Field experiments comparing 13 winter wheat varieties representing very old, old, intermediate and modern groups were carried out over three seasons. The experimental sites were on soils of high fertility and weeds, diseases and pests were controlled by applications of proprietary agrochemicals. Lodging was prevented by supporting the plants with coarse-mesh netting.

The average yield of all varieties over the three harvest years 1984, 1985 and 1986 was 7·7 t/ha (at 15% moisture content). Compared with the very old varieties which were grown by farmers during the 19th century, the modern varieties gave 59% more grain, had 14% more ears/m2 and 30% more grains per ear, but a similar mean grain mass. The modern varieties yielded slightly more biomass (total above-ground dry matter at harvest), were much shorter and reached anthesis about 6 days earlier than the older ones. In 1984, when the yields were greater than in 1985 and 1986, the yield advantage of the modern varieties was more in percentage and absolute terms than in the other years. Also, in 1984, the differences between the very old and the modern varieties in biomass was the greatest.

The genetic gain in yield measured in these experiments is compared with those estimated for other countries and reasons for the variation are discussed.

In an experiment, carried out over 3 years, two groups of dairy cows offered ad libitum access to grass silage received 8 kg/day concentrate differing only in phosphorus content (low P concentrate: 4·0–4·5 g/kg dry matter (D.M.); high P concentrate: 6·0–6·5 g/kg D.M.) during the winter feeding period of approximately 6 months. During the summer period all animals were grazed together on perennial ryegrass pastures. There were no consistent significant effects of P content on any of the variables measured. The mean calving indices were 371 (S.E. = 3·71, n = 122) and 379 (S.E. = 4·28, n = 95) days and overall conception rates were 0·70 and 0·68 for the low and high P treatments, respectively. Mean plasma P concentrations during the winter period were significantly reduced (P > 0·01 or greater) in years 2 and 3 of the study on the low P treatment. In 1 year milk yield was significantly higher (P > 0·01) on the low P treatment although the fat content was significantly reduced (P > 0·05) but these effects were not recorded in the other 2 years.

The fate of abomasal and intravenous doses of [35S]cystine and [35S]methionine was investigated in sheep to determine their relative efficacy as supplements for wool growth.

Administration of L-[35S]cystine and L-, DL- and D-[35S]methionine via the abomasum produced similar patterns of 35S labelling in blood plasma. Only 10–16% of the administered 35S was excreted in urine over 7 days and 22–32% of the dose was incorporated into wool over 21 days. All isomers were utilized by the sheep with similar efficiency and it was concluded that D- and L-methionine and L-cystine are equally effective as supplements for wool growth at optimal dose rates. Abomasal supplementation with a large amount of methionine (10 g/day) resulted in the excretion of a high proportion of the 35S in urine (55% of the dose over 7 days) and low incorporation into wool.

A comparison of the extent of incorporation of 35S into wool from intravenous doses of L-[35S]cystine and L-[35S]methionine in the same sheep, and in sheep from flocks selected for high and low wool production, showed that incorporation from methionine averaged 80% of that from cystine. It was concluded that, at moderate (2–3 g/day) methionine availability, methionine is utilized for synthesis of wool with about 80% of the efficiency of cystine, conversion of methionine to cysteine occurring via the transulphuration pathway.

Measurements of skeletal size were made at 2–3-month intervals on 30 Border Leicester × Merino castrate male (wether) sheep between 2 and 27 months of age. Fifteen sheep were fed ad libitum on a high-quality diet and the other 15 half the average amount consumed by the first group, age for age. The ad libitum group grew faster and were larger in all body dimensions on each occasion, except for leg length at 27 months which showed no statistical difference between groups. When the groups were compared over the live-weight range common to both (16–44 kg) the unrationed animals were consistently wider at the shoulders but smaller in leg and chest dimensions.

The relationship between each body component and age is described by a Mitscherlich equation and the relationship with live weight by a linear equation in which both variables are log transformed. Separate relationships were determined for each sheep and tested for differences within and between groups.

Field plot experiments were carried out for 3 years at four sites to study the effect of date of application of ammonium nitrate/calcium carbonate (CAN) and urea (U) on perennial ryegrass production in spring. Fertilizer (70 kg N/ha) was applied at weekly intervals for 10 weeks from 1 February. Herbage was cut on the same day at all sites, 3–4 weeks after the last N application. CAN and U at 50 kg N/ha were immediately re-applied and a second cut of herbage was taken after 4–5 weeks. From meteorological data, the dates after 1 February when soil temperature at 100 mm depth increased to 5·5 °C and the dates when cumulative average daily air temperatures < 0 °C from 1 January reached 200 °C were calculated for each site and year.

