A soil test is the best way to determine a need to apply copper. Other reasons to consider using copper are you:
Copper is important for pollen formation, viability and fertilization. A shortage can therefore reduce the number of grains per ear, even when physical deficiency symptoms are not visible (a slight deficiency can reduce yields by as much as 20%).
Copper is a relatively immobile nutrient in the plant and so deficiency symptoms will often manifest in the younger leaves. They become distorted and twisted or curled into a spiral as a result of a lack of lignin to stabilize the structure. Growth is typically stunted and the crop can give the appearance of sudden drought stress, frost damage or pesticide scorch. Flowering and maturation can be delayed by up to 2 weeks and lignification can also be severely impaired, making the crop more prone to lodging, particularly when higher rates of N are applied. In severe cases, new growth withers before unfolding, so that only a few heads or ears emerge. Because of the role of Copper in flowering a deficiency effects grain yield much more than vegetative growth. Subsequently, sub clinical deficiencies are common, where no visible symptoms occur but significant yield is still lost.
Increased nitrogen applications have been shown to actually reduce yields in copper deficient cereal crops. Work conducted in Western Australia by Brennan (1994) found that wheat yields in copper deficient conditions were reduced as the rate of nitrogen applied increased.
Conversely, where copper was applied, the increasing nitrogen applications resulted in increased yields. The presumption was that nitrogen applications stimulated the growth of the early developing leaves, locking up the immobile copper within these leaves and so preventing it reaching the flowering parts.
In interview to Real Ag Radio on March 28, 2019, Yara's Crop Manager Markus Braaten talks about the critical role each micronutrient plays in producing the best possible crop.