A system for automatically measuring the mean cell-wall thickness in a user-defined area of plant tissue has been developed using image analysis. The digitized grey-level image of a tissue section is first segmented using a histogram-partitioning algorithm. The resulting binary image is then repeatedly thinned until the minimum connected set of pixels, or ‘skeleton’, remains. A nearestneighbour length estimator is used to calculate the total length of the skeleton which approximates to the location of the middle lamella in the original section. The length of the skeleton and the number of nodes it contains are used to estimate the mean cell radius, and mean cell-wall thickness using the area of cell-wall material in the segmented binary image. The method has been used to estimate mean cell-wall thickness along a newly extended Zea mays internode, and the results are compared to measurements obtained manually using a micrometer ‘line’. The techniques of rapidly assessing mean cell-wall thickness and cell dimensions using image analysis are needed to assess how much of the variation in nutritive value between forage cultivars can be ascribed to changes in cell-wall chemistry and how much to anatomical differences.

The sensitivity of predicted final grain yield to changes in the mean and variance of daily temperature and daily log(radiation) was studied for the arcwheat1 crop model of winter wheat. These two climatic variables were each simulated by each of two stochastic models, the parameters of which were estimated from 12 years' data from Rothamsted Experimental Station.

When arcwheat1 was run with systematic changes in daily temperature from – 2 °C to +2 °C in steps of 0·1 °C, there were abrupt changes in predicted final grain yield which coincided with predicted increases in leaf number. These effects were smoothed out when the stochastic climate models incorporating simulated daily variation were used as inputs, in multiple runs of arcwheat1.

In such runs, the response of predicted yield to changes in the means of temperature and log(radiation) was studied both for individual developmental stages and over the entire growing season. Yield was insensitive to changes in temperature and radiation during the stage between sowing and emergence, and to changes in radiation before terminal spikelet formation.

Seven-day-old leaves of two wheat cultivars, one resistant (Cruz Alta) and one sensitive (Leones) to water stress were subjected to 100% O2 and water stress in the light for 72 h. The aim of this work was to analyse the effects of both types of stress on the chloroplast antioxidant enzymes, glutathione reductase (GR) and ascorbate peroxidase (AP) and their relationship with stress resistance.

Both GR and AP increased following 100% O2 and water stress in the resistant cultivar Cruz Alta only. Sulphydryl (SH) groups increased in the resistant cultivar and decreased significantly in the sensitive one under both stress treatments. Measurement of stress parameters (chlorophylls, proteins, lipid peroxidation and conductivity) revealed that oxidations caused by stress treatments appeared to be higher in the sensitive cultivar Leones than in the resistant Cruz Alta.

It is suggested that there may be a link between GR, AP and oxidative protection. Some cross resistance between water and oxidative stresses in wheat leaves is also indicated.

The edge effect (calculated as the mean of the two outer rows as a percentage of the mean of the four central rows of 6-row plots) was measured in ten comparative trials conducted in Cyprus during 1987–90, with two cultivars each of rapeseed, safflower and sunflower, and one cultivar each of barley and durum wheat. Grain, straw and total biological yields obtained from the outer rows were usually higher than those obtained from the central rows by 1–384%. In one trial with sunflower, the grain yield of the outer rows was only 64% of that of the central rows, which could not be explained, considering all the other data. Differences in edge effects between species and trials were significant. Grain yield estimated from the central rows was on average 53% of that estimated from whole plots, and this varied with species and season. Therefore, in comparative trials, the outer rows should be removed before harvesting. There was no significant edge effect on the oil content of the oilseed crops. Edge effects on harvest index, volume weight, 1000-grain weight and nitrogen content were generally smaller than those on yield, and on average ranged from 100 to 113, thus the values computed from the outer rows were only 0–13% higher than from the central rows.

Field experiments were conducted in Scotland between 1985 and 1987 to examine the responses of mean grain weight and nitrogen content of triticale cultivars to levels of applied nitrogen from 0 to 180 kg N/ha and the plant growth regulator chlormequat. Cultivar differences were seen for both these characters. Mean grain weight generally declined as yield increased, whether with nitrogen or with chlormequat. Nitrogen content of the grain decreased as grain size increased. Nitrogen recovery (g/m2) generally increased with applied nitrogen and was unaffected by chlormequat application. Within a grain sample from an individual treatment, however, grain size was not related to nitrogen content.

