Following germination, the ontogeny of white clover is characterized by two distinct morphological growth phases, a seminal taprooted stage followed by a clonal growth stage. Death of the seminal taproot and primary stolon initiates a process of fragmentation of the taprooted plant into a variable number of independent clonal fragments (plants) which comprise the initial population of the clonal growth stage. The objective of this study was to characterize the plant morphology of field-sown white clover populations from germination through to established clonal populations. Populations of eight white clover cultivars were assessed when sown with perennial ryegrass or tall fescue in pastures established under a common grazing regime for 16 months prior to imposition of continuous or rotational grazing treatments. One year from sowing, taprooted plants attained maximum size, with a mean plant branching order of 3·35, stolon DW of 460 mg and lateral spread of 250 mm, with some individuals having 6th order branching, 3·5 g stolon DW and 1m lateral spread. These taprooted plants were 4–5 times the size of plants in the subsequently formed clonal population. Nine months after sowing, the first individual taprooted plants fragmented into clonal plants. Twelve months from sowing, fragmentation processes were occurring at a linear rate, eliminating 6% of the original taprooted population each month. This resulted in a 12–15 month transition period when the white clover population comprised both taprooted and clonal plants. During this transition period, the initial clonal fragments produced from taprooted plants were large, and this maintained a larger mean plant size in the clonal plant proportion of the transition population than measured in the later fully clonal population. This process was also considered to act to prevent the development of the expected differences between grazing managements, as it was not until the third year when all taprooted plants had disappeared that the clonal populations developed characteristics reflecting the expected influence of grazing management. Variation due to white clover cultivar and companion grasses was minor. The substantial differences in plant size and branching structure between taprooted and clonal populations has significant implications for the evaluation of breeding lines.

The performance potentials of six perennial ryegrass varieties for conservation or grazing use were compared between 1994 and 1996 in N. Ireland. They were maintained, either under constant lax defoliation (conservation) or constant severe defoliation (simulated grazing) management for 3 years, or annually alternated between these two managements with the changeover taking place either in autumn or at the first cut of the following year. Starting one set with lax defoliation in the first year and the other with severe defoliation created duplicates of these two alternating treatments. The results showed that variety yield differed depending on the season and the management imposed and there were differential responses to the various treatments. Conducting an alternating management system that implemented the management change in autumn provided a valid estimate of yield performance potential for both simulated grazing and conservation use, compared to constant management systems. Although significant differences in variety ranking between lax defoliation and severe defoliation management yields were only observed in the third year, the study showed that if the management change was implemented in autumn, the yield potential of varieties in the following year was not affected by the preceding year's management regime. Leaving a longer sward (6 cm) to over-winter increased the spring performance of the varieties in some years, compared to a shorter sward (3 cm), which may have implications for grazing management. There were also indications that imposing an alternating management system might alter the absolute magnitude of the sward density ratings relative to a constantly managed system, though variety rankings would remain unaffected.

It was concluded that the current alternating management testing system used to evaluate candidate varieties for UK National or regional recommended lists, does not cause any variety to be unfairly advantaged or disadvantaged.

Leaves of 12 accessions of Brachiaria sp. (three accessions from each of four species) were subjected to tests of shearing strength using a Warner Bratzler meat shear. Morphological measurements of these leaves were also taken in an attempt to standardize shearing strength measurements per unit of leaf tissue. The technique was evaluated for its capacity to detect differences between accessions of the same species. All samples were also analysed for dry matter (DM), organic matter (OM), neutral detergent fibre (NDF), acid detergent fibre (ADF), lignin, crude protein, silica and digestibility in order to relate measures of physical strength to chemical components. Results demonstrated the presence of a clear relationship between shearing strength and structural (cell wall) components. Correlations between shearing strength and NDF, ADF and lignin were 0·74, 0·82 and 0·80 respectively. The technique identified clear differences between accessions of the same species (P < 0·05). These measurements can thus be used as a powerful tool for identifying forage plants of superior nutritional quality within selection programmes. Measurements of strength standardized for morphological characteristics illustrated that the ranking of the samples varied considerably depending on which morphological character was used.

The effect of sulphur (S) application on the efficiency of nitrogen (N) use was investigated using cut plot experiments on two contrasting soil types. Nitrogen was applied at 200 and 450 kg N/ha per year, with and without 38 kg SO3/ha (15·2 kg S/ha) per cut. Over three conventionally timed silage cuts for 2 years, measurements were made of herbage dry matter, the yield of N and S in herbage and losses of N and S by leaching, and N by denitrification.

