to improve corn. Globally, 10% of all Monsanto sales are invested in research and 80% of that research is conducted on seed and trait development. Specifically, research is focused on identifying traits for insect and disease resistance, herbicide tolerance, enhanced food characteristics and processing quality that can be used to improve crops. We’re also working within the existing genome of the corn plant to identify genes and genetic combinations that enhance yield and performance through improved drought and cold tolerance, disease resistance and yield.

Drawing from Monsanto’s elite corn germplasm, our corn breeders focus on factors that influence yield. This means only hybrids with the highest ratings for early vigour, early flowering, disease tolerance and straw strength are candidates for further testing. Monsanto’s Canadian corn breeding team will oversee the testing of 11,000 experimental corn hybrids. These hybrids are developed across Eastern Canada and North America.

Grain and silage from advanced technology crops such as Roundup Ready, YieldGard Rootworm and stacked trait hybrids require channeling – the process of marketing approved grain to approved markets. Monsanto is committed to ensuring that growers have a market for new technology. Before advanced technology crops are sold commercially, they must have full feed and food approval in Canada, the United States and Japan.

Part of Monsanto’s Technology Development is to continue to test DEKALB corn hybrids under reduced tillage practices. DEKALB brand Reduced Tillage hybrids have been developed, tested and proven to deliver higher yields on reduced till soils that are often cooler and wetter due to crop residue on the soil surface. Each of 14 RT DEKALB corn hybrids has demonstrated superior emergence, vigour and disease tolerance, which delivers excellent results in reduced tillage environments.   RT DEKALB corn hybrids have demonstrated consistent performance, excellent early season emergence for a vigorous, fast start inthe spring even in cool, moist soils and they possess a superior defensive package of disease resistance and tolerance that stands tall against yield-robbing diseases.


HYLAND SEEDS

Francis B. Glenn, Ph.D.,
President of Glenn Seed Ltd.

The development of leafy silage corn varieties was an evolution of thought and germplasm that has led to a revolution in silage corn management. The use of blends and “dual purpose” varieties for silage is in decline.

Corn is bred at the inbred (parent of hybrid) level. For silage, the parent should have: good germination and seedling vigour in both good and cool conditions; rapid spring development to close the canopy quickly; large leaves and extra leaves; big ears with disease free kernels; soft kernels with less of the hard vitreous and more soft starch; flexible, non-rigid stalks; good leaf-disease resistance to keep the plant green and productive; and early flowering with a long grain filling period.

Through experience and farmer “feedback”, we have developed an “ideotype” or ideal hybrid. We have been selecting extra leaf and normal leaf number families with these traits for about twenty generations, so we now have a “stable” of inbred lines. In leafy types, we have found that low ear placement is

associated with less stiff stalks, more photosynthetic leaf area to feed the ear, less stalk breakage and root lodging because of a low centre of gravity. Selection for stalks that bend without breaking has resulted in stalks with lower lignin content (better digestibility) than grain hybrids. Such stalks would be inferior in a grain hybrid.

Leafy hybrids have more young leaves above the ear resulting in high sugar content in the stalk and leaves at harvest. Growers have told us that the fermentation of our leafy hybrids is rapid and the silage has a sweet fragrance. Farmers say the cows’ intake and milk production increases. Selection for

softer kernels has produced kernels that break up during harvest, facilitating rapid starch conversion to sugar. Farmers often comment that kernels are not seen in manure of cows fed leafy silage.

We have selected hybrids that have long kernel-fill, good leaf health and slow drying after the black layer stage. Our leafy hybrids stay in whole-plant moisture range of 60 to 70% for a longer time than do grain hybrids, giving farmers more time to harvest. We have also selected for high grain content by choosing lines with consistently large ears. We do not recommend these hybrids for dual purpose (both silage and grain production).

