The effects of climate change on crop suitability in England and Wales were estimatedby systematically adjusting agroclimatic data inputs to a land evaluation model. Mean temperature risesof 1 and 2°C increased drought risk and reduced the length of time whe soil was at field capacity over-winter by raising potential evapotranspiration. In eastern England, this change had a detrimental effect on the potential for cultivating winter wheat because of drought stress. In northern and western England and Wales, drier conditions promoted greater opportunities for soil tillage, especially when temperature rises wereassociated with decreased precipitation. The effect of increased temperature was offset by increased precipitation. The magnitude of the drought stress and length of field capacity periods are similar to current mean conditions when a + 1 °C temperature change is coupled with a 10% increase in precipitation. This demonstrates that winter wheat suitability in England and Wales is affected more by changes in precipitation than by changes in temperature. The local limitations of soils and relief notwithstanding, it appears that the northern and western areas of England and Wales are most likely to gain from a warmer, drier climate.

Two field experiments at Canterbury, New Zealand during 1991–93 investigated the effect of the timing of ploughing a 4-year-old ryegrass/white clover pasture and the effect of two winter cover crops on subsequent N mineralization, nitrate leaching and growth and N uptake of the following wheat crops.

Net N mineralization of organic N (of plant and soil origin) increased with increased fallow period between ploughing and leaching. The total amount of N accumulated in the profile by the start of winter ranged from 107 to 131 and from 42 to 45 kg N/ha for fallow treatments started in March and May respectively. Winter wheat (planted in May) had no effect on mineral N contents by the start of winter, whereas greenfeed (GF) oats (planted in March) significantly reduced the mineral N content in one year.

Cumulative leaching losses over the first winter after ploughing-in pasture varied markedly between years in relation to rainfall amount and distribution. Leaching losses were greater from the March fallow (72–106 kg N/ha) than the May fallow treatments (8–52 kg N/ha). Winter wheat did not reduce leaching losses in either year. GF oats did not reduce losses in 1991/92, but losses in 1992/93, when major drainage events occurred late in the winter, were only c. 40% of those under fallow.

Incorporation of a large amount (> 7 t/ha dry matter) of pasture or GF oat residue in spring depressed yield and total N uptake of the following spring wheat, largely due to net N immobilization which could be overcome by the application of fertilizer N.

First-year treatments had very little residual effect in the second year. Leaching losses over the second winter (mean 142 kg N/ha) were largely unaffected by the extent of first year leaching losses. Second year leaching losses were greater than first year losses, probably due to the greater amount of mineral N at depth in the soil before the start of the second winter.

Disposal methods for straw from continuous winter wheat were tested on two soil types, a flinty silty clay loam and a sandy loam, over 7 years (1985–91). The methods tested were burnt or chopped straw in full factorial combination with four cultivation methods (tined to 10 cm, tined to 10 cm then to 20 cm; ploughed to 20 cm; tined to 10 cm then ploughed to 20 cm). Measurements were taken to determine the effects on crop establishment and growth, pest and disease incidence, and the consequent effects on yield. Another experiment (1985–91) on the flinty silty clay loam site, investigated the interactions between straw treatments (burnt, baled or chopped in plots that were all shallow cultivated to 10 cm) and five other factors; namely, time of cultivation, insecticides, molluscicides, fungicides and autumn nitrogen. All the straw x cultivation systems allowed satisfactory crops to be established but repeated incorporation of straw using shallow, non-inversion cultivations resulted in very severe grass-weed problems. Early crop growth, as measured by above-ground dry matter production, was frequently decreased by straw residues, but the effect rarely persisted beyond anthesis. Pests were not a problem and their numbers were not greatly affected either by straw or cultivation treatments, apart from yellow cereal fly which, especially on the heavier soil, was decreased by treatments which left much straw debris on the soil surface. Incorporating straw also caused no serious increases in the incidence of diseases. Indeed, averaged over all sites and years, eyespot and sharp eyespot were both slightly but significantly less severe where straw was incorporated than where it was burnt. Eyespot, and even more consistently sharp eyespot, were often more severe after ploughing than after shallow, non-inversion cultivations. Effects on take-all were complex but straw residues had much smaller effects than cultivations. Initially the disease increased most rapidly in the shallow cultivated plots but these also tended to go into the decline phase more quickly so that in the fourth year (fifth cereal crop) take-all was greater in the ploughed than in the shallow cultivated plots. On average, yields did not differ greatly with straw or cultivation systems, although there were clear effects of take-all in those years when the disease was most severe. In the last 2 years, yields were limited by the presence of grass weeds in the plots testing chopped straw incorporated by tining to 10 cm.

