During harvesting, grain, straw, and chaff with weed seeds are separated. The chaff is returned to the fields, resulting in weed problems in the subsequent crops. We estimated the fraction of weed seeds a combine harvester could potentially harvest and used various methods to collect the chaff and treat it with heat to kill weed seeds or reduce weed seed germination. Chaff with weed seeds was placed on top of the straw and afterwards baled with the straw as a method to remove weed seeds from the field. We exposed chaff with weed seeds to exhaust gas with various temperatures and durations to study whether this heating method could be used to reduce the input of viable weed seeds to the soil during harvesting. By collecting the shed weed seeds during the growing season, we estimated that a combine harvester could potentially harvest 41%, 11%, and 100% of the seeds produced in the growing season by Bromus hordeaceus, Cirsium arvense, and Galium aparine, respectively. When the chaff was placed on top of the straw, 45% of the weed seeds stayed in the chaff fraction on top of the straw swath after one day, 35% got into the straw swath, and 20% past through the swath to the ground. Therefore, baling straw with chaff placed on the top only had a limited effect on reducing weed seed infestation. The study showed that thermal weed seed control during harvesting could potentially be applicable and incorporated in an integrated weed management approach.

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Blackgrass (Alopecurus myosuroides Huds.) and silky windgrass [Apera spica-venti (L.) P. Beauv.] are becoming a significant problem in Europe. Due to the development of herbicide-resistant biotypes and unwanted side effects of herbicides, there is a need for new integrated weed management strategies to control weeds. Therefore, reducing weed infestations by targeting seed production during crop harvest should be considered. In 2017 and 2018, we estimated the fraction of the total seed production of A. myosuroides and A. spica-venti in a field that potentially could be collected by a grain harvester during winter wheat (Triticum aestivum L.) harvest. Twenty plants of each species were surrounded by a porous net before flowering to trap shed seeds during reproductive development. Seeds were collected and counted weekly up until and immediately before wheat harvest, and the ratio of harvestable seeds to shed seeds during the growing season was determined. Alopecurus myosuroides produced on average 953 seeds plant−1 in 2017 and 3,337 seeds plant−1 in 2018. In 2017 and 2018, 29% and 37% of the total A. myosuroides seeds produced, respectively, were retained on plants at maturity. Apera spica-venti produced on average 1,192 seeds plant−1 in 2017 and 5,678 seeds plant−1 in 2018, and retained 53% and 16% of the seeds at harvest, respectively. If a grain harvester potentially collected approximately 30% of the total seed production of the two grass weeds and removed or killed them, it would reduce seed input to the soil seedbank. However, such methods cannot stand alone to reduce weed pressure.

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We investigated if hot exhaust gas from a combine harvester could be used to reduce germination or kill weed seeds during the harvesting process. During the threshing and cleaning process in the combine, weed seeds and chaff are separated from the crop grains. After this separation, weed and crop seeds not collected can be exposed to exhaust gas before seeds are returned to the field. Seeds of some common weed species (Alopecurus myosuroides, Centaurea cyanus, Geranium pusillum, Lapsana communis, Lolium perenne, Rumex crispus, Spergula arvensis, and Tripleurospermum inodorum) were treated with exhaust gas at temperatures of 75 °C or 85 °C, 110 °C, and 140 °C for 2, 4, and 6 s, respectively. Afterwards, the seeds were germinated for 16 days. We found that 75 °C and 85 °C were insufficient to significantly reduce germination of the seeds after three durations. Some seeds were still able to germinate after 4 s exposure of 110 °C. An exposure of 140 °C for 4 and 6 s repressed germination of all species. We conclude that there is potential to develop combine harvesters that exploit the exhaust gas to either kill or reduce the ability of weed seeds to germinate before seeds are returned to the field.

Weeds are mainly controlled with herbicides in intensive crop production, but this has resulted in increasing problems with herbicide-resistant weeds and public concerns about the unwanted side-effects of herbicide use. Therefore, there is a need for new alternative methods to reduce weed problems. One way to reduce weed infestation could be to collect or kill weed seeds produced in the growing season. Crop and weeds are harvested simultaneously with the combine harvester, but most of the weed seeds are returned with the chaff to the field creating new problems in future growing seasons. During the harvesting process, the harvester produces heat. Under normal harvest conditions, the exhaust gas temperature measured directly behind the turbocharger of the engine of a combine harvester may reach between 400 • C and 480 • C depending of the size of the engine. These high temperatures indicate that there is a potential for developing a system which perhaps could be utilized to kill or damage the weeds seeds. We investigate how much heat is needed to damage weed seeds significantly and focuses on the germination patterns over time in response to these treatments. We investigated if heat treatment of weed seeds could kill the seeds or reduce seed vigour or kill the seeds before they are returned to the field. The aim is to avoid harvested viable weed seeds being added to the soil seed bank. During the threshing and cleaning process in the combine harvester, most weed seeds and chaff are separated from the crop grains. After this separation, we imagine that the weed seeds could be exposed to a high temperature before they are returned to the field. Seeds of nine common weed species were treated with temperatures of 50 • C, 100 • C, 150 • C, 200 • C, and 250 • C for 0, 2, 5, 10, and 20 s, respectively. Afterwards, the seeds were germinated for fourteen days. Seeds were differently affected by the heat treatments. We found that 50 • C and 100 • C was insufficient to harm the seeds of all species significantly at all durations. Heating with a temperature of 50 • C and 100 • C showed a slight tendency to break the dormancy of Alopecurus myosuroides Huds. and Papaver rhoeas L., but the results were not statistically significant. Seeds treated with 150 • C gave varying results depending on the duration and the weed species. The germination of A. myosuroides was significantly repressed when seeds were exposed to 250 • C for 5 s. Most species were significantly damaged when they were exposed to 250 • C for more than 10 s. Our results showed that there is a potential to explore how the waste heat energy produced by combine harvesters can be exploited to either kill or reduce the vigour of weed seeds before they are returned to the field with the chaff.