Herbage dry matter and N yields were significantly increased by the application of S at the high N level at the sandy loam site (Halse). At the clay loam site (Great Close) the application of S had no significant effect on herbage dry matter or N yields. At Halse, the pattern of response through the year was not the same in the 2 years studied, although in both, the effect of S was significant at third cut at high N. Deficiency was suggested by the N[ratio ]S ratio of herbage on the plots without S, especially at first cut, and at later cuts at Halse. Nitrate leaching was reduced by S at Halse by 72% and 58% with high N in 1997 and 1998, respectively, and by 10% and 5% on the low N treatments in 1997 and 1998, respectively. Application of S at high N at Halse reduced the peak concentration of nitrate-N in leachate from 27·3 mg N/l to 9·3 mg N/l. At Great Close, application of S had no significant effect on the amount or peak concentration of nitrate-N leached. The improvement in efficiency reported at Halse suggests that on permeable soils receiving high levels of N, the application of S could have a large effect on nitrate leaching and its associated environmental impact.

The effects of eight rates of nitrogen (N) application (0–245 kg/ha) on the performance of winter wheat over five seasons (1991–1995) on a long-term field experiment (established 1978) at Ropsley (UK) are described. In each of the five seasons, N was withdrawn from replicate plots in order to study the residual effect of fertilizer. N applications in excess of 140 kg/ha left significant residues as soil mineral N (SMN) in the autumn which, despite some loss over-winter, had a significant effect on the yield and N offtake of the subsequent crop. The amount of N carried over was equivalent to 8–20% of the fertilizer N and was observed at N applications up to 40 kg/ha lower than the optimum rate (c. 200 kg/ha). Part of the unrecovered N was also considered to contribute to the long-term build-up of fertility at the site. The results suggest that restrictions on N use to below the optimum will reduce leachable N, but may have an impact on soil fertility and future crop productivity. In addition, the rate of N applied to preceding crops should be taken into account when formulating fertilizer advice on retentive soils.

Three animal manures cross-labelled with 15N in either the urine, faeces or straw fractions were prepared. After a storage period of 86 days when the manures were exposed to either composting or to anaerobic storage, portions of the manures were incubated in six differently textured soils with clay contents ranging from 11 to 45%. Evolved CO2-C was determined during a 266 day incubation and inorganic N and 15N in soil were measured at the termination of the incubation. The mineralization of C was analysed using first-order kinetics, and two C pools with fast (P1) and slow (P2) turnover rates were estimated. The total conversion of added C (Ps) was estimated as Ps=P1+P2.

The cumulated CO2 production was considerably higher from soils incubated with anaerobically stored manure compared with soils amended with composted manure. CO2 production levelled off after c. 60 days in the three sandier soils whereas CO2 continued to be produced throughout the incubation from the three soils with the highest clay content. More C was assigned to the easily decomposable P1 pool in the sandiest soils whereas the more recalcitrant P2 pool was larger in the soils with higher clay content. Because of the different relationships between soil texture and C pools, Ps ended up being similar for five of the six soils. When taking C losses during the preceding storage into account, the accumulated C losses during storage and after incubation in soil accounted for 60 and 54% of C initially present in the composted and anaerobically stored manure, respectively.

Net N mineralization which averaged 16% of applied organic N took place in all soils amended with composted manure. Soils with anaerobically stored manure showed net immobilization after the 266 days of incubation. The amount of N immobilized accounted for up to 30% of the inorganic N applied with the manure. As anaerobically stored manure generally loses less inorganic N during storage, it may contain more inorganic N than composted manure at the time of field application. Because of the immobilization that takes place after application of anaerobically stored manure to soil, the immediate levels of plant available N in soil may not be as different from soil supplied with composted manure as could be expected from the inorganic N content in the two types of manure. However, when considering the manure as a N resource, anaerobic storage is superior to composting.

A model was developed to simulate tuber yield and water use of a potato crop by modifying and extending an existing water-use model. Empirical validation of the model was carried out with data from 37 crop-years of experiments on a wide range of cultivars at five sites within the UK (viz Silsoe, Gleadthorpe, Rothamsted, Woburn and Dundee). The study confirmed the validity of the model over a range of yields from 1·67 to 73·7 t/ha and under conditions from severely droughted to fully irrigated.

A series of experiments were carried out at 14 sites in the major arable areas of the UK, in the years 1993–94, 1994–95 and 1995–96 to determine the causes and extent of over-winter plant losses of two autumn-sown cultivars of white lupin (Lupinus albus L.).

Over the three seasons frost was the major cause of plant losses. Two mechanisms (one known and one not previously reported in the UK) of frost tolerance under field conditions were identified: (i) lignification of the root parenchyma early in the life of the seedling, and (ii) a large vernalization requirement of the main-stem apex, which delays stem elongation in older plants. A model was developed that could be used to predict the susceptibility of these lupin cultivars to the first severe frost of the winter using accumulated thermal time (above a base temperature of 3 °C).