Changes in the stolon carbohydrate contents (water-soluble and total non-structural) of four white clover populations were measured at the beginning and end of winter. Three of the populations were of Swiss origin - two of these were from high-altitude pastures, and the other from a valley location. Grasslands Huia was measured as a control variety. Levels of both types of carbohydrate declined during the winter in all populations. The Swiss populations from high altitudes contained the highest levels of both carbohydrate types.

An artificial freezing test was carried out on stolon segments collected from a range of clover populations (including the two high-altitude Swiss clovers) growing in field plots in January, and values of ‘lethal dose 50’ (LD50) were calculated. The two Swiss populations had the lowest LD50 values, indicating a greater intrinsic tolerance to freezing in those plants than in the other populations (of lowland temperate origin).

One of the Swiss populations, Ac3785, was grown under two temperature regimes and short daylength to determine whether its tolerance to freezing increased with time. Significant increases in tolerance occurred after 8 days at 2 °C, but plants grown at 8 °C showed no change in tolerance even after 30 days.

The results suggest that (i) stolon carbohydrate content is an important factor in the overwintering of white clover, (ii) there is a considerable amount of genetic variation in cold tolerance within white clover, (iii) artificial freezing tests can provide a method of predicting the survival of clover stolons during winter and (iv) the cold hardiness of white clover increases with time spent at temperatures near 0 °C under short daylengths.

Two factor designs are considered in which the levels of the two factors are linked (e.g. certain diallel cross and reciprocal transplant experiments). A common-row-and-column (CRAC) effect is defined as a systematic effect in the response when the treatment combination has the same level of the two factors. The efficiency of designs for estimating CRAC effects is discussed. Square designs with various levels of extra replication for CRAC treatment combinations and strand designs, based on omitting certain treatment combinations for which the two factors do not have the same level of the two factors, are defined and their efficiency examined.

In the design for optimal estimation of a single CRAC effect in square or strand designs, half the total replication is in CRAC treatment combinations and half in the other combinations. This gives a major increase in design efficiency, particularly where the number of levels of the factors is five or greater. Designs with equal replication of all treatment combinations are generally very inefficient. The optimal square and strand designs have equal efficiency for estimating the CRAC effect, but the strand designs are more flexible in practice, requiring fewer plots to make a complete block and this relative flexibility increases with the number of levels of the factors.

Most of the square or strand designs allow the estimation of n separate CRAC effects where n is the number of levels of the factors. Optimal square designs for estimation of separate CRAC effects give replication of CRAC combinations as ((n–1)(n–2)/2)½ times that of other combinations, which is different from the optimum value for estimating the single CRAC effect. However, for many designs, the difference is not of major importance because optimal designs for the estimation of a single CRAC effect are relatively insensitive to small deviations from the optimal ratio.

When the replication of CRAC combinations differs from that of the other treatment combinations, the designs are less efficient in estimating the factor effects. Where estimation of factor effects is also of relevance, a trade-off between increased efficiency in the estimation of CRAC effects and of factor effects may be desirable.

Strand designs are not as efficient as the optimal square design for estimating separate CRAC effects. For three-strand designs of even order examined, the CRAC effects are confounded with factor effects and are not all separately estimable. When factor effects and/or separate CRAC effects are of interest, square are preferable to strand designs.

Some of the results generalize readily to three linked factors. There is a three-factor CRAC effect and three two-factor CRAC effects. The replication allocation for CRAC relative to other combinations, to optimize the estimation of the three-factor CRAC effect is (n–1)(n–2)/2 for the three-factor CRAC combinations and (n–2)/2 for the two-factor CRAC combinations. Selection to optimize the estimation of the two-factor CRAC effects gives very different optimal allocation.

