A joint model for plot yield in response to fertility trends and interplot competition is described. The model combines the mixed model representation of a cubic smoothing spline to model fertility and a regression model with auto-regressive terms to model competition. Estimation is based on a generalization of residual maximum likelihood. The methods were applied to a series of 70 sugar beet trials conducted by the Plant Breeding Institute, Cambridge, UK, and the results summarized.

An experiment was conducted over seven sites in eastern England sown to winter cereals, whereby the soil mineral nitrogen (N) and N recovery in the crop were measured frequently during the active growth period. Plots received 100 kg/ha fertilizer N on one of three dates in spring (28 March, 11 April and 25 April) on each site in each of the three years (1994 to 1996), and these were compared with controls with no applied N. At one site, uncropped plots were included, and measurements of gaseous N emission, soil respiration, soil microbial biomass and root tissue N concentration were also made. The fertilizer applications boosted yields by at least 3 t/ha at all sites, but apparent recovery of fertilizer N varied throughout the previously known range of 45–85%. Soil type and timing of application had no effect on N recovery, or on final yields. N was mineralized from soil sources throughout the growth period, but mineralization was outweighed by ‘‘immobilization’’ of large amounts of N (around 30 kg/ha) in the soil, chiefly during May when crop growth was most rapid. This occurred on all sites with a crop, but not where the crop was absent. Measured losses of N2 and N2O were very small (<70 g/ha/dayN) on the site where they were measured, and no other evidence of loss or storage of N was found. The apparent recovery of fertilizer N at each site, was almost exactly explained by the amount immobilized during May. We conclude that the poor recovery of spring-applied N was due, not to losses as previously assumed but to temporary immobilization during the period of most active crop uptake. Immobilization was caused primarily by the presence of the crop.

Eighteen experiments were carried out over a 6-year period (1989–94) on three different soil types to compare the effects of a broad-spectrum fungicide, applied as 1-, 2- or 3-spray programmes at different growth stages, to control foliar diseases on early and later-sown Blenheim malting barley. Powdery mildew, caused by Erysiphe graminis (Blumeria graminis) f. sp. hordei, was the dominant disease in all 6 years. Sowing date had a major effect on grain yield and ex-farm quality of the malting barley. The earlier-sown treatments gave significantly greater yield than the later-sown in 13 of the experiments, and significantly lower grain N concentrations and screenings (small grains <2·2mm) in 14 and 16 of them, respectively.

Early and more frequent fungicide applications improved grain yield and reduced grain screenings but had no significant effect on grain N. Early-applied fungicide (i.e. before GS 33) was generally the most effective in controlling disease, increasing yield and reducing screenings. The magnitude of the response depended on disease severity. In those years when disease was low on the early-sown crops (1989,1991 and 1994), the early-applied 1-spray programme and 2-spray programmes increased grain yields by relatively small amounts in both early and late-sown crops. In the other 3 years (1990,1992 and 1993), when disease severity was greater, the early-applied 1-spray programme and 2-spray programmes gave much greater grain yield increments on both early and late sowings. The 3-spray fungicide programme gave the greatest yields in most of the experiments but they were not significantly greater than the best of all the other treatments. Grain screenings were reduced by fungicide applications in both early and late-sown crops, but the early-applied 1-spray programme and 2-spray programmes were generally the most effective in reducing screenings.

This paper reports an analysis of the yield of sugar beet crops grown under experimental conditions between 1993 and 1995 in the UK. Crops were either healthy (unstressed) or subjected to drought, infection with Beet yellows virus (BYV) or a combination of both. The study investigated whether the large differences in yield between the crops grown in different seasons and subjected to different stresses could be accounted for by simple relationships between total biomass and radiation interception (εs), transpiration (εw) or εs and εw adjusted for mean saturation deficit (Ωs and Ωw respectively). Mean values of εs, εw, Ωs and Ωw in healthy crops were 1·42 g/MJ, 0·89 g/kg, 6·76 g/kPa/MJ and 4·29 g/kPa/kg respectively. Variations in the dry matter yield between seasons were best accounted for by Ωw and less well by εw or εs. Ωs accounted for least variation in yield between seasons. None of these relationships remained constant in stressed plants; both drought and BYV-infection decreased εs (and Ωs) but Ωw was increased by drought and decreased by BYV-infection. However, in common with healthy crops, seasonal variation in yields was best accounted for by Ωw. Mean values of εs, Ωs, εw and Ωw for all healthy, infected and droughted crops accounted for 61, 50, 88 and 97% of the variation in dry matter yield between experiments respectively. Accurate prediction of the yield of stressed plants requires a knowledge of their infection and drought status. If this information is unavailable then the mean value of Ωw for healthy, infected and droughted crops will provide a reasonable prediction of the yield of such crops.

