Small Grains Focus

Does Nitrogen Fertilizer and Seeding Rate Affect Wheat Tillering?

By Jochum Wiersma,
U of M Small Grains Specialist
wiers002@umn.edu

As part of the recent Minnesota Wheat & Barley Growers Associations Prairie Grains Conference, I took part in a panel discussion on “Intensive Wheat Management.” To prepare for the occasion, I went back to the research literature and reviewed the relationship between nitrogen fertilizer, seeding rate, and number of tillers. I focused on this specific interaction because:

1) It is one of the key (and most contentious) components of an intensive wheat management system that is being marketed in the region.

2) My own research has delved into the optimum seeding rate to maximize grain yield for a number of modern hard red spring wheat cultivars. See column and research summary “Variety Specific Seeding Rates” online at www.prairiegrains.com . Click on the “Prairie Grains” link, then “2001,” and “March 2001.”  The research summary is under link “Production Tips by Jochum Wiersma.”

Bauer et al (1984) stated that an adequate seeding population has a significant impact on maximizing grain yield of spring wheat, because this initial seeding population largely determined the number of heads at harvest.  Their research showed that the number of tillers a plant produced was determined by: 1) the variety itself, 2) the seeding rate used, and 3) the amount of available nitrogen. 

Their research showed that the number of tillers increased with additional nitrogen if the soil was deficient in available nitrogen.  These findings do lend support to the claim of commercial IWM advocates, that the number of tillers can be regulated with the amount of pre-plant nitrogen fertilizer applied. Other research findings do not support this, however.

Longnecker et al (1993) demonstrated that the initiation of tillers was not reduced if adequate nitrogen was available to produce the normal number of leaves.  Only when leaf initiation was delayed or halted as a result of severe nitrogen deficiencies did it also reduce the number of tillers. 

Inducing such severe nitrogen deficiencies that leaf initiation is halted or delayed is not a desirable management tactic, in my opinion, because of our short growing season, and also because it is not likely we have such low levels of available N in many of our soils that it would result in a delay or halt of normal leaf development.  Further, nitrogen is not likely to be the only factor influencing tiller formation. The amount of available water, the amount of available light (interplant competition in which neighboring plants shade one another), and temperature also determine the number of tillers that will be initiated.

Power and Alessi (1978) as well as Bauer et al (1984) found that nitrogen deficiencies early in the growing season were particularly detrimental for tiller survival, while increased availability of nitrogen allowed for a greater survival rate of tillers.  An increase in available nitrogen allowed the additional tillers to be more competitive and produce grain.

In their studies, grain yield was closely related to the number of tillers that produced grain.  With adequate nitrogen fertilizer, the number of heads per acre that produced grain increased. This indicates again that other factors besides N determined tiller survival and the number of tillers that produced grain.

Bremmer et al (1969) demonstrated that tillers which did not survive had an insignificant effect upon plant growth, and no effect on final grain yield.  With nitrogen fertilization and seeding rate according to current University recommendations, wheat will likely initiate more than maximum number of tillers it can support to the end of the season. However these unproductive, higher order tillers do not have a negative effect on the final grain yield.

The findings of both Bauer and Longnecker support results of the research I conducted to determine an optimum seeding rate for new spring wheat cultivars. First, individual varieties differed for the optimum plant population that resulted in maximum grain yield.  Secondly, the initial plant population needed to increase as planting was delayed.

So what’s this all mean? Well, this data points out that it is questionable to expect a larger wheat yield by increasing your wheat seeding rate and using split N applications to reduce excess tillering. A lot of factors affect yield, but you can go a long way to maximizing yield potential by fertilizing according to soil test recommendations, and calculating the right amount of seed to plant.

Seeding rate is a function of the number of kernels per pound of seed, percent germination of the lot, expected stand loss, and desired stand.  An average optimum stand of spring wheat when planted early is generally between 28 to 30 plants per square foot or about 1.25 million plants per acre.  This number should increase by one to two plants per square foot for every week planting is delayed past the optimum seeding date.  Keep in mind that expected stand loss even under good seedbed conditions is 10%-20%, and will increase with a poor seedbed or seed placement.

Still have questions on the effect of seeding rates and split N rates on plant tillering and yield? If you have a yield monitor, then consider conducting your own side-by-side field comparisons the next growing season.