An experiment at Rothamsted in 1985–89 and another at Whaddon in 1986 studied the effects of incorporating straw on diseases of winter barley. Net blotch (Pyrenophora teres) and leaf blotch (Rhynchosporium secalis) were initially less severe where straw was burnt or incorporated by ploughing than where cultivations only partially buried it. However, by summer both diseases were usually more severe where straw had been burnt than where it had been incorporated. At Whaddon, eyespot (Pseudocercosporella herpotrichoides) tended to be less severe in tine-cultivated plots where straw was incorporated than where it was burnt, but at Rothamsted, where the straw treatments were confounded with cultivations, there was no consistent effect. The disease was usually more severe where straw was incorporated by ploughing than where it was incorporated using other methods. In contrast, the severity of take-all was generally decreased by ploughing. Seedlings usually grew better where straw had been burnt rather than incorporated and grain yields were often larger. However, yields at Rothamsted in 1987 were unusually, and inexplicably, smaller after burning the straw so that the 5-year mean yields showed no significant differences between treatments.

Winter non-leguminous cover crops are included in crop rotations to decrease nitrate (NO3-N) leaching and increase soil organic matter. This study examined the effect of incorporating a mature cover crop on subsequent N transformations. A field trial containing a winter cover crop of Merced rye and a fallow control was established in December 1991 in Salinas, California. The rye was grown for 16 weeks, so that plants had headed and were senescing, resulting in residue which was difficult to incorporate and slow to decompose. Frequent sampling of the surface soil (0–15 cm) showed that net mineralizable N (anaerobic incubation) rapidly increased, then decreased shortly after tillage in both treatments, but that sustained increases in net mineralizable N and microbial biomass N in the cover-cropped soils did not occur until after irrigation, 20 days after incorporation. Soil NO3-N was significantly reduced compared to winter-fallow soil at that time. A 15N experiment examined the fate of N fertilizer, applied in cylinders at a rate of 12 kg 15N/ha at lettuce planting, and measured in the soil, microbial biomass and lettuce plants after 32 days. In the cover-cropped soil, 59% of the 15N was recovered in the microbial biomass, compared to 21% in the winter-bare soil. The dry weight, total N and 15N content of the lettuce in the cover-cropped cylinders were significantly lower; 28 v. 39% of applied 15N was recovered in the lettuce in the cover-cropped and winter-bare soils, respectively. At harvest, the N content of the lettuce in the cover-cropped soil remained lower, and microbial biomass N was higher than in winter-bare soils. These data indicate that delayed cover crop incorporation resulted in net microbial immobilization which extended into the period of high crop demand and reduced N availability to the crop.

Methods are described for estimating interplot interference in variety trials. The effects on variety yield performance of interference from varieties in adjacent plots was studied for more than 600 UK National and Recommended List variety trials in the years 1987–1991. The crops examined were barley, wheat, oats, grain peas, beans, oilseed rape, forage maize and sugarbeet. Estimates of differences in variety performance may be biased by as much as 4% in some crops. Factors associated with interference included plant height, standing ability and disease susceptibility. An approach is outlined for limiting the effects of interference through controlled randomization within trials.

The proportion of sward area rejected for grazing under different stocking intensities in grass-white clover (Trifolium repens L.) swards continuously grazed by steers toconstant sward height was determined and the effect of these areas on clover stolon developmentwas investigated.

In a randomized block field experiment, grazing treatments imposed from 1988 to 1991 comprised target sward surface heights (SSH) of 5, 7 or 9 cm from April to July followed by 7 or 9 cm from July to October (i.e. six treatments). The percentage area rejected was higher in 9 cm than in the shorter swards, the proportion of the total areas rejected increasing generally over the year. Within the rejected areas in spring the number of branches on the main axes of white clover plants was only 17% and the mean number of nodes per branch was 14% of those in grazed areas. Corresponding percentages in summer in rejected areas were 56 and 46% compared with grazed areas.

In a glasshouse experiment involving stolons grown for 18–21 days, the difference between those from grazed and rejected areas in branch number per stolon and mean node number per branch was much reduced. Branching and rate of branch development (node number) were slower in stolons maintained under dark conditions.

