An area of plant science that is still uncertain is precisely what determines plants' demand for nitrogen (N), that is the amount of N they need to take up from the soil to meet their requirements for potential growth and synthesis of new tissue. A robust and unequivocal physiological basis from which to determine N demand is lacking. Yet N dominates plant nutrition. No nutrient is needed in larger quantities and, in most environments, no nutrient is in such limiting supply. Knowledge of the factors governing N demand is essential to predict the needs of crops under a wide range of field situations, so that growers can be given more reliable fertilizer recommendations (Greenwood 1982; van Keulen et al. 1989). This is important, not just for economic reasons, but because of the risks to the environment that can arise from the over-application of N fertilizers, in particular the problem of nitrate leaching (Addiscott et al. 1991).

Observations were made in 1994 and 1995 in Hertfordshire of the flowering phenology and attractiveness to beneficial insects of two commercial mixtures of flowering plants intended for set-aside land. These were the Tübingen Mixture from Germany and Ascot Linde SN from the Netherlands. The mixtures were visited by 14 species of Hymenoptera, 14 species of syrphid Diptera and six species of Lepidoptera. Although the mixtures contained 12 and five plant species respectively, Phacelia tanacetifolia was the dominant species to establish, flower and attract insects in both mixtures. The other plants contributed little to flower density or insect diversity. These mixtures are therefore not suitable for UK needs using the present proportions of plant species.

Yield responses of some economically important crop plants due to foliar applications of low concentrations of 28-homobrassinolide (HBR) were studied in experiments conducted at research stations and in farmers' fields in India during 1989–95. Foliar sprays of different concentrations of HBR were applied at tillering and spike/panicle initiation in wheat (0·5 and 1·0 mg/l) and rice (0·25, 0·50 and 1·00 mg/l); at flowering and pegging in groundnut (0·25 and 0·50 mg/l); 30 and 45 days after sowing (DAS) in mustard (0·25 and 0·50 mg/l); 25 and 35 days after emergence in potato (0·25 and 0·50 mg/l); and 30, 50, 70 DAS in cotton (0·1 and 1·0 mg/l). The HBR treatments significantly (P<0·05 and P<0·01) increased grain yields in wheat, rice and mustard, pod yields in groundnut, tuber yields in potato and seed cotton yields, over control. The extent of yield improvement due to HBR was influenced by crop species, concentration of HBR, plant growth stage at application and frequency of application.

15N-labelled fertilizer was applied at different rates (0, 30, 60, 90 kg N ha−1) and in different forms (urea or ammonium sulphate) to wheat grown in Syria in three seasons (1991/92, 1992/93 and 1994/95).

Recovery of 15N-labelled fertilizer in the above-ground crop at harvest was low (8–22%), with the amount of 15N-labelled fertilizer recovered in the crop increasing as the rate of application increased. Fertilizer application caused a significant increase in the amount of unlabelled soil N in the crop, suggesting that the application of N fertilizer caused a ‘real’ added nitrogen interaction. Recovery of 15N-labelled fertilizer in the crop was unaffected by the form of the fertilizer.

On average 31% (14–54%) of the 15N-labelled fertilizer remained in the soil at harvest, mostly in the 0–20 cm layer. At the lowest application rate (30 kg N ha−1) most of the residual fertilizer was as organic N, but at the higher application rates (60 and 90 kg N ha−1), a greater proportion of the 15N-labelled fertilizer was recovered as inorganic N, presumably as the result of top-dressing N in dry conditions in the spring. The amount of 15N-labelled fertilizer remaining in the soil increased as the fertilizer rate increased, but was unaffected by the form of fertilizer applied.

Losses of 15N-labelled fertilizer were large (>35%), probably caused by gaseous losses, either through volatilization of N from the calcareous soil, or through denitrification from wet soils rich in organic residues.

N fertilization strategies in the West Asia/North Africa (WANA) region should take note of the low recovery of N fertilizer by the crop in the season of application, and the resultant large quantities of residual fertilizer.