The response of nitrogen content is interpreted as differential dilution of nitrogen in the grain by different amounts of carbohydrate. This is probably related to the frequent occurrence of shrivelled grain in triticale.

Wheat, barley and two morphologically contrasting cultivars of peas (leafy and semi-leafless) were grown in pure stands, at standard agricultural densities, and in additive mixtures of cereals with peas. The stands were grown in boxes in the field, and partitions were used to separate the effects of root and shoot interactions. The cereals and peas were either planted at the same time, or one species was planted 10 days before the other. The origin of the N present in each species was determined by applying N fertilizer labelled with 15N.

Both cultivars of peas had greater shoot and root competitive abilities than wheat or barley, probably because of their larger seed size; leafy peas had greater shoot and root competitive abilities than semi-leafless peas. Sowing peas after cereals reduced their competitive ability.

The relative yield total (RYT) of cereal-pea mixtures, based on total biomass, averaged 1·6 when only the root systems interacted, and 1·4 when only the shoot systems interacted, but did not differ significantly from 10 when both root and shoot systems interacted. RYT values were greater when peas were grown with wheat, rather than with barley, and when peas were sown at the same time as the cereals.

Shoot competition from peas increased the N% of cereals, but substantially reduced their total N content, because biomass yield was reduced. Shoot competition from cereals had no effect on the N% of peas, and only slightly reduced their total N content. Shoot competition between cereals and peas had no significant effect upon the proportion of N derived from various sources by either cereals or peas.

Root competition from peas significantly reduced both the N% and total N content of cereals. Root competition from cereals had little effect on the N% of peas, but significantly reduced their total N content and increased the proportion of N derived from rhizobial fixation from 76 to 94%. Since cereals and peas largely used different sources of N, resource complementarity for N was probably an important component of intercropping advantage, when the roots of cereals and peas shared soil resources.

Field studies were conducted in order to determine the components of yield and response of maizeto soil applied copper and sulphur in the rainforest-savannah transition zone and Guinea savannah zone of Nigeria. Five CuO treatments, 0, 1, 3, 6 and 9 kg/ha, were applied to maize in 1988–90, and grain yield was consistently and significantly increased by the 1 kg/ha CuO treatment. Five S treatments, 0, 10, 20, 40 and 80 kg/ha, were applied to maize in 1989–90, and all treatments increased grain yield significantly in 1989, but only the 10 kg/ha S treatment increased grain yield significantly in 1990. Higher amounts of applied S increased leaf S significantly in the Guinea savannah zone.

Field scale variability in the grain yield of barley in 1989 was investigated in 62 field plots in a Dutch polder area, and compared to soil- and simulation-type characteristics. Total grain mass varied between 3409 and 6019 kg/ha, and grain moisture content between 131 and 14·7%. Soil profile descriptions and soil characteristics were used as basic input data for simulations. Soil water flow was simulated at 119 locations with the LEACHM model, for the purpose of quantifying spatial variability in transpiration deficits in the growing season. Both soil- and simulation-type characteristics were translated from point values to spatial averages for the harvested fields, using kriging. Kriged characteristics were correlated with yields, and used to construct transfer functions. Simulated transpiration deficits during sensitive crop development phases showed negative correlations with grain yield. Transfer functions explained at maximum 68·2% of the variance in the yields.

The morphology of independent plants of perennial ryegrass (Lolium perenne L.) in mixed swards under intensive sheep grazing was studied at Palmerston North, New Zealand in 1988/89. Intact plants of two cultivars, ‘Grasslands Nui’ and ‘Grasslands Ariki’, were sampled from pastures under rotational grazing, set stocking, and a combination of both systems, every 2 months for 16 months, by taking turves (250 × 250 mm) and washing out the plants. Characters measured for each plant were: number of flower heads, leaves and tillers; basal stem and internode stolon length; flower head, leaf blade, leaf sheath and stolon dry weight (DW). Additional pasture data on tiller density and dispersion and herbage DW to ground level were also collected.

