Despite the importance of potato tubers as a source of food and a means of propagation, the initiation, growth and development of tubers and the factors affecting these processes are not well understood. There are many reported studies of the effects of various factors on the initiation of tubers but, for a number of reasons, few have sought to understand the initiation of tubers under field conditions. Most reports are concerned with experiments carried out in growth-rooms, which often used very short stem sections as planting material and in which environmental conditions, especially light intensity, differed greatly from those normally prevailing during the period of initiation in the field. In almost all field experiments, the timing of initiation was not defined or properly assessed; number of tubers was recorded infrequently and maximum number of tubers was rarely, if ever, established. Despite these deficiencies, the initiation of tubers is still widely regarded as a key developmental stage in the crop's life, having profound implications for subsequent growth and development (Ewing 1990; Ewing & Struik 1992). This view was first expressed 30 years ago (Ivins & Bremner 1965; Bremner & Radley 1966) and its general acceptance has greatly influenced studies of the growth and development of the crop. During the intervening years, the study of tuber initiation has been an important aspect of research programmes at the University College of Wales (UCW) Field Station, Tenby, Pembrokeshire (1972–84) and Cambridge University Farm (CUF), Cambridge (1982–95), which have now produced sufficient data for a re-evaluation of the significance of tuber initiation in field-grown potato crops.

The responses to fertilizer N of winter wheat and winter barley grown on sandy soils were measured in 74 experiments in England from 1991 to 1994. A series of curves was fitted to the data from each of the experiments to determine which best described the yield response to fertilizer N. The linear plus exponential model, adopted for data from a range of other soil types, with the rate parameter (r) determined for each experiment, proved to be the most satisfactory for these data. The median value of r at 0·987 was less than has usually been determined for other soil types, and was reduced by increasing moisture stress in June, suggesting that the reduced efficiency with which recovered N produces assimilate is partly a result of moisture stress. The model produced a mean Nopt of 143 kg/ha following cereals.

Soil N supply in spring (SNSs) was small at c. 45 kg/ha and commensurate with a large requirement for fertilizer N. Apparent fertilizer recovery (AFR) at 0·70 was greater than that previously measured on clay or chalk soils, suggesting that AFR was not restricted by lack of soil moisture. The larger AFR is considered to be a consequence of more rapid remobilization of fertilizer N in these sandy soils. The greater recovery of fertilizer N reduced the optimum requirement for fertilizer N. Nitrogen harvest index (NHI) was decreased from 0·76 to 0·70 as fertilizer N increased from 0 to 300 kg/ha, whereas dry matter harvest index (HI) decreased from 0·53 to 0·50. The economic optimum requirement for fertilizer N, at 143 kg/ha, was less than reported for other soil types.

The increase in grain yield from applied fertilizer N was less in these experiments than in those reported on other soil types in the UK. However, this overall observation masks two counterbalancing differences. Fertilizer N was recovered more efficiently on these soils; the recovered fertilizer N was used with similar efficiency to produce grain dry matter. The explanation proposed is that on these soils a large proportion of soil water is readily available to crops, and this was taken up rapidly in May accompanied by fertilizer N. However, shortage of water in June restricted the ability of the crop to produce assimilate from fertilizer N applications >c. 150 kg/ha.

Twenty-eight F1 hybrids of wheat and their parents were grown in field trials at Trumpington, Cambridge during 1986/87 and 1987/88. They were derived from crosses between seven ‘modern’ varieties, used as female parents, and either two ‘old’ (Squareheads Master and Partridge) or two ‘modern’ varieties (Bert and Motto), which were used as male parents. Grain yield, yield components, biomass and height were determined. The male parents were chosen to provide contrasting phenotypes and genetic backgrounds for the F1 hybrids. Mid-parent advantage, the increase of a hybrid for a given character above the mean of its parents, and heterosis, the increase of a hybrid above the ‘better’ parent for that character, were calculated. Most F1 hybrids showed mid-parent advantage for the characters studied. This tended to be greatest for hybrids derived from parents with the largest phenotypic differences in that character. In contrast, where heterosis occurred it tended to be greatest where the phenotypic difference between the parents was least. This suggests that the beneficial effects of hybridization, resulting from the dispersion of dominant genes between the parents, was insufficient to overcome the detrimental effects of other genes present where the ‘less good’ parent was substantially lower than the ‘better’ parent. Hybrids derived from the ‘modern’ male parents had greater heterosis for grain yield and mean grain weight than those from the ‘old’ parents. Of the yield components, positive heterosis for mean grain weight resulted in heavier seeds and was the most important yield component in determining heterosis in grain yield. Heterosis for the number of grains/ear was small or did not differ significantly from zero while number of ears/m2 showed negative heterosis resulting in fewer ears/m2 in the hybrids.