Four extra-short-duration (ESD) lines in 1991 and eight ESD lines in 1992 were grown with adequate soil moisture throughout their growth or subjected to drought coinciding with the vegetative, flowering and pod-filling stages under rainout shelters. In both years, drought stress treatments significantly reduced dry matter accumulation and grain yield. The extent of reduction in grain yield varied with the line and stage of stress imposition. Drought stress at the flowering stage caused greater reduction in total dry matter and grain yield than the stress imposed during the pre-flowering and pod-filling stages. Drought stress coinciding with the flowering stage reduced grain yield by 40–55% in 1991 and 15–40% in 1992 in different lines. ESD genotypes could extract moisture from up to a metre depth during pre-flowering and flowering stage stress but less so during the pod-filling stage stress. Genotype ICPL 88039, followed by ICPL 89021, showed consistently lowest sensitivity to drought stress at flowering. Protracted drought stress commencing from the pre-flowering to flowering or from the flowering to pod-filling stages was more harmful than stress at the individual stages. The reduction in yield under drought stress could be attributed mainly to less total dry matter accumulation, but also increased abscission of plant parts. The results suggest variation in sensitivity of ESD lines in relation to timing of stress, which should facilitate targeted screening for different intermittent moisture stress environments.

Field experiments were conducted during two successive seasons in the Agricultural Research Center, Giza, Egypt (a clay loam soil), to study the effects of phosphorus fertilization (addition at rates of 44 and 74kg of P2O5/ha) and foliar application of zinc (at 0·0 and 40ppm of Zn applied twice, 75 and 90 days after planting) and calcium (at 0·0, 20, 40, and 60ppm of Ca applied twice, 80 and 95 days after planting) on seed, protein and oil yields and oil properties of the Egyptian cotton cultivar Giza 75. Application of P at the high rate and spraying plants with zinc and calcium at any concentration increased cottonseed yield/ha, seed index, seed oil content, oil and protein yields/ha, seed oil unsaponifiable matter, and total unsaturated fatty acids (oleic and linoleic). Conversely, decreases in oil acid value, saponification value and total saturated fatty acids occurred. The seed protein and oil refractive index tended to increase when the high P rate and Zn were applied. Seed protein content tended to decrease slightly with Ca application. Regardless of the slight decrease in protein content as a result of Ca application, the yield of protein increased, due to the fact that there was an obvious increase in seed yield which counterbalanced the negative effect of the decrease in protein content. The highest Ca concentration (60ppm) gave the highest values of cottonseed yield/ha, seed index, seed oil and protein yields/ha and oil unsaponifiable matter, followed by the concentration of 40ppm. The concentration of 40ppm gave the lowest total saturated fatty acids. The effect of Ca concentrations on oil refractive index was very limited.

The effects of different plant arrangements (conventional single rows v. paired rows at a standard density of 55–67 thousand three-bud setts/ha) with different intercropped green manuring strategies with Sesbania aculeata (none v. ploughed in after 4 and 6 weeks) and with different amounts of nitrogen (N) fertilizers (0,75 and 150kg/ha) were studied in sugarcane crops planted after wheat or mustard and in its first ratoon. The yield of millable cane from the planted sugarcane was not affected by intercropped green manuring or plant arrangement but was increased by fertilizer N after both wheat and mustard. The residues from the green manuring and fertilizer N both increased the yield of the following crop of ratoon sugarcane by 9–10% with the residues increasing the numbers and length of millable canes and the fertilizer N the number, girth and weight of the millable canes. None of the treatments affected the quality of cane juice in either the planted cane or the ratoon crop. Residues from the green manures and the N fertilizer treatments increased the organic carbon content and available N in the soils.