The effect of sowing at the beginning or end of a sowing window, calculated to optimize plant architecture the following summer, therefore varied with the weather during the autumn/winter period. A combination of cool autumn weather and late sowing (outside of the sowing window) resulted in plant losses due to a lack of lignification of the root parenchyma. In unusually warm autumn conditions (1994–95 and 1995–96) plants sown at the beginning of the sowing window were well developed before the first frosts occurred, with consequent stem elongation and plant losses.

Although losses due to pests were not severe (c. 3 to 5 plants/m2), plants were attacked during the autumn by grey field slugs (Deroceras reticulatum Muller), bean seed flies (Delia platura Meigen), and probably thrips (Thrips angusticeps Uzel, Noctuidae), although this has not been confirmed. The fungal pathogens Fusarium spp. and Botrytis cinerea Pers. caused losses in conjunction with frost damage.

A split-plot experiment was conducted in 1992 and 1993 at Mokwa in the southern Guinea savanna zone of Nigeria to determine the responses of maize yield components to infestation by parasitic weed Striga hermonthica (Del.) Benth. and identify the most important yield components under Striga parasitism. Infestation treatments (infested v. non-infested) and hybrids [9022-13 (highly tolerant), 8644-31 (moderately tolerant), 8321-21 (moderately susceptible) and 8338-1 (highly susceptible)] were the main and subplots, respectively.

Striga significantly reduced grain yield and all the yield components considered. Pre-flowering stress due to Striga parasitism was higher than post-flowering stress and resulted in higher reduction for ears per plant (44%) than reductions for other yield components (12–29%). The numbers of yield components for which the hybrids exhibited the highest percentage reduction increased with the degree of susceptibility to Striga. Thus, 8338-1, the highly susceptible hybrid, suffered more severe reductions in a larger number of yield components than the other hybrids. Number of ears per plant was a major yield component and accounted for an overwhelming proportion (R2 = 0·95) of the variation in grain yield under Striga. Reductions in maize grain yield and its components were more severe under the moisture stress that occurred in 1992.

Experiments were conducted during 1992/93 and 1993/94 to determine the effect of different planting dates and dinocap applications on control of powdery mildew and yields of early and late maturing coriander cultivars. Early plantings resulted in lower overall disease severity values for both the cultivars, but the late maturing cultivar S-33 benefited more than RD-44, an early maturing cultivar. Early planting resulted in smaller increases in yield in RD-44 but provided considerable economic gains in S-33. Further, it was possible to achieve similar levels of control of powdery mildew and yields in coriander with fewer dinocap applications by early plantings of both the cultivars. Therefore, to reduce the fungicide inputs, it is important that emphasis should be placed on early plantings for managing powdery mildew especially in environments conducive for its development.

A key need of the global change research community is to be able to synthesize the data collected by scientists from a wide range of disciplines and use it to make predictions about our future environment. In an agricultural and forestry context, this will allow predictions of yields under elevated atmospheric CO2 levels and/or changed climate using data from studies of both the direct effects of global change on plant growth and physiology and indirect effects on soil fertility, water resources, pests and diseases.

A conference held at the University of Reading, UK in September 1999, and organized under the auspices of Focus 3 (Global Change Impact on Agriculture, Forestry and Soils; http://mwnta.nmw.ac.uk/GCTEFocus3) of the IGBP Core Project “Global Change and Terrestrial Ecosystems” addressed many of these issues.

Thirty-two pregnant ewes (Saudi breeds) selected during 1998 and 1999, were studied throughout the whole gestational length and around parturition. Concentrations (mean±S.E.) of plasma progesterone (3·9±0·28 ng/ml), oestradiol-17β (20·5±2·33 pg/ml) and cortisol (4·8±0·58 ng/ml) remained fairly constant throughout the early stages of pregnancy. Characteristic changes in these steroids were evident during the prepartal period involving a steady decrease in plasma progesterone beginning 8 days before parturition; a remarkably consistent increase in oestradiol-17β 4–6 days prior to delivery, peaking in excess of 250 pg/ml at the birth day; and a fivefold increase to 23·7±2·12 ng/ml in plasma cortisol 2 days preceding delivery, showing a large increase during the day of lambing. The timing of the marked reduction in the platelet count and the coagulation screening tests including prothrombin time (PT), activated partial thromboplastin time (APTT) and thrombin time (TT), during the prepartal period, were significantly correlated with the temporal changes in the circulating steroid hormone concentrations. The results suggest that the characteristic prepartal hormonal changes might provide the basis for shifting the fairly balanced coagulable state throughout gestational periods to a mild to moderate degree of hypercoagulability during parturition which is needed to limit the possible intra- and post-partum uterine haemorrhage.