The progress of leaf emergence, external morphology and apical development stages were recorded in sowings of triticale (cv. Lasko) made from February to November 1990 at Aberdeen (57° N). Leaf appearance and the number of primordia were related to thermal time (above a base of 0°C) except when photoperiods were <c. 11 h. The thermal time per phyllochron varied between leaves and the combined times for all the phyllochrons at a particular sowing accounted for the apparent response of average phyllochron to sowing date. The thermal time requirements for progression to the double ridge stage, terminal spikelet stage, onset of stem elongation and anthesis were similar except where photoperiods of <11 h occurred. The rate of grain primordium production was constant when photoperiod had been increasing at seedling emergence but the rate was reduced when the seedling experienced shortening photoperiods at emergence.

The effect of four seed rates on the grain yield, grain N content and grain size of Blenheim malting barley sown on two dates in the spring was tested on three different soils over a 4-year period (1991–94). The four seed rates (120, 160, 200 and 240 kg/ha) gave mean plant populations of 222, 308, 374 and 430 plants/m2, respectively, for the first sowing date (February to early March) and 235, 340, 405 and 470 plants/m2 for the second sowing date, 3–5 weeks later (late March to mid-April). The earlier-sown crops generally gave greater yield, lower grain N and smaller amount of screenings, but in two of the twelve experiments, the first sowing gave lower yield and greater grain N.

The influence of seed rate on yield and quality depended on sowing date. The three higher seed rates (160, 200, 240 kg/ha) had little influence on yield or grain N, irrespective of sowing date, and there was no significant difference in yield between the lowest (120 kg/ha) and the three higher seed rates when the crop was sown in February and early March. But when the crops were sown in April, the lowest seed rate (120 kg/ha) gave the lowest yield in all seven comparisons and gave significantly reduced yield in three of the seven comparisons. There was a significant increase in grain N in three of the comparisons. Seed rate had little effect on grain screenings or 1000-grain weight. Soil differences did not influence the effect of seed rate on the yield and quality of the grain.

A field experiment was established at the Institute of Grassland and Environmental Research (IGER), Aberystwyth, to compare the seasonal growth and annual yields of two white clovers when grown with each of five perennial ryegrasses. Swards were subjected to two contrasting defoliation managements, namely cutting only and continuous grazing with sheep. In addition to data recorded on dry matter yields of the swards, the effect of management on growth and development of white clover stolons and tillering of ryegrasses was also studied in the third harvest year. The management of the swards had a significant effect on ryegrass tiller number and on the ingress of weed grasses. Differences in persistency and tillering among the ryegrasses were significantly greater when continuously grazed by sheep than when swards were cut. It is argued that ingress of weed grasses under grazing could be used as an indicator of ryegrass persistency. Among those studied, only two of the ryegrasses were able to provide dense persistent swards under continuous sheep grazing. The white clovers included in this experiment differed in leaf size with Gwenda smaller than Ac 3785. Greater differences existed between white clover components when grazed than when under cutting management. Under continuous sheep grazing there were large differences between the clover varieties in stolon length, stolon weight, number of growing points and root weight.

The data indicate the importance of the persistence of mixture components under continuous sheep grazing and, since ryegrass is the main component of a clover/grass seed mixture, greater emphasis in breeding and evaluation programmes should be given to persistence, especially under grazing.