Perennial rye grass exhibited a strong clonal growth pattern similar to that of white clover (Trifolium repens L.) but with a better balance between growth at the apex and death of old basal stem. Release of branch stolons to form new plants was regular, which maintained population stability. Formation of internode (I) stolon appeared to be a response of heavily shaded tillers for survival by elevation of the growing point to a more favourable position. The total DW of ryegrass stolon in pastures was greater than that of the companion white clover.

There was little seasonal variation in number of tillers or basal (B) stem length per plant. During the reproductive period (late spring/early summer) plants were heavier and had more leaves and internode stolon than at other times of the year. Grazing management had little effect on plant structure, but plants under rotational grazing had 75% greater DW, less basal stem and reduced probability of internode stolon formation than plants under set stocking. Nui plants had heavier but fewer tillers than Ariki did, resulting in little difference in plant DW. Ariki had fewer single-tiller plants and a higher proportion of multiple-tiller (> 7) plants than Nui had. The structure of the populations was not affected by season, management or cultivar and remained stable throughout the year, with a log-normal distribution of plant size. Because of the uniformity of plant structure, plant density in pastures paralleled tiller density, averaging 1900, 3360 and 2330 plants/m2 for rotationally grazed, set stocked and combined grazing treatments respectively.

In view of the strong influence that grazing management can have on ryegrass performance and persistence in pastures, the lack of response at the level of plant and population structure to grazing management suggests that some other level of organization within the ecology of the pasture is of greater importance.

A study, conducted in the Peace region of Canada, determined the effects of the stand and management treatments of four herbage experiments (1986–88) on the yield and quality of subsequent crops of barley (Hordeum vulgare L.) harvested in 1989 and 1990. Quantitative relationships indicated that the crop management treatments most conducive to good yield and quality of herbage were also the most beneficial for subsequent high yield and quality of the barley grain. Unless designs orthogonal to those previously used can be found, covariance adjustment procedures are recommended to minimize the residual effects of previous experiments.

The growth of the tree legumes Leucaena leucocephala, Gliricidia sepium, Sesbania sesban, Sesbania grandiflora, Albizia falcataria and Calliandra calothyrsus was evaluated at four sites in Indonesia between 1984 and 1986. Tree seedlings were planted at 10000 trees/ha and, after an establishment period of 9–12 months, harvested every 3 months for a 2-year period.

At Bambu Apus, West Java (2040 mm annual rainfall, 1 month dry period, lowland, latosol with pH 5·6 decreasing with depth), G. sepium and C. calothyrsus produced 13 and 10 t/ha per year of leaf dry matter, respectively. Other species did not grow well at this site and fewer than 60% of the trees of these species survived until the end of the experimental period. At Sei Putih, North Sumatra (1550 mm annual rainfall, 3 months dry period, lowland, podzolic with pH 5·5 increasing with depth), no species produced > 6 t/ha per year of leaf dry matter. The highest yielding species were L. leucocephala and A. falcataria. At Cisarua, West Java (3340 mm annual rainfall, no dry periods, 925 m altitude, latosol with pH 5–6 increasing with depth) only C. calothyrsus grew well and produced 9 t/ha per year of leaf dry matter. At Grati, East Java (1500 mm annual rainfall, 6 months dry period, lowland, alluvial with pH 6·6 increasing with depth) L. leucocephala produced > 8 t·ha per year of leaf dry matter, while none of the C. calothyrsus and A. falcataria trees survived the first dry season.

Although L. leucocephala has been the most widely grown tree legume species in the tropics, C. calothyrsus and G. sepium performed better at the wetter sites and at high altitude.

The effects of varying the timing of nitrogen application and the frequency of fungicide sprays on the incidence of foliar disease and on the grain yield of winter barley (cv. Panda) were investigated at two sites in Ireland over three years, 1986–88. Nitrogen application timing had little effect on the amount of foliar necrosis due to disease but delaying application of all the nitrogen until April significantly reduced grain yields in two of the three years when compared with standard split dressings in March and April. The reductions in yield reflected small differences in 1000-grain weight.