Mean growing point density over six sampling dates from summer 1990 to autumn 1991 was significantly higher in the 5 cm than in the 9 cm spring SSH treatment. Although rejected areashad only 34% of the growing point density of grazed areas in autumn, the latter comprised the majority of the total area in most sward height treatments and so reduced the overall effect of rejected areas on growing point density.

Field and glasshouse studies were conducted in Durango, Mexico in 1987 and in East Lansing, Michigan, USA in 1989, respectively, to determine the effects of moisture deficits upon N-use efficiency (NUE), N partitioning and remobilization, and N harvest index (NHI) in edible beans (Phaseolus vulgaris L.). Four indeterminate, semi-prostrate genotypes adapted to the semi-arid high plains of Mexico, Pinto Nacional-1, Durango 222, L1213–2 and Bayo Madero, were used in the field study and Pinto Nacional-1 and Bayo Madero were used in the glasshouse study. A Xerosol Haplic soil was used in the field study and a Spinks loamy sand in the glasshouse study. A moisture deficit was induced by use of temporary rainshelters in the field and curtailment of water in the glasshouse. Plants were sampled periodically and subdivided into leaves, stems, pods and flowers, and roots (in the glasshouse study only) for determination of dry weight and total N content. Water-use efficiency was determined in the glasshouse study. A moderate moisture deficit (drought intensity index 0·41) reduced yield by 41% in comparison with non-stressed yield (from 134·3 down to 79·2 g/m2) and resulted in a greater percentage of seed-N derived from N that had been redistributed from the leaf, indicating that N partitioning was not impaired by this degree of stress. In contrast, N remobilization was greatly reduced by a more severe moisture deficit (drought intensity index 0·92), which resulted in yield losses of 92% (from 2·19 down to 0·17 g/pot). These results suggest that N remobilization may be an important drought adaptation strategy under moderate or intermittent moisture deficits. Severe moisture deficits reduced NHI, harvest index (HI), NUE and water-use efficiency (WUE) when WUE was expressed as seed dry weight per litre water used. Genotypic variability was observed for NHI, HI and NUE.

Omitting or changing various inputs to winter wheat, cv. Norman, were studied over the 1985/86, 1986/87 and 1987/88 crop years in Northern Ireland. Control plots received chemical inputs as commonly applied to intensively managed wheat, including herbicide, two broad spectrum fungicides and chlormequat and yielded 7·5, 9·3 and 6·3 t/ha in 1986, 1987 and 1988 respectively. Other treatments included no herbicide, no chlormequat, no late fungicide, no fungicide, no sprays, half seed rate, late sowing and late sowing with no sprays. Later sowing decreased take-all in 1986 and increased yield. All other treatments either reduced yield or had no effect. Omitting fungicide gave consistent and large decreases in yield by depressing the 1000-grain weight but the importance of early and late applications varied between years. The yield reductions from omitting fungicide were generally larger than those reported elsewhere. Lodging occurred in all years but chlormequat reduced it only in 1988 and gave a yield increase only in 1986, although the number of grains/ear was increased in all years. There was no evidence of any interactions between different pesticide inputs or with seed rate or sowing date.

Under the short-day conditions and cool temperatures prevailing from January to March in the Nile delta, Egypt, potato (Solatium tuberosum) seedlings grown from true potato seed formed tubers in the nursery when the plants were less than 5 cm in height. In order to examine the hypothesis that slow field establishment after transplanting and low tuber yields of these seedlings were caused by premature tuberization, different treatments to delay tuberization were applied in the nursery before transplanting.

Increasing the air and soil temperature in the nursery accelerated both the development of the above-ground and below-ground shoot organs, but did not improve field establishment of the seedlings after transplanting. Extension of the photoperiod with incandescent dim light, exogenous application of gibberellic acid (GA), and removal of the shoot apex to encourage the growth of lateral branches from the leaf buds increased biomass allocation to the above-ground shoot organs, decreased tuberization in the nursery and accelerated field establishment of transplanted seedlings. Extension of the photoperiod was the most effective treatment to delay tuberization, and doubled the final tuber yield from transplanted seedlings.

The results are in agreement with the hypothesis that poor field establishment and low final tuber yields from potato transplants may be caused by the presence of strong tuber sinks at transplanting, which reduce assimilate partitioning towards the roots and above-ground shoots.

