Coumestrol produced by alfalfa (Medicago sativa L.) can reduce the ovulation rate of ewes. This study isolated agronomic factors that affect coumestrol levels in alfalfa. Fungal diseases explained most differences in coumestrol. Alfalfa inoculated with Stemphylium vesicarium contained 169 ± 25.1 mg kg -1 dry matter (DM) compared with 3.4 ± 0.84 mg kg -1 DM in controls. However, in field-grown alfalfa there was a low relationship (R 2 = 0.388) between coumestrol content and visual fungal damage score which indicates a need for alternative methods of prediction. In the field, a range of cultivars all reached coumestrol levels reported to pose a risk to ewe reproductive performance (>25 mg kg -1 DM). In S. vesicarium-inoculated leaves, the modern cultivar 'Stamina 5' had 396 ± 82.4 mg kg -1 DM compared with the 40-yr-old industry standard 'Wairau' at 143 ± 35.6 mg kg -1 DM. Flowering in isolation did not produce a coumestrol response, with increases detected or not detected simultaneously in vegetative and flowering plants. Pea aphids had a minor effect on coumestrol content. When alfalfa was subjected to ~5 aphids per stem for 4 wk coumestrol increased from 2.4 ± 0.39 to 5.3 ± 0.65 mg kg -1 DM. This study highlights fungal pathogens traditionally not considered problematic in alfalfa stands due to limited effects on stand persistence and yield, as important causal agents of elevated coumestrol levels. Plant breeding to reduce their impact is recommended.

A gronomy J our n al • Volume 10 8 , I ssue 4 • 2 016 P otato is a staple crop worldwide, ranking fourth aft er maize (Zea mays L.), rice (Oryza sativa L.), and wheat (Triticum aestivum L.). As potato consumption increases there is a need to understand the mechanisms that drive its yield. However, the origins of yield diff erences are rarely made clear in cultivar comparisons (Van Der Zaag and Doornbos, 1987) and breeding programs.Potato crop growth and tuber yields have been linked to the duration of the growth cycle, which depends on climate, cultivar, and crop management (Kooman et al., 1996a(Kooman et al., , 1996b. Th erefore, assuming optimum crop management, diff erences in tuber yield among potato cultivars can be explained by differences in accumulated intercepted radiation, the utilization coeffi cient of foliage followed by DM distribution within the plant (Van Der Zaag and Doornbos, 1987). Indeed, the amount of radiation intercepted by a crop and the RUE provide a useful basis for investigating yield variation (Monteith, 1977). From this concept, a simple model to express potato DM growth has been reported (Jamieson et al., 2004) as:where Y is the tuber yield, R o is the daily incident solar radiation received, R/R o is the daily fraction of R o which is intercepted by the canopy (R being the daily average radiation intercepted by the plant), and RUE is the overall photosynthetic effi ciency of the crop (i.e., the effi ciency of conversion of radiant to chemical potential energy). Th e HI is the fraction of the dry matter produced which is allocated to the tubers. Th e integration of these components over time (t; days) from crop emergence (em) allows the description of biomass accumulation in terms of plant development processes. Th is is a common approach used for potato yield analysis (Van Der Zaag and Doornbos, 1987;Spitters et al., 1989;Ellissèche and Hoogerndoorn, 1995) and simulation models of potato yield (Jamieson et al., 2004). Th e amount of radiation intercepted by the crops is reportedly the central component to explain ABSTRACTUnder ideal growing conditions, yield is the product of intercepted photosynthetically active radiation (PAR i ) and its conversion effi ciency to dry matter (radiation use effi ciency, RUE). For potato (Solanum tuberosum L.) the ability of the leaf to convert the PAR i into carbohydrates (source) and the storage capacity of the tubers (sink) aff ect the potential growth of individual tubers and therefore crop yield. Th is study describes these mechanisms for three commercial potato cultivars (Bondi, Fraser, and Russet Burbank) grown in non-limiting fi eld conditions. At fi nal harvest Bondi had the largest tuber yield and produced heavier but fewer tubers compared with Fraser and Russet Burbank. All crops had similar total accumulated radiation interception (R cum ), and yield diff erences were explained by the RUE which was highest for Bondi, lowest for Fraser, with Russet Burbank intermediate. Fraser had the lowest rate of canopy senescence, maintained the lowe...

Abstract:The effects of five rates of nitrogen fertiliser applications on the performance of the cereal aphid Metopolophium dirhodum on winter wheat, within the range of rates recommended for UK crops, were investigated over two seasons in field-grown crops and also on plants grown in the glasshouse. Longevity was unaffected by the level of fertilisation, but aphid intrinsic rate of increase and fecundity increased with each level applied. In the second field season, when a higher upper limit was used, many of these increases were significant. A previously unreported finding for this species was that there was a significant decrease in fecundity for the highest rate of fertilisation. Results for the glasshouse-reared aphids followed a similar pattern to those in the field, and overall they underline recent reports in the literature of the negative effects of high nutrient concentrations on the performance of herbivorous insects. The underlying reasons for these are discussed.

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