The number of fungicide sprays applied had significant effects on the amount of foliar necrosis and on grain yields. One-spray and two-spray programmes increased average yield by 16 and 32%, respectively. The yield increases were attributable to more ears/m2 and larger grains (greater 1000-grain weight). Fungicides also delayed the onset and significantly reduced the amount of brackling. Delaying nitrogen application in the spring did not decrease the number of fungicide sprays required to achieve maximum yield.

A 3-year field study (1986–88) was conducted in central Alberta to discover how diverse soil-plant systems function under cryoboreal subhumid conditions. Barley, fescue, faba (field) bean and a barley/field pea intercrop were grown continuously on different soils at Ellerslie and Breton using two distinct tillage methods. The agronomic performance, weed-crop interactions and below-ground productivity of these cropping systems were examined. The main findings were as follows: different soil properties did not affect yields of barley, barley/field pea and fescue fertilized with N and P; silage yield of faba bean at Breton was greater than at Ellerslie; barley/field pea and faba bean could be grown without tillage at Ellerslie; barley/field pea plots had the lowest weed counts; fescue root biomass was greatest at all depths followed by faba bean and barley; and soil properties appeared not to induce differences in root production of a cereal, an annual forage legume and a perennial grass. Increasing the use of annual legumes into rotations, either as sole crops or as intercrops with cereals, may be a viable alternative to continuous cereal cropping because annual legumes contribute N through biological N fixation, reduce weed competition and increase the input of root mass in soil.

In two field experiments in Suriname, competition between weeds and sorghum (Sorghum bicolor (L.) Moench) was studied in the rainy seasons of 1982 and 1983. The crop was kept either weed-free or without weed control for each of six different periods of time from planting. Observations made at the end of each period allowed an analysis to be made of the growth and development of a crop with and without weed control.

Due to the crop canopy structure and fertilizer placement near the row, weed growth was concentrated in the rows. Starting at c. 20–30 days after planting, competition with weeds reduced ground-cover and leaf area index of the crop. Competition reduced growth rates, leading to lower yields. Plant population density was not affected, but competition reduced the number of leaves present. Stem length initially increased with competition in one season, but was retarded in another. Competition for nutrients was strong and was found as early as 15 days after planting. Nitrogen was the element most competed for initially. Competition for water occurred, but competition for light seemed unlikely. About 20 weed-free days after planting were necessary to avoid yield losses and around 30 weed-free days to attain negligible weed growth at harvest. Yield reduction was mainly due to a decrease in number of grains per panicle. Competition during floret establishment (c. 30–40 days after planting) should be avoided.

The rumen degradation and gas production characteristics (methane, carbon dioxide) of leaves of 20 accessions of multipurpose trees (MPTs) from six genera: Acacia, Cajanus, Chamaecytisus (L. fil.) Link, Erythrina, Leucaena and Sesbania and some poisonous plants were investigated in vitro and in sacco in rumen fistulated cows fed on a diet of grass hay ad libitum supplemented with cotton seed cake. The degradation constants (i.e. the soluble fraction (a), the slowly degradable fraction (b) and the rate of degradation (c) and the potential degradability (a + b) (PD)) were calculated. The gas production constants were estimated following the equation: Volume = bg(1—e-cgt) where bg is gas production and cg is the rate of gas production. Between and within genus comparisons of degradability and gas production constants were done.

The soluble fraction (a) was significantly higher for Sesbania (59) (P < 0·05) than the other genera, which had values between 45·18 and 40·38 units. There were no significant differences in the slowly degradable fraction (b) between genera. Sesbania was degraded significantly faster (P < 0·05) than either Acacia, Leucaena or Cajanus. Similarly, the potential degradability was significantly higher (P < 0·05) for Sesbania (92·7) than for the other genera. Acacia was the least degradable genus.

Degradation characteristics were similar between species within the genera Acacia, Erythrina and Leucaena. However, within the Leucaena genus, L. revoluta had the highest soluble fraction (50·02) and the cross L. leucocephala × L. diversifolia had the lowest (29·24). L. leucocephala had the highest slowly degradable fraction (57·32) and L. revoluta had the lowest (42·37). L. leucocephala × L. pallida had the highest rate of degradability (0·0626) and L. pallida had the lowest (0·0221). L. leucocephala had the highest potential degradability (92·23) and the cross L. pallida × L. diversifolia the lowest (84–81).