A 4-year field experiment was conducted from 1986 to 1989 in Niger to determine the effects of pre- and post-harvest tillage and rotation of pearl millet (Pennisetum glaucum (L). R. Br.) and cowpea (Vigna unguiculata (L.) Walp.) on crop yield, crop water use and soil fertility on a sandy soil. Presowing tillage increased grain and crop residue yields over the traditional system of no tillage. Post-harvest tillage resulted in lower yields than pre-sowing tillage during the last 2 years of the experiment. Millet-cowpea rotation increased yield and there were no important tillage × rotation interactions. Crop water use was similar for a dry and a wet year, but less water was held in the profile after the harvest of cowpea. Compared to pre-sowing tillage and no tillage, post-harvest tillage increased the amount of water conserved until the next season. In all treatment combinations, soil organic matter and total N content both declined on average by 62%, and soil pH (KCI) decreased from 4·6 to 4·3.

The possible interacting effects of shading and N supply on number of grains of Triticum aestivum L. (cv. Buck Ñandú) were investigated at Balcarce, Argentina, during the 1988/89 and 1989/90 growing seasons. Shading was imposed from c. 13 days before anthesis to 6 days after, and four rates of N fertilization were supplied within each shading treatment around the date of terminal spikelet formation. Water and other nutrients were not limiting.

Total grain yield was strongly correlated with grain number/m2, regardless of shading or N supply. At the highest N rates, grain number and dry weight of spikes at anthesis were linearly related to a photothermal quotient, i.e. the ratio of intercepted photosynthetically active radiation (PAR) to mean temperature minus 4·5 °C, during the period from 20 days before anthesis to 10 days after. The response of grain number to the photothermal quotient was interpreted in terms of the supply of assimilates to the spike at anthesis, which determined flower survival. The response of dry weight of spikes to photothermal quotient was interpreted in terms of crop growth rate since there was a linear relationship between crop growth rate and intercepted radiation. The lowest N rates reduced the number of grains/m2, at any given photothermal quotient. Since the reduction in grain number also occurred at any given dry weight of spikes, it cannot be explained by a reduced supply of assimilates to the spikes. Grain number responded directly to the supply of N to the spike, probably through the survival of differentiated flowers. The relationship between spike growth rate and crop growth rate was not affected by N supply. Crop growth rate was reduced by reduced N supply, because less radiation was intercepted and because radiation-use efficiency was lowered. These results indicate that current models for determining yield and number of grains/m2, based on crop growth, are not adequate when N is deficient.

Field experiments were undertaken at the University of Wales, Aberystwyth over two seasons to investigate the effects of N fertilizer rate and timing on the development of Septoria diseases (principally Septoria tritici) in winter wheat and its subsequent effect on crop growth. Rates of 100, 200 and 300 kg/ha N were applied at GS23, GS30, or split (GS23/30/38) in 1985/86 and 100 and 250 kg/ha N at GS23, GS30, GS23/30, or GS23/30/37 in 1987/88. Foliar diseases were allowed to develop or were controlled with fungicide. Early disease levels were temporarily enhanced soon after N application and were greater at the higher rates of N. In April and June a significant correlation between disease infection and concentration of N in leaf tissue was shown. Despite differences in the level of early infection, dry matter production up to and including anthesis was unaffected by disease. Infection during grain filling caused premature senescence of the flag leaf which significantly reduced grain yield, primarily through a reduction in grain size. Loss of yield was directly related to severity of disease on the flag leaf in early July, with a yield loss of 0·45% for every 1% increase in disease infection. The severity of Septoria post-anthesis increased with increasing rate of fertilizer N by an average of 11·1% per 100 kg/ha N (translating to a potential yield loss of 5% for each 100 kg/ha N given as fertilizer), but was unaffected by its timing of application.

