Today I heard a news story on local radio about a farmers protest campaign in Hamburg (Germany). Currently in Germany the discussion about sustainability, climate change and environmental protection is very active. The background of the protest is that many (conventional) farmers feel that they are blamed for all the environmental problems we have and don't get enough credit and support from society. One of their main points of critique is that there are too many new laws that restrain them from doing their work. In general I can't comment on that since I am not a framer. During the show there was an interview with a farmer that claimed that the restriction of pesticide use on wheat would reduce the protein content of the wheat and thus make it unsuitable for baking bread. I don't understand the logic behind this statement. I would rahter think that the restriction of pesticide use would reduce the yield per hectar but not alter the properties of the grain itself. So my question is: What is the link between the protein content of wheat (or any other grain) and the use of pesticides?
Farmers wanting (or being forced) to avoid the use of pesticides have a wide variety of choices at their disposal. Most of them will in fact alter the quality of the crop, but most of the alternatives are often not known or considered.
Switching to a different variety is obviously one of the easiest, but it also has a big effect on the final product. Altering planting time is also very often considered, and that also has an effect. Types of pesticides, types of insects, types of flour, types of wheat, and many other factors are involved, but I'll try to hit the main ones here.
For making bread, hard winter wheat or hard red spring wheat tend to contain higher protein content. The protein in bread wheat might be between 10% in some soft wheats to over 15% in hard wheat. Hard red winter wheat normally has around 13% protein, and hard red spring wheat often has well over 15% protein. If your pest avoidance strategy involves switching your planting time from Fall to Spring, you will be using a different species and will obtain a different quality flour.
The current impetus in Europe is leaning towards reduction of all agrochemicals, some more than others, but a change in nitrogen available in the soils will also have an effect on the protein content of wheat. A farmer can increase nitrogen in various ways, but many only consider the chemical options. In some cases, increasing nitrogen can have the opposite effect by simultaneously increasing competitive weeds. Those weeds could then, in turn, actually reduce the nitrogen, but if legumes are planted to outcompete the weeds, then an important balance can be restored. All of this in turn has an effect on the types and population sizes of local insects.
It's hard to say what particular issues the farmer in that particular interview was facing in is fields, and what strategies he considered or failed to consider. A lot depends on the overall condition of soil, climate, season, local ecosystem, and intended use of the crop. All we know is that he was trying to grow bread flour in Germany and was complaining about forced reduction of pesticide use and the effect that has on protein content.
Current restrictions on neonicotinoids restrict their use on wheat sown between January and June, so it's possible that we're talking about autumn sown Spring wheat. Winter wheat, compared to spring wheat, will generally be more productive with higher yields and more consistent results, but it requires significantly higher levels of inputs.
The European Food Safety Authority (EFSA) tends to have a rather scientific perspective when considering the impact of agrochemicals on farmland. They find that encouraging natural predators is an important alternative to insecticides which could kill the very species of insects needed to control the target pests. Many farmers don't take that into consideration because their primary concern is the condition of the crop they currently have in the ground, and if a pest threat arises, they will spray. Some may wait until they feel it's needed, while others may spray as a precautionary measure.
For the Orange Wheat Blossom Midge, quantities and frequency of spraying can mean the difference between one midge per three wheat ears for feed wheat, but as low as one midge per six ears for milling wheat. Crops grown for milling wheat may fail to meet certain specifications which means they get sold as feed wheat for less profit. Some parasitoid wasps can be introduced to prey on these midges, and some may even arrive naturally after reduction in spraying. The trick is to perfectly time the spray because wheat heads are susceptible to wheat midge only between heading and flowering. Spraying at other times kills the wasp but not the midge. Information such as this needs to be in the hands of farmers, and entomologists can be provided to help them assess their options.
Nevertheless, bug damage tends to affect the protein quality of wheat more than its protein quantity, and this is a big concern when growing wheat for bread. Flour that has high-protease activity caused by damage by some Heteropteran insects are not suitable for breadmaking at all. Some species of Eurygaster and Aelia, and the "wheat bug" (Nysius huttoni) attack developing wheat kernels and inject salivary secretions into the developing kernel which dissolves some nutrients with proteases and breaks down the protein and gluten structure, especially evident during mixing and fermentation of the bread. In general, varieties of wheat with low baking quality show the effects of bug proteinase more than cultivars considered to be higher baking flour quality.
It's a very complex issue with countless possible approaches, but it's certain that the farmer in question could be assisted with good research based information to help overcome his concerns and possibly produce a superior crop with a better profit margin using fewer chemicals. Incentives towards long term solutions are most needed, and they can also show good results in a single season, but restricting his options in some areas can only be countered by increasing them in others. The same holds true for agriculture worldwide, but understanding bread flour in Germany is a good start.
Summary: either there is no link, or no one did any research on this. The latter doesn't seem likely given the amount of research there has been on grain protein content. Most likely the farmer meant 'use of herbicides' as this does have an effect on protein content.
I don't have access to the full article, but in this abstract they say that
the grain protein content variation was mostly explained by the baking quality grade of the cultivar, crop nitrogen status and weed density at flowering. ...to a lesser extent, climatic factors also explained grain protein content variability.... grain protein content of organic winter wheat could be increased by improving fertilization management, using an improved baking quality grade cultivar, choosing a legume fodder crop as preceding crop, or by avoiding late sowing dates
There is no mention that the use of pesticides has an influence, but in all fairness this research was on organic winter wheat so it could be that they didn't consider pesticides.
However this article about different types of wheat also claims that wheat genetics and growing conditions are major factors that determine protein content. The paper also says that
Various studies have compared the protein contents of wheat varieties from the early part of the 20th century with those of recent varieties. When grown under comparable conditions, there was no difference in the protein contents
Sadly the term 'comparable conditions' is not explained further, but I'm assuming it refers to climatic conditions. Since wide-spread usage of synthetic pesticides started after the second world war, this suggests that protein content did not change due to the use of pesticides.
None of the other research I found mentions a link between pesticide usage and protein content. That doesn't necessarily mean there is no effect, but it does make it improbable given the large number of other factors that have been investigated. I did find this paper where researchers found a relationship between the use of herbicides (so not pesticides!) and raw protein in grain. This led me think that the farmer made an error and actually meant herbicide usage.