We assessed the seed production and shedding pattern of 10 common weed species in two oat fields in Denmark. The aim was to evaluate the possibility of harvesting retained seeds on weeds at crop harvest by a combine harvester based on estimation of weed seed retention. Before flowering, ten plants of each weed species were selected and surrounded by a seed trap comprising of a porous net. When the plants started shedding seeds, the seeds were collected from the traps and counted weekly until oat harvest. Just before oat harvest, the retained seeds on the plants were counted and the ratio of harvestable seeds and shed seeds during the growing season was determined. The seed production and shedding patterns varied between the 2 years. Across both years, Anagallis arvensis L., Capsella bursa‐pastoris L. Medik, Chenopodium album L., Geranium molle L., Persicaria maculosa Gray, Polygonum aviculare L., Silene noctiflora L., Sonchus arvensis L., Veronica persica Poir. and Viola arvensis Murray retained on average 61.6%, 52.7%, 67.2%, 58.4%, 32.05%, 59.5%, 95.7%, 23.5%, 51.7% and 33.9%, respectively, of their produced seeds at crop harvest. Silene noctiflora was classified as a good target for harvest weed seed control; C. bursa‐pastoris, C. album, G. molle, P. maculosa, S. arvensis and V. arvensis were classified as intermediate targets; and A. arvensis, P. aviculare and V. persica were classified as poor targets.

If seeds retained on weeds at crop harvest could be collected and removed by the combine harvester, weed infestation could be reduced in the following years. We estimated the proportion of weed seeds that could be removed at oat harvest. The seed production and shedding pattern of Fallopia convolvulus, Sinapis arvensis, Spergula arvensis and Stellaria media, were assessed in two spring oat fields in Denmark during 2018 and 2019. Ten randomly chosen plants of each species were surrounded by a porous net before flowering. The start time of seed shedding was recorded, and the seeds were collected from the nets and counted weekly until oat harvest. Just before harvest, the retained seeds on the weed plants were counted. The ratio between harvestable seeds and shed seeds during the growing season was determined. On average 260, 195, 411 and 316 seeds plant −1 were produced by F. convolvulus, Sinapis arvensis, Spergula arvensis and S. media, respectively, of which in average 44%, 67%, 45% and 56% of the seeds were retained on the plants at harvest. There was a strong, positive correlation between the weed biomass and the total seed production.Agronomy 2020, 10, 46 2 of 10 pods shatter readily, and some seeds are harvested with the crop [5]. Sinapis arvensis flowers six weeks after emergence with the peak in June and July in northern latitudes, but it can continue flowering until the frost starts [6]. The average number of seeds plant −1 reported for plants grown without competition varied from 2000 to 3500 [7], and in dense plant populations, from 10 to 590 [8]. Seed longevity of up to 75 years has been reported [7].Stellaria media is native to Europe, and exists as one of the most distributed weeds in the world. It is one of the most common weeds in spring and winter cereals in northern Europe. It quickly colonizes disturbed fields [9], but it is considered as a weak competitor [10]. The number of seeds plant −1 has been reported to be in the range of 500 [11] to 2500 [12]. It has numerous, small, easily dispersed seeds, and can flower and set seed throughout the year [13].Spergula arvensis is a weed of cereals in almost all areas of the world. The plant flowers from June to November and will shatter mature seeds from July onward. Flowering and seeding continue until the plant dies. A large plant may have 500 capsules and releases 7500 seeds. Capsules produced early in the season may contain 25 seeds, but later capsules may only contain five [2].Sinapis arvensis and Spergula arvensis can only reproduce by seeds, while F. convolvulus and S. media also can reproduce by creeping roots and creeping stems rooting at the nodes, respectively [14]. The soil seed bank is the primary source of weed infestations. Thus, information on their seed production and shedding pattern is necessary for applying proper weed management strategies. Weed seeds disperse when the seeds ripen, detach and fall to the ground [15]. Late season production of weed seeds has gained particular attention because of the development of herbicide-resistant...