We conduct our yield trials at 69,000 plants/ha (28,000 plants/ac), because our research has shown that increasing population may give more yield under best conditions, but less grain and lower stalk digestibility under more adverse growing conditions. At this population, our leafy hybrids produce

large canopies, ensuring high silage yield and quality under dry soil conditions. Although leafy plants have a large leaf area, our leafy hybrids are selected to withstand drought. We yield-test new hybrids at several locations until we determine their climatic or maturity zone, and their ability to perform under stress.

We have conducted a huge amount of silage quality analysis in testing our screening and advanced varieties. We conducted a detailed comparison of different quality tests to determine the most useful quality testing procedure for silage hybrids. Our conclusion is that all techniques give very variable

results and that no technique gives a precise enough measurement on which to base hybrid selection. Currently we do not use any quality analysis in our hybrid selection.

Leafy silage hybrids have changed farmers’ views. A revolution in corn silage is happening and leafy silage hybrids are a big part of the revolution.


MYCOGEN SEEDS FROM DOW AGROSCIENCES

For over 15 years, we’ve been offering Silage-Specific™ corn hybrids that have defined and set industry standards. Bred to deliver nutritional feed quality and high silage yields, our award-winning lineup helps producers boost dairy and beef production. In addition, our local dealers help ensure that producers get sound silage management advice.

World-class research & development drives discovery
At Dow AgroSciences, we are continuing to evolve the development of forage hybrids to even higher levels. We are one of the few companies with the dedicated resources in place to conduct Silage-Specific™ corn hybrid seed and trait research and development. Our breeding and research facilities are located throughout the North American corn-growing region. All of our facilities provide the perfect breeding ground for hybrid and trait advancements that add real value to every ration. A broad distribution of research and breeding stations allows Dow AgroSciences to test hybrids and traits in a variety of growing conditions. We are continuously advancing our hybrid genetics and nutritional knowledge to maximize animal performance.

Get Silage-Specific™ corn hybrids for unsurpassed feed advantages
Better rations mean more milk or beef. That’s why Mycogen Seeds Silage-Specific™ corn hybrids are such assets for any operation. Our exclusive lineup includes TMF™ and FullTime Forage™ hybrids bred to create better rations to maximize production of dairy or beef cattle. Our forage hybrids provide many advantages:

  • High yields
  • Greater cell wall digestibility
  • Higher dry matter intake and total nutrient intake
  • Ability to utilize higher forage diets, which results in healthier animals, reduced feed costs and increased milk and beef production.

Utilizing our Forage Quality™ Evaluation Program to ensure quality
Mycogen Seeds Silage-Specific™ corn hybrids are the industry’s finest for many reasons. Bred specifically for silage production, they’re selected based on forage quality, yield and grain characteristics. Adhering to the most stringent standards set by our Forage Quality™ Evaluation Program (FQEP), we are able to evaluate each and every Silage-Specific™ hybrid to ensure it delivers top-quality forage. Take a look at the three key points of emphasis in our FQEP process:

1) Drawing on silage evaluations and trials for nutritional data.  Working with leading universities and USDA research facilities, Dow AgroSciences conducts yearly silage evaluations on nearly 10,000 samples. Each is thoroughly analyzed to assess cell wall digestibility, lignin content, whole-plant digestibility, non-fibre carbohydrates and other characteristics. They’re also compared head-to-head against competitive forage hybrids.

2) Measuring animal performance through in-vitro testing.  For the most accurate prediction of how hybrids will perform in an nimal, we measure silage quality through “in- vitro testing”. These detailed tests provide firm data on the digestibility and nutrient value of silage yielded from our Silage-Specific™ corn hybrids. These data allows us to confidently determine how animals will perform with our silage.

3) Tonnage Evaluation Plots help us review hybrid yields.  In order to truly understand the agronomic conditions where each hybrid thrives, we plant and harvest Tonnage Evaluation Plots (TEP’s) in leading dairy regions. From these plots, we’re able to assess silage quality and see how hybrids perform in a variety of environments.

“When you’re talking leadership in silage, you’re talking Mycogen Seeds from Dow AgroSciences.”