The date of application for maximum dry matter (D.M.) yield at the first cut differed with site and year, but for 11 of the 12 site/years was in February. The number of dates of application resulting in optimum yield (i.e. at least 90% of the average maximum yield response to CAN or U) varied also with site and year from one to six. Soil and air temperature predictive systems implied a precision in choice of application date that was unjustified and were no more successful at predicting the optimum application date than a simple date range. The first fertilizer application had a pronounced positive residual effect on D.M. yields at the second cut. Less precision on date of the first application was required to obtain optimum cumulative yields over both cuts than optimum yield at the first cut only.

Differences in performance between CAN and U were only significant for three of the 120 fertilizer applications at the first cut. On these occasions, all in one year at two sites, U gave higher yields than CAN. Correlations were sought between D.M. yield response and growth period, air temperature, long-term rainfall and short-term rainfall for CAN and U separately. Factors relating to rainfall had no significant effect on response to U but response to CAN showed a significant negative correlation with short-term rainfall. The short-term weather forecast may therefore be another criterion to be considered in deciding when to apply N in early spring.

Maize was grown on the same Clay-with-Flints site at Rothamsted for 14 years (1974–87) in an experiment which tested cumulatively all combinations of three rates of N fertilizer (50, 100 and 150 kg N/ha) and six agrochemical treatments (aldicarb, benomyl, dazomet, phorate, benomyl + dazomet + phorate and none). Permethrin and pirimicarb were also tested in 3 of the years. The crop was grown for forage from 1975 to 1987 and for grain from 1974 to 1976. With 150 kg N/ha, the average forage yield was 10·5 t dry matter/ha and there was no overall decline during the experiment; annual yields ranged from 5·9 to 14·5 t/ha. From 1975 to 1983, yields were on average 0·2 t/ha less with 100 kg N/ha and 1·0 t/ha less with only 50 kg N/ha than with 150 kg N/ha. The crop grown for grain in 1974 failed but in 1975 it gave 3·5, 3·6 and 3·8 t/ha with 50, 100 and 150 kg N/ha, respectively, and 3·3 t/ha in 1976 with all N rates. The cultivars grown, Cargill Primeur 170, Caldera 535 and Fronica, matured too late to ensure a grain harvest at Rothamsted.

Pests observed on the crop included nematodes of the genera Pratylenchus, Tylenchorhynchus and Tylenchus/Ditylenchus and initially some Heterodera avenae, the aphids Metopolophium dirhodum and Sitobion avenae, and frit fly (Oscinella frit). Common smut (Ustilago maydis) was the only fungus disease recorded and Microdochium bolleyi, possibly a minor pathogen, occurred on roots. There was no overall increase in the incidence of pests and pathogens during the experiment. Agrochemical treatments increased yields in all years; the most effective was the combined treatment with benomyl + dazomet + phorate which gave on average 1·4 t/ha more forage than the untreated crop with the largest N rate.

Forage yields were generally larger with sowing in early May rather than later and maturity was earlier in years with the most accumulated day-degrees.

Statistically sound methods for analysing the results of germination tests were used to quantify and assess differences between seed lots of perennial ryegrass (Lolium perenne L.) subjected to accelerated ageing. Ageing altered not only the proportion of seed not germinating but also the mean germination time and the synchrony of germination. Seed lots differed in these three parameters before and after ageing. The responses to ageing of mean germination time and synchrony were not regular and a simulation of trie possible effects of ageing on germination suggested this irregularity might be due to changes in the proportion of viable seeds and to a non-normal distribution of seed ages.

In field experiments in 1983–85 in Northumberland, UK, early sowings achieved a leaf area index (LAI) of 3·5, capable of intercepting 90–95% total incident solar radiation, earlier than late sowings. As there was a close relationship between total dry weight, bud dry weight and amount of intercepted solar radiation, early sowings invariably outyielded later ones. The efficiency of energy conversion of radiation was 1·28, 2·05 and 2·11 g/MJ for total dry weight and 0·97, 0·83 and 0·67 g/MJ for bud dry weight in 1983, 1984 and 1985, respectively. Harvest index ranged from ca. 25% in 1985 to 40% in 1984.