Experiments were conducted in Nigeria to evaluate the optimum plant population density for okra (Abelmoschus esculentus) sown in monoculture or intercropped with cassava (Manihot esculenta). The crops were sown as late and early season crops in 1997 and 1998. Okra was sown at varied spacings to achieve final plant densities of 25000, 35000 and 50000 plants/ha in both monoculture and mixed stands. Intercropping had no significant effect on okra phenology (time to vegetative growth, flowering and fruiting). However, it significantly reduced weed growth by 25–45% and nematode infection in okra by 36%, and kept the canopy environment of cassava cooler by 3·0–4·4°C and more moist by 5–10g/kg, compared with monoculture. The durations of vegetative and reproductive growth and the weed-control ability of okra in both cropping systems were influenced by population density. Okra sown at 50000 plants/ha took the longest time to reach specific phenological stages, controlled weeds best and gave the highest fresh pod yields in both seasons. There was little increase in pod yields between 35000 and 50000 plants/ha in mixed stands in late season sown crops. The growth and tuber yield of cassava were not significantly affected by intercropping and okra population density.

The growth of okra and cassava were better with early than late season sowing, irrespective of okra population density or cropping system. This appears to have been due mainly to the effect of temperature and rainfall differences between the seasons. The substantial variation in the weather during the two seasons also had greater effect in monoculture than mixed stands. It is concluded that when rainfall is limiting, okra could be intercropped with cassava using population density up to 35000 plants/ha to allow the vegetable to develop and to maximize pod yield under the relatively dry conditions. The optimum target suggested is 50000–60000 plants/ha during the wet periods.

In order to compare the growth potential of buds of two contrasting cultivars of white clover (Trifolium repens L.) (cv. AberHerald and Grasslands Huia) during the winter–early spring period, we adapted a method to assess the potential for activity of shoot buds and root primordia. The growth potential was evaluated following the excision of individual stolon nodes sampled in the field, from experimental plots located in the eastern part of France. Two populations of nodes were sampled: (i) the youngest node emerged from the apical bud (node 1), (ii) the third node behind the apex (node 3). The excised nodes were placed in controlled conditions and the outgrowth of axillary buds and roots was recorded every day for a week. The study demonstrated a seasonal influence on the potential for activity of axillary buds and root primordia, particularly for buds at node 3. The proportion of potentially active shoot and root buds was less than 50% at the end of March, and then increased from end of March to reach 80% in April. The data indicated a genetic variation in potential for outgrowth during winter. AberHerald had a higher percentage of potentially active buds than Huia during winter, irrespective of the node position. Moreover AberHerald buds had faster rates of outgrowth than Huia buds. Differences between cultivars in winter survival and potential for growth in spring were linked to identified cultivars' differences in developmental characteristics of buds.

The effectiveness of alternative fertilizer strategies to control nitrate leaching was investigated in a field experiment at IGER, Aberystwyth using simulated pastures to represent swards grazed by dairy cattle between May and October 1997, following an initial silage cut. Cut-plots with and without applications of artificial urine were used to represent the separate components of grazed pastures managed with the following nitrogen (N) fertilizer strategies: standard (fertilizer applied uniformly to all areas of the sward at a total rate of 180 kg N/ha between May and October), tactical (fertilizer rates adjusted to match the average soil mineral-N content of the pasture to that of ungrazed reference plots receiving the standard rate) and patch-avoidance (fertilizer applied at the standard rate but withheld from areas that had received urine). Calculated stocking rates derived from herbage yields indicated that 12% of the pasture would have been affected by urine up to the end of ‘grazing’ in October. The presence of urine patches increased the nitrate-N content of the 0–90 cm soil profile in October from 61 kg N/ha for ungrazed pasture to 104 kg/ha for ‘grazed’ pasture receiving the standard fertilizer rate. Although the patch-avoidance strategy was more effective than the tactical in reducing the accumulation of nitrate in soil under urine patches, they were both of limited effectiveness in reducing the content over the pasture as a whole. Profile contents in October for the simulated pastures managed with the tactical and patch-avoidance strategies were equivalent to 99 and 97 kg nitrate-N/ha, respectively. The tactical strategy achieved a 26% saving in overall fertilizer use. Under the conditions of the experiment this did not significantly reduce herbage yields. The patch-avoidance strategy reduced fertilizer use by only 3%.