NK SILAGE HYBRIDS

“Maximizing feed value per acre”

Selecting and growing corn for silage is different than grain. Hybrid selection is the first step but optimum harvest moisture, chop length, ensiling and ration-fit play essential roles in realizing the total silage value.

Hybrid Selection
NK brand, Syngenta Seeds selects and rates silage hybrids based on a combination of agronomic characteristics, tonnage and feed quality characteristics. Hybrids designed for maximum beef production and profitability are high in whole-plant digestibility and starch content. Hybrids recommended for dairy are high in fibre digestibility, which results in increased dry matter intake for increased milk production.

NK also utilizes a ‘Ration Fit’ rating to assist in hybrid selection for dairy operations. Our strategy is to build the optimum diet formulation for maximum animal production while minimizing potential herd health concerns from excessive starch loads. Hybrids recommended for rations containing greater than 60% corn silage should be high in NDF (Neutral Detergent Fibre) with low starch and high fibre digestibility while hybrids recommended for rations with greater than 60% alfalfa are low in NDF with high starch and fibre digestibility.

If a portfolio of hybrids is required due to crop rotation or soil adaptability, selecting hybrids of similar forage quality characteristics and ration-fit maintains a consistent level of forage quality. The net results are:

  • Silage predictability
  • Silage consistency
  • Reduced diet reformulation
  • Reduced production swings

Production Recommendations
In general, silage planting recommendations are similar to those for grain corn. Considerations should be made for soil type, tillage and crop rotation. Final plant populations should be based on soil productivity and, at most, 5,000 to 10,000 plants/ha (2,000 to 4,000 plants/ac) greater than those recommended for grain. Maintain good fertility levels and be aware of any herbicide sensitivities.

Harvest Management
Harvest timing is critical for silage quality. Wholeplant moisture between 65-70% allows for maximum dry matter accumulation with minimal impact on digestibility (see Fig. 1). Chop length should be consistent (1.3 to 2.0 cm or 0.5 to 0.75 in) for optimum packing and fibre utilization. A storage structure that is filled quickly, packed and sealed with silage at optimum moisture levels, ensiles efficiently with reduced respiration losses, heating and spoilage.

Hybrid selection and base genetics make up only 20% of the quality story. Without good crop management, harvest timing, optimum storage and ration fit, even the best hybrid can produce disappointing results. It takes genetics and the total management package from planting to ration fit to realize true silage quality value.


PICKSEED

Since grain in the silage makes the greatest contribution of energy to the animal, some argue that the best silage hybrid is tall with high grain yields. Decades ago that might have been true, before plant breeders started selecting specific traits to put into grain hybrids, such as hard stalk rind enabling plants grown at high populations to resist lodging, and hard kernels with dense starch so that the grain would have high test weight and remain intact during harvesting. To improve yields without delaying harvest, breeders developed grain hybrids that silked late but dried down rapidly.

The current idea is that we need corn hybrids dedicated to silage use. “While excellent silage hybrids with high forage yield and high quality exist, dual purpose hybrids that are excellent for both silage and grain do not. This is because characteristics that make an excellent grain hybrid such as fast rate of kernel drydown and hard kernel texture are undesirable for silage production since they reduce the digestibility of starch in the grain. Kernels in corn silage should have high moisture and be of soft kernel texture to increase digestion. (Dr. M.S. Allen, Assis. Professor, Department of Animal Science, Michigan State University “Selection of Corn Hybrids for Silage- A nutritionist’s perspective.”)