Increasing plant density from 2·22 to 6·66 plants/m2 advanced and increased maximum LAI and total and bud dry weight per m2 but had an adverse effect on distribution of dry matter. Maximum total dry weights were achieved at or slightly after maximum LAI. The onset of rapid bud growth coincided with maximum total standing dry weight and was advanced by early sowing but largely unaffected by plant density.

Early-sown crops produced more buds than late-sown ones because of a longer growing season. Plant density had a large effect on the number of buds per m2, which was almost directly proportional as the number of buds per plant was not severely affected. However, individual bud size was restricted as a result of competition for assimilates. Approximately 80% of buds finally recorded had been produced before significant bud growth had occurred.

Total bud fresh yields averaged over all sowing dates reached 17 t/ha in 1983 and 31 t/ha in 1984. The lower yield in 1983 was the result of late sowing caused by unfavourable weather. Early sowings significantly outyielded late ones because of earlier onset of rapid bud growth which gave a longer growing period. The effect of plant density on total sprout yield was less pronounced than that of sowing date but effects on yield per plant were large.

Yields of buds in the freezing grade (20–30 mm) increased rapidly between late September and early to mid-November in both 1983 and 1984 and reached 7·5 and 8·8 t/ha, respectively. The difference between freezing-grade yields in the two years (1·3 t/ha) was much less than the difference between total yields (14 t/ha). Late sowing in 1983 restricted bud growth resulting in a higher proportion in the freezing grade. Plant density had a greater effect on freezing-grade yield than on total yield. Low planting densities gave high yields of small buds at early harvests but denser planting gave higher yields at later harvests. Generally, increases in bud fresh weight over the harvest period were greater than those in bud dry weight because of water uptake. The average dry matter content of buds declined by 2–5 % from October to January.

The experiments confirmed that manipulation of sowing date and planting density is an effective way of spreading harvest date throughout the season in order to achieve an orderly sequence of crops for the fresh market and for processing.

The mineralogy of the clay fraction (<2 μm) of nine soil samples representing three physiographic positions of a toposequence in Ochon, south-eastern Nigeria, was determined by transmission electron microscopy and X-ray diffraction.

Soils in the crest and middle slopes (pedons TE1 and TE2, respectively) of the toposequence, formed over fine-grained sandstone, were coarse-textured, well-drained and acidic and had low effective cation exchange capacity and percentage base saturation. Kaolinite, mica, quartz, goethite and gibbsite were the principal minerals identified in the clay fraction of these soils. Soils in the valley bottom (pedon TE3) formed over arenaceous shale were, on the other hand, fine-textured, poorly drained and weakly acidic with higher cation exchange capacity and percentage base saturation. Their clay fraction mineralogy was similar to that of the well-drained upland soils but additionally included feldspar and smectite/vermiculite mixed layer minerals.

Mixed cropping of yam, cassava, maize and other annuals under traditional methods was carried out on the drier, well-drained crest and upper slopes while sole cropping of lowland rice occurred in the poorly drained, more fertile valley-bottom site. The valley-bottom soils show greater potential for improvement in agricultural productivity under good management than the upland soils.

Yields from lucerne cultivars grown on land infested with Verticillium albo-atrum, a cause of wilt disease, were studied over the period 1969–77. Field experiments established in 1969, 1971, 1972 and 1973 were maintained for periods of 4–6 years. Reactions by cvs Europe and Vertus were of principal interest. Europe is considered to be moderately susceptible, initially producing yields exceeding those of resistant types on uninfested land. In comparison cv. Vertus has considerable resistance derived from a polygenic system. In the first 2 cropping years on infested land, Europe yielded similarly to Vertus with only slight symptom expression. In year 3, Europe maintained comparable yields to Vertus but with increasing disease expression. During succeeding years Europe was badly affected by discase and yields were below those achieved by Vertus. Data from other cultivars indicated a positive correlation between symptoms in years 4 and 5, a negative correlation between dry-matter yield in year 5 and symptom severity in year 4, and a negative correlation between symptom severity and yield in year 5. Symptom expression and pathogen build-up appear to occur at similar rates in the latter years of a lucerne crop. Disease development in one season has a considerable effect on future productivity. These findings are discussed in terms of the primary (extra-vascular) and secondary (intra-vascular) determinants of resistance to wilt pathogens. It is suggested that the apparent moderate susceptibility of cv. Europe in seasons two and three relates to a rapid growth rate.