Between the genera, more gas was produced from Sesbania than from any other genus. The effect of genus was significant (P <0·0003). On the other hand, the rate of gas production (cg) was higher with the genus Chamaecytisus than with any other genera (P < 0·0001). Although the two methods used agree in the position of a forage of a high degradability like Sesbania, they fail to agree on the relative positions of the forages of lower degradabilities. For some genera, drying of the tree foliage reduced the volume of gas produced, but increased the rate of gas production.

Fifteen grassland species were grown in a heated glasshouse in February-April 1985. The harvested herbage was separated into leaf and ‘stem’ and analysed for neutral detergent fibre, digestibility, physical breakdown when macerated, fibrosity, water soluble carbohydrate and N, P, K, Ca, Mg and Na.

Lolium perenne was digestible, but rather high in neutral detergent fibre and not readily broken down by maceration. The other grasses tested, Holcus lanatus, Poa annua and Glyceria fluitans, tended to be less digestible and higher in neutral detergent fibre than L. perenne.

The leaves of all 11 dicotyledonous species were much lower in neutral detergent fibre than the leaves of the grasses, and most broke down readily when macerated. The stems of Medicago saliva and of two shrub species (Lonicera periclymenum and Prunus spinosa) were the highest in neutral detergent fibre.

Rumex obtusifolius, Spergula arvensis and Stellaria media were high in Mg; Plantago lanceolata, S. arvensis and S. media were high in Na; and S. arvensis was high in P.

Three perennial and two annual herbaceous, dicotyledonous non-legumes were selected as being of sufficient promise and interest to be compared with Lolium perenne in feeding experiments: Plantago lanceolata, Taraxacum officinale, Rumex obtusifolius, Spergula arvensis and Stellaria media.

Six plant species were compared as food for sheep: chickweed (Stellaria media (L.) Vill.), dandelion (Taraxacum officinale Weber), dock (Rumex obtusifolius L.), ribwort (Plantago lanceolata L.), spurrey (Spergula arvensis L.) and perennial ryegrass (Lolium perenne L.). In two experiments at Aberystwyth in 1985 and 1986, the species were fed to lambs as the total diet, after artificial drying. In another experiment in 1987, they were fed fresh, in 1·5 minute meals, to yearling sheep.

The dicotyledonous species had a lower proportion of cell wall (neutral detergent fibre) in dry matter than perennial ryegrass. The true dry matter digestibility of chickweed, dandelion, dock and leafy ribwort was as high as that of ryegrass, but the in vivo digestibility of all five dicotyledonous species was lower than that of ryegrass. The voluntary intake of the dicotyledonous species was higher than might have been predicted from their digestibility. The voluntary intake of spurrey was > 20% higher than that of ryegrass.

When the species were fed fresh, spurrey had the highest rate of intake as a consequence of a high chewing rate and a low requirement for number of chews/g of dry matter consumed. Chewing rate was higher on all the dicotyledonous diets than on ryegrass. The rate of intake of dandelion was above average, but that of dock was low and variable, particularly when the plants had been chopped.

Previous research has suggested that fry colour of processed potatoes is inversely correlated with tuber reducing sugar content, and agronomic practice should therefore be tailored to minimize amounts of tuber sugars at harvest and during storage as a means of maximizing processing quality. A total of 30 experiments from three sites in England from 1985 to 1988 examined this hypothesis for two cultivars using a wider range of husbandry than that used in practice and long periods of storage. No tuber sugar value was sufficiently well correlated with fry colour for the relationship to be interpreted as causal and of use predictively, irrespective of whether it was measured early in growth, at harvest or at the time of processing. Fry colour at harvest was not correlated with fry colour after storage. No agronomic practice (variables used were physiological age, date of planting, irrigation or date of harvest) consistently caused fry colours to be darker than the acceptable colour limit, whether at harvest or after long term storage. Crops producing unacceptable fry colours early in storage generally improved to produce acceptable fry colours after long term storage. Agronomic practices within the range normally found in commercial production are therefore considered to have little effect on long-term processing quality, although harvesting, handling and transport operations were not investigated.