The effects of suppressing couch grass (Elytrigia repens L.), through integrated management, on soil biological quality and N and P nutrition of barley (Hordeum vulgare L.) were evaluated in a field experiment (1987–92) on a Dark Grey soil in Alberta, Canada. The management practices consisted of combinations of herbicide application, crop rotations and tillage treatments. The 3-year crop sequences consisted of continuous barley, canola (Brassica rapa L.)–barley–barley, fallow–barley–barley, and barley or canola undersown with red clover (Trifolium pratense L.)–red clover green manure–barley. In 1992, the sixth year of the experiment, soil and plant samples were analysed for nutrient content. Treatments that consisted of spring and autumn tillage only did not suppress couch grass and produced low barley yields and N and P uptake. More N was immobilized in couch grass shoots and rhizomes and soil microbial biomass with these treatments than with similar herbicide-treated crop rotations. Tillage-plus-herbicide treatments effectively suppressed couch grass and enabled the barley crop to compete for soil N, however, both spring and autumn tillage were required for weed control. The fallow treatment impaired soil quality by reducing soil and microbial C and N, but produced similar barley yields as continuous cropping with tillage-plus-chemical control. Red clover ploughed in for green manure enhanced soil quality indicators such as soil and microbial biomass C, total and mineralizable soil N, and microbial N, but did not increase barley yield compared to continuous grain cropping.

Two aspects of yam storage practice were tested from 1994 to 1995 in central Côte d'Ivoire. The first experiment compared the storage environment in pits or sheds with that of the traditional yam open barn method. The pits allowed the daily range of variation in temperature to be reduced while maintaining a high relative humidity (RH), whereas the temperature and RH in the sheds were similar to those of the yam barns. Differences in temperature and RH between the storage systems had little effect on the fresh weight losses of the tubers. The more effective protection against solar radiation and rain afforded by the sheds and pits reduced fresh weight losses in D. cayenensis rotundata by 12% over 6·5 months in comparison with the barns. The second experiment tested the regular removal of the sprouts. Sprout removal reduced fresh weight losses equally over 5 and 6 months' storage. The reduction in fresh weight loss was 12% for cv. Gnan (D. cayenensis rotundata) during 6 months' storage. Monthly sprout removal gave an equal benefit in pits, in sheds and on yam barns. The monthly cutting of sprouts at the base should be encouraged in places where this is not already done. The total loss is mainly determined by the growth of the sprouts and the reduction in weight after onset of sprouting, which are highly dependent on species and cultivar. Bètè bètè yams had better keeping qualities than all the other cultivars tested.

Drought stress and virus yellows disease are two of the major problems of sugarbeet crop production in the UK. We have calculated the annual national drought losses from 1980 to 1995 by using long term data sets for two sites (IACR-Broom's Barn, Suffolk and ADAS Gleadthorpe, Nottinghamshire) to relate yield loss to cumulative potential summer moisture deficit, and combining these relationships with regional meteorological records, soil type and crop distribution data.

Experimentally measured relationships between yield losses and the timing of virus yellows infection were combined with annual survey data of the extent of the problem, and calculated infection dates from the UK aphid suction trap network, to calculate actual national annual losses to the disease. Potential losses in the absence of control measures were then calculated by use of data from trials and surveys of pesticide use.

The results showed a mean annual loss of production to drought stress of 141000 t/year of sugar, 10·5% of production, with a loss to the industry of £27·9 million. Losses in individual years varied from zero to 2·5 times the mean figure. Actual losses to virus yellows were much smaller, due to the efficacy of treatments, averaging 24700 t/year of sugar (1·8% of national yield, financial loss £5·5 million). Average potential virus yellows losses in the absence of control measures were approximately double this.

Control of virus yellows is a major, cost-effective contributor to rising and consistent sugarbeet production. Nationally, irrigation has made little impact on drought losses and, due to constraints in surface water supply, this situation appears likely to continue. Improved drought stress tolerance represents the largest single opportunity for yield and profitability improvement of the sugarbeet crop in the UK at present. Predicted climate change appears likely to increase the severity of both drought and disease stresses. Drought stress appears relatively less important in other NW European sugarbeet-growing areas.