Rates of intake of herbage and grazing time of beef cattle are essential components of simulation models of grassland agroecosystems. We studied the effects of herbage allowance on rates of intake and ingestive behaviour of twelve 2-year-old Angus heifers (Bos taurus)(364 ± 12 kg) grazing pastures of tall fescue (Festuca arundinacea Schreb.). A balanced change-over design and a novel tethering technique were used to estimate direct, residual and permanent effects of three herbage allowances on intake per bite, rate of biting and rate of intake. Herbage dry matter (D.M.) allowances, which were established by varying tether lengths, were 5, 9 and 13 kg (1·4, 2·5 and 3·6 kg/100 kg of live weight) and made available in circular plots of 15, 27 and 38 m2, respectively, for a single measured 2 h grazing session each day. Heifers grazing Kenhy tall fescue swards, composed of vegetative tillers and free from the endophyte Acremonium coenophialum, with herbage D.M. masses (> 5 cm) of 2000 kg/ha and D.M. allowances of 9 and 13 kg/2 h per heifer, ingested D.M. at 1·9 kg/h by taking bites averaging 800 mg D.M. at 38 bites/min. Allowances of 5 kg/2 h per heifer slowed the rate of intake to 1·3 kg/h by limiting D.M. intake per bite to 654 mg and biting rate to 35/min. Rate of D.M. intake of cattle grazing vegetative temperate grass swards appears to be ca. 0.5% of live weight per hour when allowance and availability of herbage are not limiting.

An experiment in which all combinations of four rates of N fertilizer soon after sowing, four in early spring and nine in late spring were applied to the same plots of winter wheat for 6 years is described and the effects discussed.

It was found that although the yield varied considerably from year to year the regime leading to greatest yield was fairly consistent. If the farmer had used the regime which gave the greatest average yield (0, 90, 180 kg N/ha in autumn, early spring and late spring, respectively) he would have averaged only 0·12 t/ha less than the maximum attainable each year.

Application of N increased protein concentration in the grain more or less linearly but the latest application increased it most. Since the regime which gave greatest yield on average produced grain which qualified for bread-making premium from the protein point of view in all years except 1981, adopting the regime which gave maximum yield would reduce the potential maximum gross margin by only £30/ha.

Analysis of the soil showed a reduction in pH down to 15 cm and of available P2O5 down to 10 cm.

In field experiments, infections by okra mosaic virus (OMV) and okra leaf curl virus (OLCV) reduced trie growth of okra more than when the plants were infected singly by either virus. The critical growth component responsible for low performance was leaf area in OLCV-infected plants and chlorophyll content in OMV-infected plants. However, the two viruses had a synergistic effect in mixed infection.

Expressed as percentage yield loss, mixed infections significantly reduced edible fresh fruit weight more than single infections in all okra cultivars. In single infections, OLCV caused higher yield loss than OMV. Since both viruses cause more damage in mixed infections, it may be preferable to breed for combined resistance, particularly if the genes for resistance are linked.

Two field experiments and one pot experiment were carried out to test the hypothesis that competition and dominance relations between and within inflorescences control seed yield. The experiments compared untreated field bean (Vicia faba L.) plants with others in which the numbers of inflorescences and flowers had been surgically reduced to 18 defined positions at lower or at upper nodes. Growth conditions were varied by sowing date and plant density. Seed yields, yield components and 14C-labelled assimilate distribution were determined.

Most untreated plants set no more than 18 pods while treated ones set 4–7 pods fewer. As variation in seed set per pod was small, the resulting number of seeds remained about 30% less in treated compared with untreated plants. In plants with pods set only at upper nodes, 14C-assimilate distribution shifted in favour of leaf and root production, presumably supporting seed production later on. Concerning seed yield per plant, full compensation was reached as mean weights per seed increased up to 40%. In interactions with growth conditions, small overcompensation effects on seed yield per plant were indicated. Results are discussed in relation to storage capacity and yield stability.

Measurements of the area of individual leaves in crops are useful in the analysis of canopy architecture as they allow determination of the structure of leaf area index in a vertical profile. This information may be of use in modelling leaf growth and the assessment of photosynthetic potential of different strata of the canopy with ontogeny (cf. Firman & Allen, 1988).

Atomic Absorption Spectroscopy (AAS) is the usual method for the direct determination of small amounts of manganese in plant materials (Christian & Feldman 1970; Heanes, 1981) and other substances (Carnrick, Slavin & Manning, 1981). However, it uses the highly sophisticated and expensive Atomic Absorption Spectrometer, which requires a reasonable degree of technician expertise, and is therefore not suitable for routine chemical analyses in Third World countries.