Slurry pH is of great importance for the regulation of ammonia volatilization from livestock slurry, and therefore more knowledge of the buffer system controlling pH is urgently needed for modelling ammonia losses from stored and surface-applied slurry. The composition of 17 different Danish cattle, pig and biogas plant-digested slurries was studied. The results were used to describe the main buffer components in the slurries, and to discover the most important chemical components necessary for modelling slurry pH. The results showed that the pH of slurry was mainly controlled by the species NH4+/NH3, CO2/HCO3-/CO32- and CH3COOH/CH3COO-, and that ion pair formation did not change the ionic balance significantly. There were only trace amounts of Ca2+, Mg2+ and inorganic phosphates in solution due to precipitation of CaCO3 (calcite) and MgNH4PO4.6H2O (struvite). Measured electrical conductivities were found to be strongly correlated with the calculated ionic strength.

The transmission of silver scurf (Helminthosporium solani) disease of potatoes was examined in field experiments at Cambridge University Farm in 1988–90. Treatment factors examined were seed size, seed age, seed incubation, soil moisture regime and planting date. A laboratory experiment investigated the viability of conidia of Helminthosporium in soil stored under different conditions.

Incubation of seed at high humidity before planting increased sporulation of Helminthosporium on seed tubers after planting and fewer conidia were produced from small seed than from larger seed. Delay in planting caused more rapid growth of Helminthosporium on seed tubers after planting.

Early planting and late harvesting increased the severity of silver scurf on progeny tubers. Severity of silver scurf was also increased by ageing seed and by incubating seed. Weight loss of potato tubers during storage tended to be greater from treatments with most severe silver scurf in all years but a significant linear regression of weight loss on silver scurf severity was found in only one year out of three from a late harvest. The viability of conidia added to soil was found to decrease rapidly so that by 10 weeks after addition, < 1% of conidia were apparently viable.

Seed yields of caraway vary considerably between years, indicating that weather affects the yield-determining processes. Pollination could be one of these processes, because pollinator activity and efficiency are negatively affected by cold or wet weather. From 1990 to 1992 at the Research Institute for Agrobiology and Soil Fertility in Wageningen, The Netherlands, field and glasshouse experiments were performed to study the importance of pollination for caraway seed yields and the effects of some environmental factors. Preventing insect pollination by placing gauze cages in field plots reduced the yield of caraway by c. 15–20%, but under normal field conditions > 90% of hermaphrodite flowers were fertilized and additional hand-pollination did not improve yield. Removal of competing umbels enhanced the low seed set percentages in higher-order umbels, indicating that pollination did not limit seed yield in these umbels. Assays suggested that wind transfer, in addition to insects, plays a role in the pollination of caraway. It was concluded that assimilate availability and not pollination limits caraway seed yield.

A simple model of vernalization, originally developed to quantify the vernalization response of fieldgrown carrots, was fitted to previously published experimental results for winter wheat cv. Norin 27. The optimum temperature for vernalization indicated by the model was c. 5·2 °C, as this induced the fastest progress to final leaf appearance, expressed as the reciprocal of number of days from sowing to final leaf. This rate decreased linearly with temperature rise or fall on either side of the optimum, extrapolating to zero at –4·8 °C (Tmin) and 26·6 °C (Tmax). When all the treatment temperatures and durations were expressed as vernalizing degree days > –4·8 °C (V °C d), there was a linear increase in post-treatment development rate with increasing vernalization up to c. 275 V °C d. Ending the effective treatment duration for vernalization at the estimated time of initiation of the final leaf primordia brought many of the data points closer to the linear trend which described the rest of the data.

Effects of using leaf number, which is linearly related to thermal time, instead of days as the unit of time to compensate for temperature differences in the original experiment were examined. Unvernalized plants had the potential to produce 18 leaves before flowering and therefore rates were expressed as the fraction of the potential total leaf number that each new leaf represented. All plants were assumed to have an initial development rate of 1/18 per leaf. This rate was assumed to increase linearly with time during the vernalizing treatment periods and then remain constant after treatment until the final leaf appeared. Leaf numbers reported from the original experiment were used with these assumptions to estimate the rate at the end of each treatment. The relationship between these rates and treatment temperatures was similar to that for rates based on post-treatment durations. There was an optimum temperature c. 5·5 °C and Tmin and Tmax of –5·1 and 18·8 °C estimated by extrapolating the decreasing linear trends to the base rate of 1/18. When plotted against V °C d calculated from these temperatures, the rates from the full data set were well represented by the model line which had been fitted to the data from just one treatment duration.