A laboratory experiment was designed to determine the fate of 15N-labelled slurry ammonium (15NH4-N) and compare soil inorganic-N distribution following surface applied or injected pig slurry. A system of cylindrical volatilization chambers equipped to allow continuous trapping of ammonia (NH3) was used. Undisturbed soil columns were placed in the chambers prior to the application of slurry. A nitrogen balance including soil, air and plant analysis was established for both treatments, 8 days after application. Average cumulative emissions of NH3 were 15% and 11% of the total ammoniacal-N added with the surface and injected treatments, respectively. After 8 days 55% of the 15NH4-N applied through slurry injection was recovered in the soil inorganic-N pool: 37% as 15NH4-N and 18% as 15NO3-N. These figures compare with only 25% 15NH4-N recovered with the surface applied slurry treatment: 7% as 15NH-N and 17% as 15NO3-N. Immobilization into soil organic-N accounted for 8% of the 15NH4-N applied for the injected treatment and 6% of the surface applied slurry-15N. 15N uptake by the grass was 2% and 7% for the injected and surface applied treatments, respectively. The percentage of added 15N accounted for was 76% for the injected treatment and 53% for the surface applied slurry treatment.

An experiment was carried out from August to early November 1994 to examine differences in diet selection, herbage intake, grazing behaviour and animal performance between weaned lambs rotationally grazing swards of annual ryegrass (Lolium multiflorum)/white clover (Trifolium repens) and Yorkshire fog (Holcus lanatus)/T. repens with or without Lotus corniculatus. There were four replicate groups of six lambs per treatment. The effects of condensed tannins (CT) on lamb production were assessed by twice-daily oral administration of 10g polyethylene glycol (PEG; molecular weight 4000) to half the lambs on each sward. The Lotus content of all swards was very low, and results are presented here for main sward comparisons meaned over lotus treatments. Overall mean estimates of pre-grazing herbage mass and sward surface height for the annual ryegrass and Yorkshire fog swards respectively, were 5820 v. 4360±190 kgDM/ha (P<0·001) and 29 v. 21±0·6cm (P<0·001). The coefficient of organic matter digestibility (OMD) of the diet selected and herbage intake were higher on Yorkshire fog than on annual ryegrass (0·78 v. 0·74 ± 0·080g/kg; P<0·05, and 1070 v. 860±57g OM per lamb per day, P<0·05 respectively), reflecting the higher content in the diet of grass green leaf (980 v. 930g/kg±14g/kg, P<0·05) and the lower content of dead material (80 v. 110±15g/kg, P<0·08). Lambs grazing on Yorkshire fog swards had higher clean wool growth rate (1470 v. 1280±30mg/cm per day, P<0·01) and greater fibre diameter (31 v. 29±0·2μ, P<0·001), greater liveweight gain (152 v. 108±5·5g/day, P<0·001), final weight (42 v. 38±0·5kg, P<0·001), carcass weight gain (89 v. 69±2·5g/day, P<0·001), carcass weight (19 v. 17±0·3kg, P<0·001) and soft tissue thickness (GR value 11 v. 8±0·5mm, P<0·01), and lower faecal egg counts (FEC; square root transferred values 9·2 v. 11·0±0·4 eggs/g fresh faeces, P<0·01) than lambs grazing annual ryegrass swards. Similar dietary concentrations of condensed tannins (CT) between Yorkshire fog and annual ryegrass swards (4·2 v. 3·7 DM±0·2g/kg, P<0·08) increased clean wool growth (1440 v. 1310±32 mg/cm per day, P<0·05), fibre diameter (30·7 v. 29·5±0·21μ, P<0·01) and liveweight gain (141 v. 120±4·3g per lamb per day, P<0·01), although differences in carcass weight (17·9 v. 18·2±0·3kg) and FEC transformed values (9·6 v. 11·0±0·6 eggs/g fresh faeces) were not significant. The effects of CT on animal performance were greater in Yorkshire fog swards. CT had no significant effects on diet selection, herbage intake and grazing behaviour patterns.