Paula R. Westerman

Comparing Estimates of Seed Viability in Three Foxtail (Setaria) Species Using the Imbibed Seed Crush Test with and Without Additional Tetrazolium Testing

Estimates of seed viability using the imbibed seed crush test, a method performed by applying pressure to imbibed seeds, were compared with estimates obtained from using the imbibed seed crush test supplemented with tetrazolium staining. The seeds of three weed species, giant foxtail, green foxtail, and yellow foxtail, were collected from three different crops and tested by each method. The results from the two approaches were strongly and significantly correlated. The imbibed seed crush test requires considerably less skill and time to perform and is a reasonable alternative to tetrazolium staining to test the seed viability of newly produced foxtail seeds. Nomenclature: Tetrazolium, 2,3,5 triphenyl tetrazolium chloride; giant foxtail, Setaria faberi Herrm. SETFA; green foxtail, Setaria viridis (L.) Beauv. SETVI; yellow foxtail, Setaria glauca (L.) Beauv. SETLU.

Weed seed survival during anaerobic digestion in biogas plants.

Anaerobic digestion using animal manure and crop biomass is increasingly being used to produce biogas as a durable alternative to fossil fuel. The sludge, the leftover after processing, is returned to the field as a crop fertilizer. If weed seeds survive anaerobic digestion, the use of contaminated sludge poses a phytosanitary risk. The conditions that seeds are likely to encounter in biogas plants, and the effect of these, in particular temperature, on seed viability were reviewed. Knowledge on seed defence mechanisms and how these might protect seeds from inactivation in biogas reactors was summarized. Mechanisms of seed inactivation can be classified as thermal, biological and chemical. Weed species with hard seeds (physical dormant), high thermoresistance, a thick seed coat or adapted to endozoochory were identified as high-risk species. Specific seed traits could be used in future tests to circumvent extensive testing of seeds in biogas reactors.

The probability of maize biomass contamination with weed seeds

Anaerobic digestion in biogas reactors generates a semi-solid leftover, the digestate, which is frequently used as a crop fertiliser. If weed seeds in the maize biomass that is used for feedstock survive the process of anaerobic digestion, the use of digestate could contribute to the spread of weeds. We investigated here the probability that weeds enter the biogas production chain via maize biomass. Five organic and four conventional maize fields were sampled to determine the identity, density and biomass of the weeds present, and the number and viability of seeds produced. This was done separately for plant parts below and above the anticipated cutting height of the maize crop, close to the anticipated harvest date. Weed density varied from 56 to 568 weeds m−2 and weed seed production from 886 to 229 256 seeds m−2. Conventional fields tended to have lower weed biomass, lower seed production, and lower proportion of the seeds above the cutting height of maize, with lower seed viability. The main weeds were Chenopodium album and Echinochloa crus-galli. In general, either weed plants were small and produced few seeds that were largely found below the cutting height of maize, or they were tall and produced numerous seeds that were largely found well above the cutting height. Given the numbers of weed seeds produced per hectare in this study, it is likely that some will survive anaerobic digestion in biogas plants, although in low numbers, as seed survival during anaerobic digestion tends to be low.

Exploring the scope for improving biocontrol of black vine weevil, Otiorhynchus sulcatus, with Heterorhabditis spp. at low temperatures: A simulation study

Control success with heterorhabditid nematodes varies with nematode species, isolate, production and storage conditions, and environmental conditions after application. These factors affect nematode behaviour. A model was developed that simulates movement of a nematode population in space and time from the moment of application on a sand column until penetration into a host. It was used to identify, (1) which nematode traits can best be used for improvement, and (2) what is the most promising strategy of improvement. The sensitivity of simulated control success of Heterorhabditis spp. against larvae of the black vine weevil, Otiorhynchus sulcatus, at low temperatures, to changes in nematode behavioural parameters was quantified and related to genetic or environmental variation found in the nematodes. Parameters characterizing nematode movement had little influence on simulated control success. Parameters characterizing aggregation and arrestment had a large effect on control, but there is no variation in Heterorhabditis for these traits. Parameters characterizing penetration had a moderate effect on control. The most promising option to enhance control by Heterorhabditis in this system would be to raise the proportion infectious nematodes of an isolate up to its genetic maximum, by improving production and storage conditions. Variation in biocontrol would be reduced, resulting in a more reliable product.

Density-independent reproductive success of the hemiparasitic plant Striga hermonthica , despite positive and negative density-dependent phases: Density dependence and independence in the hemiparasitic plant Striga hermontica

The hemiparasite Striga hermonthica is a major constraint to smallholder farmer livelihoods and food security in sub‐Saharan Africa. A better understanding of its life‐cycle can help developing more effective management strategies. Here, we studied density dependence in S. hermonthica on Sorghum bicolor. We exposed two genotypes of S. bicolor that differed in the level of tolerance and resistance to S. hermonthica to a range of seed densities of the parasite. We evaluated multiple host and parasite performance parameters through periodic, destructive harvests and related these to the initial seed density using model selection. Initially, the probability for attachment was positively density‐dependent, suggesting facilitation of new infections. However, at host maturity, S. hermonthica infection probability showed strong negative density dependence, indicating severe competition, in particular in the early developmental stages. Although parasite shoot dry weight showed a strong negative density dependence at host maturity, flower production per parasite exhibited positive density dependence again, suggesting compensation. The two host genotypes had similar responses to increased parasite densities, indicating differences between the genotypes in tolerance but not resistance. Consequently, despite density dependence in life‐cycle components, the per capita reproductive output of S. hermonthica, R₀ (flowers seed⁻¹) was density‐independent. Apparently, management of the hemiparasite can neither benefit from a negatively density‐dependent bottleneck, nor from a positively density‐dependent Allee effect. The most promising suggestion to obtain S. hermonthica population decline (R₀ < 1) and long‐term control is to increase host shading by growing a vigorous, competitive crop.