The effects on succeeding crops of nitrogen in residues returned to the soil of brassica vegetable crops (Brassica oleracea) were studied on a nitrogen-retentive silt loam soil at Horticulture Research International, Kirton, Lincolnshire, UK. A sequence of four crops was started in 1988 and again in 1989. In the first sequence, two successive cauliflower crops (Brassica oleracea cv. botrytis L.), crops 1 and 2 in the first year, were followed by Brussels sprouts (Brassica oleracea cv. Gemmifera D.C.), crop 3, in the second and spring barley (Hordeum vulgare), crop 4, in the third year. The second sequence, started in spring 1989, was on an area adjacent to the first, but with spring wheat (Triticum aestivum) as crop 4. The sites followed an unfertilized, 1-year grass ley (1987) or spring barley (1988) with 73 and 107 kg N/ha soil mineral nitrogen (SMN, NH4+NO3) in the 0–90 cm soil profile at the start of each sequence. The marketable yield of the first cauliflowers in both sequences increased with fertilizer nitrogen up to 240 or 300 kg/ha. The response of the second cauliflower crop to fresh N declined with increasing amounts of SMN (0–90 cm) at planting, with no response when SMN exceeded c. 270 kg N/ha. Crop 3 apparently responded to fresh N in sequence 1 but not in sequence 2 even though SMN at planting ranged up to c. 400 kg N/ha. This difference in response was largely explained by the amount and distribution of SMN in the 0–30 and 30–60 cm layers when the sprouts were planted. Soil mineral N to 90 cm when the cereals were sown was only c. 100 kg N/ha, which did not reflect the large amounts of N applied to the previous crops. In spite of this, barley yield without any fresh fertilizer N did vary with the amounts of N applied to the previous crops. The results showed that SMN was a useful predictor of fertilizer response in some, but not all, situations. To use SMN more generally requires interpretation using dynamic simulation models.

In two experiments conducted at Badgery's Creek, NSW (Latitude 34°S) the roles of testosterone and the thyroid hormones triiodothyronine (T3) and thyroxine (T4) as possible initiators of the change in voluntary feed intake (VFI) associated with the seasonal reproductive behaviour (rut) in male fallow deer were investigated.

In Expt 1, the association between changes in these hormones with the onset of the rut was assessed in deer in which the timing of this event was manipulated by changing the photoperiod, or by melatonin administration. Groups were maintained under either natural photoperiod (n=6; control), a constant long daylength (16 h light[ratio ]8 h dark; n=9; LD group) or a constant long daylength and implanted with melatonin capsules (n=5; LD+M group) for 7 months from December until the following July. Blood samples were obtained weekly and VFI recorded.

Feed intake decreased by 94% from the last week of March for 3 weeks in the control group. Changes of a similar magnitude were measured in the LD and LD+M groups but these changes were advanced by 1 and 9 weeks respectively in these groups. In all groups, circulating testosterone concentrations increased markedly at a time corresponding with the decrease in VFI. The concentrations returned to basal levels with the resumption in VFI. A distinct decrease in plasma concentrations of T3 and T4 in all three groups was associated with the decrease in VFI, however, the relationship with T3 was less apparent.

In Expt 2, the role of testosterone in the regulation of the decline in VFI was investigated. Fallow bucks were treated with testosterone enanthate every 4 or 5 days for 28 days up to 6 weeks prior to the expected onset of the rut. Plasma testosterone concentrations, which were increased 13-fold, resulted in a decline in VFI which was comparable to that observed in the subsequent rut. Plasma free fatty acid concentrations were correlated negatively with the decline in VFI.

Thus, the seasonal increase in circulating testosterone concentrations plays an important role in initiating the fall in VFI associated with the rut. As the rut was still apparent in animals maintained under an extended photoperiod, it is possible that factors other than decreasing daylength act as the cue for the timing of the rut.

Growth and reproduction of the snail Helix aspersa (Müller) var. maxima were examined at four combinations of temperature and artificial photoperiod and in transparent and opaque containers at two temperatures for over 40 weeks. Effects of temperature on both processes predominated. However, following 10 weeks of oviposition at optimum temperature (20°C), absence of photoperiod inhibited egg-laying throughout a further 15 weeks. Following inhibition of oviposition, the albumen gland atrophied, indicating the storage of secretory products.

Snail growth was improved by rearing in opaque rather than transparent containers in a long-day photoperiod. Snail oviposition was unaffected by container type throughout a 28-week period, demonstrating an acute perception of photoperiodicity by snails maintained at low light intensity.