Nitrogen Use Efficiency in Maize

Timing and form for better yield

Maize is a C4 crop, with a metabolism evolved to optimise growth in hot arid conditions. Understanding how this metabolism works can give us an insight into getting higher yields from our fertiliser inputs.

 What is a C4 crop?

To photosynthesise plants need to extract carbon from CO2 to make sugars, most plants use a type of carbon fixation called C3 where the whole process takes place in the mesophyll cells where CO2 is fixed using RuBiSCO enzymes.

Maize uses a different system called C4, where CO2 is incorporated into the organic acid malate, and transported to bundle-sheaf cells where it is released and then fixed with RuBisCO.

This C4 process uses more energy, but protects RuBisCO from photorespiration (where RuBisCO combines with oxygen instead of CO2 which reduces its efficiency).

Maize and other C4 plants evolved this mechanism as a way of adapting to arid conditions, as C4 photosynthesis uses less water with 277 molecules of water used for each molecule of CO2 fixed instead of the 833 molecules of water used in C3 plants like wheat. So the process uses higher energy but less water to fix carbon.

So how does this impact nitrogen use?

 Maize has evolved C4 photosynthesis to conserve water, and to be able to use CO2 efficiently where access is limited. Plants growing in hot dry conditions conserve water by limiting access to CO2 via stomata. The plant is prioritising water efficiency, by adopting a more efficient way of processing CO2.

 Nitrogen form also affects carbon use efficiency, with different forms of N requiring different levels of carbon to process. Nitrate nitrogen takes 7.4g of carbon to create 1g of protein, whereas amine and ammonium N require only 0.6g of C to make 1g of protein. This is due to C inefficient nitrate reductase enzyme systems that plants use to turn nitrate to more useable amine forms of N.

When we feed crops nitrate, we increase the amount of carbon they need in order to grow. In crops like maize that have evolved to be carbon efficient we reverse this efficiency when we feed them in a way that makes them less efficient. Diverting energy from photosynthesis to nitrogen conversion.

Growth stage effects N efficiency in Maize

Maize responds differently to forms of N at different growth stages. If we look at dose response to nitrogen inputs we see there is a ‘linear plateau’ response. Initially adding N gives a good grain yield response, but this flattens out eventually with additional N not turning into extra yield.

 When we look closer at growth stage we can pick out difference in response to different kinds of N, which give us clues as to how to use agronomy to push yield up.

At early growth stages corn responds well to nitrates, as vegetative growth is key for growth stages V1-V11. However after V12 additional nitrogen starts to have little effect, leading to nitrate accumulation in the stem and not converting into growth or yield.

Research into response to different forms of N between V12 and V15 growth stages shows big differences between response to nitrate, and ammonium and amine N. Nitrate has little or no effect on R1 ear weight, but by contrast amine N has a large impact on R1 ear weight.

Maize can take up nitrates faster than it can turn them into protein, this results in nitrate accumulation in stems and a negative impact on photosynthesis. Malate builds up in leaves (malate is the organic acid corn uses to move CO2 to bundle sheaf cells to fix carbon) as the plant diverts energy from photosynthesis (using RuBisCO enzymes) to nitrogen assimilation (using nitrate reductase enzymes) and growth of grain stalls.

Research shows that foliar application of urea, and particularly stabilised urea during V12-V15 is more effective than 10 times the quantity of nitrate at improving ear size at R1

 LimiN chemistry was developed by Levity CropScience to stabilise N in the amine form. Application of products using LimiN like Lono are far more effective at influencing ear growth than conventional N applications applied more than 5 weeks post emergence.

 Use of Lono on maize allows the crop to use the additional N for reproductive growth, rather than just creating a backlog of N to process. With Lono growers can rescue ear weight where conventional fertiliser would be ineffective.

Recommendations

 Apply 2L per Ha Lono between V12 and V15 to increase ear weight. In drought conditions additional applications between V16 and R2, and between R3 and R5 will protect yield.