In two field experiments conducted in 1991 and 1992 at Akure, Nigeria, 0–12 and 0–24 ml/l of the plant growth regulator, paclobutrazol, and 0, 75 and 150 kg N/ha were applied to maize in factorial combinations. Substantial vegetative growth occurred with increasing N rates in 1991 and this accelerated 50% silking date. Paclobutrazol influenced maize growth at this stage only in 1992 when 12 and 24 ml/l depressed plant height and increased both stem girth and leaf number/plant.

Higher paclobutrazol application rates decreased plant height at maturity in both years while, in 1991, 150 kg N/ha enhanced it, and also stem girth and shoot dry matter yield. In 1991, both ear and grain dry weight yield/m2 increased with higher N rates while in 1992 4–24 ml/l paclobutrazol enhanced ear dry yield/m2 compared with untreated maize.

Both chlorophyll a and b in the ear leaf increased with higher rates of paclobutrazol and N fertilizer, except in 1992 when high leaf N tended to mask the effect of paclobutrazol. N fertilizer in 1991 enhanced the contents of N and Ca in the ear leaf while depressing P, compared with untreated maize.

Six white clover genotypes that were easily distinguishable from each other on the basis of leaf lamina marks and morphology were grown at Lodi, Italy, during 1990 and 1991 in dense swards, under field conditions and a mowing regime, as (i) pure stands, (ii) a complex mixture of all genotypes, (iii) binary mixtures of each genotype with each of two ryegrass varieties, and (iv) complex mixtures of all clover genotypes in binary association with each grass variety. The grass components were of known, different vigour. The study assessed both intergenotypic and interspecific interference and related dry matter yield responses to morpho-physiological traits of the clovers, and also determined whether a high level of morpho-physiological heterogeneity conferred a yield advantage on clover populations.

Greater heterogeneity (i.e. a complex mixture of clover genotypes) did not produce higher clover yields either in the presence or absence of interspecific interference from grass; thus, the use of blends of varieties or the development of varieties with a fairly high degree of heterogeneity was not recommended for short-term meadows in environments with relatively low spatial and temporal variability. Interactions for yield occurred between clover genotypes and the presence or absence of intergenotypic interference (P<0·001), and between clover genotypes and the presence or absence of interspecific interference from the grass variety characterized by greater vigour and aggressiveness (P<0·01). The variance of the former interaction tended to be consistently larger than that of the latter interaction, indicating that competitive effects were greater between clovers than between the clover and grass components. A lower Spring [ratio ] Summer yield ratio and taller canopy tended to confer a competitive advantage under intergenotypic interference. Relatively better performance under interspecific interference was related to higher stolon density, suggesting that selection for this trait may increase the general ecological compatibility of large-leaved white clover types grown with vigorous grass companions.

Growth, shoot water relations and root hydraulic conductivity were studied in tomato plants (Lycopersicon esculentum cv. INCA9) subjected to different salt concentrations in the root medium. Two experiments were carried out at Instituto Nacional de Ciencias Agrícolas (INCA), Cuba, during May and June 1995. In the first experiment, plants were grown for 13 days in a nutrient solution with 0 or 100 mM NaCl. In the second experiment, the hydraulic conductivity was measured on roots submerged in nine different concentrations of NaCl up to 200 mM. The effect of temperature treatments between 0 and 50 °C on root hydraulic conductivity was also examined. Shoot growth, leaf water potential, leaf stomatal conductance, leaf relative water content and root hydraulic conductivity values decreased more rapidly in the treated plants than in control plants. A strong correlation was found between the root hydraulic conductivity and leaf water parameters, indicating that water flow through the roots was the main factor controlling shoot water relations.

In 74 experiments carried out in England from 1990 to 1994, cereal test crops were grown on sandy soils prone to nitrate leaching, following cereals, sugarbeet and potatoes. Measurements were made of the effects of the previous crops on soil mineral N, and on N recovery by the cereal test crop.

