With each bump in price, wheat becomes an ever more valuable addition to the Southeast crop mix.
So this year, more than ever, variety selection and planting date are vital for insuring top yields next spring.
Planting the right variety at the right time best insures that wheat flowers at the optimal time, namely when the risk for freeze damage is at its lowest but before the onslaught of heat stress, says Brenda Ortiz, Auburn University Extension specialist.
“The key to maximizing wheat yields rides on two related decisions: choosing the right variety and planting it at the right time to ensure optimal flowering and, ultimately, maximum yield,” says Ortiz.
The problem with variety selection and planting is that there are no “hard-and-fast” rules, she says. Both decisions are essentially a compromise with Mother Nature. Opting to plant early can improve crop establishment but can also cause early flowering, which increases the risk of frost damage.
Early planted wheat is also more prone to insect pests, such as the Hessian fly and the fall armyworm and diseases such as barley yellow dwarf.
“An added challenge in the Southeast is that wheat planting dates are closely tied to the harvest of summer crops such as cotton and soybeans,” says Ortiz. “Delayed harvest of these summer crops due to wet soil conditions may force producers to plant wheat later than preferred, increasing the likelihood that flowering will occur in hot, dry conditions.
“The end results are likely to be reduced yields and test weights.”
Much remains to be learned, she says, about the effects of variety and planting dates on wheat yields and related issues.
Between 2009 and 2012, researchers in Alabama conducted studies at various locations throughout the state to better understand how these decisions, coupled with seasonal climate variability, affect wheat growth and yield.
Research has shown that winter wheat growth is reduced when average fall/winter temperatures drop below 50 degrees F., say Ortiz. Wheat may even become dormant to protect itself from cold injury.
Low temperatures needed
“To overcome this dormancy, plants require a period of exposure to low temperatures. The process is called vernalization and results in the formation of flower heads. Wheat head size, or the number of spikelets per head, is determined when the vernalization requirements are met,” she says.
In the Southeastern United States, most winter wheat varieties require three to seven weeks of vernalization, according to Ortiz. To vernalize, early maturing varieties require less time (less exposure to cold temperatures) than do later-maturing varieties.
Both warm and cold weather conditions can affect wheat growth development, she says. Prolonged warm winter weather can cause wheat to lose some accumulated vernalization units. In such weather, varieties with longer vernalization requirements (approximately six weeks) will not fully vernalize.
“A wheat variety receiving only a partial amount of cold weather required for vernalization either will produce few heads — between 1 and 10 percent — or it will produce full heads, but mature when the weather is very hot, resulting in poor yield and test weight,” she says.
In the Southeast, maximum vernalization occurs around 40 degrees F. An ideal vernalization day would have a nighttime low of around 38 degrees F. However, vernalization occurs even if maximum temperatures on a cold day are above 35 degrees F. and nighttime low temperatures are below 50 degrees F.
The required length of low-temperature exposure decreases with colder temperatures and advanced plant development.
Day length — the number of hours between sunrise and sunset — also can affect the length of time required to reach flowering and heading stages, says Ortiz. Variety selection should closely tie to a day length’s anticipated effect on heading time.
There are some other special issues to consider with day length, she says.
“For example, day length interacts with vernalization and heat units, which makes it difficult to predict its effect on wheat development, especially during mild winters. Also, day length–sensitive wheat varieties are long-day plants, which means that flowering is induced by longer days. The longer the days, the fewer heat units are required to initiate flowering.”
When a long-day wheat variety is exposed to above-average temperatures, early flowering and a short grain-filling period may occur, she cautions. This might result in low yield and reduced grain quality.
Late planting may cause yield losses, especially on medium and late-maturing varieties, says Ortiz. Moreover, late-planted wheat has less time to tiller during the fall.
“Most tiller development occurs in the spring with late planting of late-maturing varieties, though spring tillers contribute less to yield potential than fall tillers. Late planting also may shorten the grain-filling period and delay it until the weather is warmer.”
By contrast, early planting causes excessive tillering during fall and spring, which increases the risk for spring freeze injury, she says. “Excessive growth also may cause wheat to grow taller in the spring, which promotes lodging. The impact of planting date on final yield varies by location in Alabama.”
Data from an Alabama field study conducted between 2010 and 2012 demonstrated the negative effects of delayed planting on wheat yield.
Three wheat varieties, AGS 2060 (early maturity), AGS 2035 (medium maturity), and Baldwin (late maturity) were planted at different planting dates, each one 15 days apart, in north, central and south Alabama.
In north Alabama, yield losses of 21 percent, 12 percent and 21 percent were observed for the 2010, 2011 and 2012 seasons, respectively.
In south Alabama, yield losses for the same years were 16 percent, 20 percent and 55 percent.
In central Alabama, no yield differences were observed in 2012; however, yield losses between 26 percent and 28 percent were observed in 2010 and 2011.
The data also showed that planting 15 days earlier than the producers’ planting date resulted in yield increases in 2010 and 2011 across all locations, though no benefits to this earlier planting date were observed in 2012.
The effect of planting date on yield was also studied using data from 14 years of trials in Tifton, Ga. Data showed yield declines in varieties that develop heads late in the season. Yield losses were higher for later planting dates than for the standard planting date.
Research results confirm that choosing the right variety and planting it at the right time can positively affect wheat development and final yield, says Ortiz. This means choosing varieties and planting dates best suited to your specific growing season and location.
The following are important points to consider:
• The differences among varieties are most expressed at heading date and least expressed at maturity when the grain moisture is almost the same.
• In selecting varieties and planting dates, be aware of freeze damage and vernalization.
• Producers in the southernmost areas might choose early plantings for short vernalization varieties. This carries some risk because early plantings increase the likelihood of early growth which, in turn, may result in severe winter kill or damage from late-spring freezes.
• Early maturing varieties are good options for planting late in the season. In cases where you don’t have information about a variety’s vernalization, remember that, as a general rule, it requires little to vernalize.
• Take special care with delayed plantings of early maturity varieties in the southernmost areas. In cases where planting is delayed a month with respect to the standard planting date, yield losses tend to run higher than they run in northern areas.
• Later maturing varieties, which are more likely to avoid freeze damage, are generally better suited to the northernmost regions. Data collected in Belle Mina, Ala., between 2010 and 2012 show that early plantings for medium and late-maturity varieties resulted in higher yields compared to the early maturity variety. Yield losses and low test weight might be expected if choosing late-maturing varieties for late plantings because of lack of proper vernalization or late grain-filling occurring in hotter weather.
• A delayed planting date may result in reduced yield, reflected in reduced seed weight, though these effects vary between varieties and locations.