It is important to think of silage corn in terms of forage and not grain production, and assess it in terms of high dry matter yields, palatability, and nutritive quality. At Pickseed, we believe that there should be “different horses for different courses” so we set out to develop distinctive grain and silage varieties. In the late eighties we began working with the “leafy” gene to develop silage hybrids with comparably high grain yields and plant size but more total leaves than traditional grain hybrids. The silage hybrids would also have high whole-plant dry matter yields at lower plant populations than conventional hybrids as well as soft starched grain. Our experience and observations are as follows:

  • The leafy types continue to initiate leaves in later growth stages and these younger leaves are higher in protein than older leaves.
  • The younger, healthier leaves produce extra carbohydrate, which is not stored in the grain (useless for a grain hybrid) but remains as soluble sugars in the stalk, to produce more palatable silage.
  • The leafy hybrids are huge and produce high dry matter yields with corresponding grain content, at populations of 59,000 to 69,000 plants/ha (24,000 to 28,000 plants/ ac).
  • Similar yields can be achieved by planting a conventional grain hybrid at higher plant populations but this would reduce the digestibility and crude protein of the silage while increasing the NDF and ADF. It is also an inefficient use of resources since sunlight, water and soil nutrients are employed to support non-productive portions of corn plants (e.g. additional roots would be needed in dry years) not to mention added seed cost.
  • The softer and larger kernels of the leafy hybrids fracture more readily during harvest and chewing, to reduce fecal starch.
  • The leafy silage hybrids although genetically early, tend to silk late since silking cannot occur until all the leaves have been initiated and these hybrids produce on average 3 to 4 more leaves than non-leafy hybrids of similar maturity. Although the grain moisture of leafy hybrids is high, wholeplant moisture is low, which is ideal for silage varieties.


PIONEER HYBRID

Dave Harwood, Research Coordinator, Johnston IA

Pioneer Hi-bred has been conducting specific research and development of corn hybrids for use as silage for the past 25 years. The result is a product line-up that leads the industry in bite for bite feed value and feed value per acre. In addition to having excellent nutritional characteristics, these hybrids have agronomic stability.

Pioneer’s dedicated effort to develop new silage products represents the industry’s largest such program. Conventional hybrid development continues with extensive multi-location evaluation for silage yield and quality. New techniques, however, are being deployed with the objective of increasing genetic gain for silage performance. These approaches include: improved nutritional characterization, associative breeding and transgenics.

The ability to make genetic gain for any characteristic is to a great degree a function of the ability one has to measure it. In the case of silage performance, a measurement is only valuable to the extent that it accurately and with adequate precision, predicts the way an animal will respond when fed the feed. Pioneer continues to address this important issue by continually refining the methods used to measure nutritional quality of corn silage at its world-class Livestock Nutrition Center in Iowa. This is done by staying abreast of the latest industry trends such as the use of the University of Wisconsin Milk 2000 equation for predicting nutritional value per ton and per acre. At the recent 2002 Cornell Nutrition Conference, Pioneer scientists introduced the next wave of nutritional evaluation that takes into account the site, rate and extent of forage digestion. Adopting this approach will further enhance Pioneer’s ability to make genetic gain for silage performance as realized by enhanced animal performance.

Associative breeding is a relatively new term that describes the concept of identifying the specific genetic components of a plant that contribute to specific characteristics. If one can make these sorts of associations, one can use the genetic information to identify desirable plants without having to grow and measure the plant at many locations. Pioneer is developing this technique to improve traits like the digestibility of the corn plant’s fibre. This approach can increase the rate of genetic gain for silage performance by greatly increasing the amount of genetic material that can be evaluated and the accuracy of the evaluation.

Transgenic technology is commonplace in North American corn production. The value of the application of transgenic technology to deliver traits from other species, like resistance to European corn borer and resistance to herbicides, is well understood. Perhaps less well recognized is the opportunity to use this technology to manipulate the expression of genes from within the target species. Pioneer researchers are investigating the isolation of genes native to corn that are responsible for characteristics valuable in silage hybrids. Through this approach it is hoped that the expression of these genes can be optimized to result in genetic gain for feed value of the wholeplant corn crop.

Advancing corn genetics for silage performance remains a high priority for Pioneer, recognizing the importance of corn for silage in the key corn production areas around the world. Pioneer is dedicated to delivering value to corn growers world-wide through improved plant genetics.