Soil N supply in autumn (SNSa) was greater following potatoes (c. 100 kg/ha N) than following sugarbeet or cereals (c. 60 kg/ha N). However when potato crops to which organic manures had been applied were excluded, mean SNSa after potatoes was only c. 60 kg/ha. Soil N supply in spring (SNSs) following sugarbeet and potatoes was similar and at c. 56 kg/ha, c. 10 kg/ha greater than following cereals. Seasonal differences in SNSs were related to excess winter rainfall and soil water-holding capacity. Modelled leaching losses gave good agreement with overwinter changes in SNS (r=0·87), although SNSs was usually greater than predicted by the model. This discrepancy was considered to be due to overwinter mineralization, which was estimated from the intercept of the regression lines to be c. 40 kg/ha following cereals and potatoes, and c. 50 kg/ha following sugarbeet.

Apparent net mineralization (AM) of N during the test crop growing season was c. 37, 53 and 63 kg/ha following cereals, sugarbeet and potatoes respectively. However, AM was c. 24 kg/ha N greater if the preceding potato crop had been given organic manures, but there was no difference in AM following manured and unmanured sugarbeet crops.

Nitrogen offtake by the cereal test crop without fertilizer N (NoffN0) was c. 15·20 kg/ha greater following potatoes and sugarbeet than following cereals. The greater N offtake following potatoes is considered to be due to mineralization of organic manures applied to the potato crop, while the effect following sugarbeet appears to be due to mineralization of sugarbeet residues. At harvest, SMN was c. 38 kg/ha and similar for all three previous crops. It is concluded that mineralization of sugarbeet residues has taken place more quickly on these sandy soils than previously reported on soils of greater clay content. Only potatoes grown with organic manures leave greater N residues than cereals.

Water balance components under wheat were assessed by employing two simple models, differing in their structure and data requirements, namely the soil-plant–atmosphere–water (SPAW) model of Saxton (1989) and the water balance model (WBM) of Arora et al. (1987). A few modifications based on the SPAW model procedure for the estimation of green canopy were used in a modified WBM and its performance was also tested. Soil water loss (the sum of interception evaporation, soil evaporation, plant transpiration and deep drainage) from sowing to harvest, simulated with the WBM, modified WBM and the SPAW model, had a close correspondence with the measured sum of profile water depletion, rainfall and irrigation for values ranging between 18·3 and 42·7 cm. Estimates of drainage with the WBM and modified WBM using empirical coefficients were greater than those calculated using the SPAW model for situations where the upward flow of water into the root-zone was negligible. Estimates of soil water evaporation using the WBM and modified WBM were invariably smaller than those using the SPAW model. A comparison of simulated and measured soil water storage and a correlation analysis of simulated transpiration with measured biomass at harvest showed that the performance of the WBM was the most realistic of the three models. However, it requires the input of leaf area index values to infer green canopy for each water supply regime. In the absence of this information, the modified WBM and SPAW models are more useful for assessing water balance components in cropped soils.

Individually-penned Matebele does were randomly allocated to three dietary treatments during gestation (0·66, 0·50 and 0·33 MJ ME/kg metabolic weight) and subsequently offered either 0·95 or 0·6 MJ ME/kg metabolic weight during lactation in Gwanda, south west of Zimbabwe, in 1990. The diets offered consisted of veld and lucerne hay and maize grain in proportions of 0·56, 0·24 and 0·2, respectively.

Reducing the amount of energy consumed from 0·58 to 0·26 MJ ME/kg metabolic weight in the last 7 weeks of gestation resulted in does suffering a net loss in body weight (−2 kg) and body condition score, although it had no significant effect on the birth weight of single kids.

Reducing feeding in the first 15 weeks of lactation from 0·95 to 0·59 MJ ME/kg metabolic body weight only had minor effects upon liveweight and body condition score change of the does and had no effect on the growth of kids. Amount of feed offered in lactation had a positive effect upon milk yield and there was a trend for yields to increase proportionally to pre-partum dietary intake. Diet had no effect upon milk composition. It was concluded that the benefits of supplementary feeding are small and may not justify the extra costs entailed in smallholder systems.

