Weeds are one of the main challenges that farmers face to grow crops. Weeds are plants that grow at the same time and on the same place as the crop. If there are too many, they can cause yield losses because they compete for resources such as water, space, nutrients or light. In my research, I am interested in understanding the ecology of weeds to be able to identify what drives their abundances and distribution. My work on weeds has mainly focused on two different topics: weed seed predation and using cover crops for weed management, but I am currently starting to look at how soil fertility impacts weed-crop competition.
1. Weed seed predation
In many agricultural systems there are organisms that eat seeds, and they also can eat weed seeds. My research on weed seed predation has focused on understanding who the main seed predators are in each system, when they are active relative to when weed seeds are available and how management can help promote weed seed predators. Most of my weed seed predation worked happened in Spain, in both rainfed and irrigated cereal systems and the main predators I encountered were harvester ants, granivorous mice and carabid beetles. My research was one of the first ones to study weed seed predation in semi-arid systems and showed that harvester ants can take huge amounts of weed seeds from the rainfed systems, and seemed to be favored by low tillage intensity. Ants are active and looking for seeds when the main weeds are producing seeds, around June and July, right before cereal harvest. So, they are able to remove and eventually consume many weed seeds, thus helping managing weeds more sustainable. Most of my studies were conducted on commercial fields and so many farmers collaborating with me wanted to know if ants were also eating their crops (mainly barley). So, I measured crop damage in up to 36 winter grain fields and concluded that crop losses due to ants are usually very low, although they can increase if crop harvest is delayed and there are no other seeds for the ants to consume.
The latest work on weed seed predation was trying to understand if and how the distribution of harvester ants nests differed between tilled and no-till fields. We were able to show that in no-till fields ants nests were more regularly distributed whereas in tilled fields, nest distribution was more random. A regular nest distribution in this case means that ant nests were spaced in a way that they were covering a larger area of the field, so they can eventually be more effective in locating and removing weed seeds. This research made to the news (click here to see article in Science Daily) and has opened many doors for me to share my work, and I'm so grateful for that!
2. Cover crops to manage weeds ... and much more
Cover crops and crops that are usually sown in time periods between "cash crops", when the fields would otherwise be left bare. Contrary to cash crops, which are harvested for money (corn, soybean, alfalfa, wheat, etc.), the purpose of cover crops is primarely to cover the ground. But they can do much more than that, by covering the soil they protect it from erosion, they can retain nutrients that otherwise would be washed away, they can control weeds, they can provide habitat for beneficial organisms or, once they are terminated, they can provide nutrients for the following crop. My latest paper reports on benefits and challenges associated with weeds in cover crops (you can see it here).
Cover crops are increasingly being adopted in countries around the world and so there is a growing need for research to understand how to better grow them, what species to use in each particular situation and what benefits can be expected from them. My research on cover crops looked at if and how cover crop mixtures can provide more benefits that cover crop monocultures (one species). And why the same cover crop mixture can be so different accross sites. Our results showed that what you sow is not allways what you get because soil nitrogen and climate (mainly growing degree days) can largely change mixture composition expression across sites.
All this work on cover crops was part of a large multi-disciplinary project that we call the "Cover Crop Cocktails" project, funded by the Organic Research and Extension Initiative (OREI) at Penn State University (research site is on the picture). In this project we grow cover crops as a part of an organic crop rotation of winter wheat, corn and soybean. Because it is organic, we cannot use synthetic herbicides and fertilizers, so we need to better understand the ecological processes that operate within the system to manage weeds and get good crop yields.
My research within the project focused on understanding how different cover crop species and mixtures can help manage weeds. We have found that some species like oats or mixtures with oats and cereal rye are very good weed suppressors when sown in the summer-fall, after wheat harvest. However, the timing of cover crop establishment also can largely determine the weed pressure in the cover crop. The earlier you plant your cover crop, the more likely is that it will be full of summer annual weeds that germinate at the same time as the cover crop. So, if cover crops need to be planted earlier, you need to use a highly competitive species (alone or in a mixture) that can deal with high weed pressure. These results have been published in many scientific journals (see publications) and on some extension pieces (see Extension and Outreach).
3. Weeds and the soil
My latest research is focusing on how soil fertility impacts weed - crop competition. Some research has shown that with the same amount of weeds, organic farms experience lower yield losses compared to conventional farms. One of the hypothesis for this is that soils in organic farms have more resource "pools" from where crops and weeds extract their nutrients from, and so, competition is lower. Those diverse pools would come from diverse rotations, organic amendments and higher soil fertility. So, I am currently testing if this is the case and if we can move beyond the organic - conventional dicotomy and find soil managements that decrease weed - crop competition and improve soil fertility.