At Mount Derrimut Field Station from 1982 to 1985, growth and body composition was studied in three groups of wethers from the same flock. A control group (C) and a restricted group (R) were housed from the age of 1 day until slaughtered. The first slaughter was at the age of c. 1 year (reported elsewhere) and the second at the age of c. 3 years. Apart from the first 36 days, when group R was fed a severely restricted ration, both groups were fed a high quality diet ad libitum throughout the experiment. At the end of the second year, a group of 2-year-old wethers (G) was introduced, to compare the growth of these sheep with those in groups C and R. Group G sheep were reared on their dams, kept as grazing animals and killed at the second slaughter.

At the age of c. 3 years there was no significant difference between groups C and R for mean fleece-free body weights, but the sheep in group G were significantly lighter (P<0·01) in spite of reaching mature size for grazing sheep in the general flock.

After the age of 7 months there was no significant difference between groups C and R in rate of wool production. Rate of wool production in group G sheep was not significantly different from that of sheep in the other groups except during the last 6 months, when they produced significantly less wool (P<0·05).

Regression analyses for surface and X-ray measurements against fleece-free body weight (independent variable) showed no differences between groups C and R, except for width of metacarpal (X-ray measurement). In this case, the regression coefficients were different (P<0·01) but, at slaughter, actual bone measurements were the same. Both X-ray measurements and those at dissection showed that the metacarpals of group G were significantly (P<0·05) longer and narrower, and had a narrower cortex, than those in the other two groups (P<0·05).

There were no significant differences between groups C and R in mean weight and chemical composition of the parts and tissues examined; however, the carcass fat content of sheep in group G was significantly less (P<0·01) than in the other two groups. The sheep in group G had higher weights (P<0·01) for the brain, the digestive tract (DT), kidneys and liver than those in groups C and R.

Cell size in the semitendinosus muscle, as indicated by the protein[ratio ]DNA ratio, increased (P<0·01) between the first and second slaughters. There were no differences between groups in protein[ratio ]DNA ratios, but the DNA and protein contents were significantly higher (P<0·05) in the kidneys and liver in group G than in the other groups. Although the weights of the semitendinosus muscle were heavier (P<0·01) in groups C and R than in group G, on a fat-free basis, there were no significant differences between treatments in muscle weight or in DNA and protein contents.

It is concluded that sheep will make a complete recovery after a severe nutritional check in early post-natal life if given the opportunity to do so. Even though large differences in body weight may develop between sheep, this appears to have little effect on mature skeletal dimensions. However, at maturity, with grazing sheep that grow relatively slowly, the metacarpals are likely to be narrower and longer than those of the same genotype fed to grow faster. Even with sheep fed ad libitum throughout life, the use of X-ray and surface measurements in young sheep at a particular body weight are unlikely to be useful for the accurate prediction of mature body weight.

Experiments were carried out at Uhříněves, Czech Republic, in 1995 and 1996 to evaluate the effect of salinomycin, monensin, lasalocid and maduramicin on in vitro rabbit caecal fermentation. In cultures supplied with starch, hemicellulose and pectin, all ionophores in the range of concentrations tested (1–10 μg/ml) stimulated methanogenesis. Ionophores at 5 μg/ml increased the production of methane by 26–35% and decreased the production of acetate (P<0·05). Significantly more acetate was produced in mixotrophic acetogenic cultures of caecal micro-organisms with glucose (2 mg/ml) under hydrogen atmosphere than under nitrogen. The H2-dependent formation of acetate was blocked in cultures with ionophores. The stimulation of methanogenesis by salinomycin, monensin, lasalocid and maduramicin can thus be related to the inhibition of acetogenesis, which is an alternative method of H2 utilization in the rabbit caecum.