Starting with the F2 generation of a single-cross hybrid, a maize population was improved from cycle 0 (= F2) to cycle 4 by mass honeycomb selection and from cycle 5 to cycle 8 by pedigree honeycomb selection. Seven half-sib families were derived from cycle 8. The grain yield of these, their corresponding S1 lines, testcrosses with B73 and diallel crosses were studied. The results showed (i) that the seven half-sib families contained half the percentage of the load of deleterious genes present in the F2 generation, (ii) that four of them outyielded their corresponding single-cross hybrids, (iii) that they could constitute an open-pollinated variety, which could be used as a source population and (iv) that they revealed different patterns for general and specific combining ability. Honeycomb selection for grain yield was successful in favouring the existing additive genetic variation in the derived families.

Field experiments at Askov and Lundgaard Experimental Stations, Denmark, were carried out in 1987–91. Winter and spring applications of pig slurry at two application rates and an unfertilized treatment were combined with three rates of mineral nitrogen fertilizer; 15 treatment in all. Grain and straw yield, nitrogen content and nitrogen uptake of spring barley were measured at harvest. All main effects were significant, resulting in increasing yield and nitrogen uptake both when increasing the rate of mineral nitrogen fertilizer and slurry, and when slurry was applied in spring rather than in winter. The nitrogen content of the barley was affected by mineral nitrogen fertilizer application, and by the rate of slurry. The nitrogen efficiency of applied ammonia-N in the pig slurry was estimated by a quadratic polynomial yield response function and expressed relative to a reference function based on the application of mineral nitrogen fertilizer. The efficiency was estimated at 10 for spring-applied pig slurry. The value for winter-applied pig slurry was lower (0·6) and more variable. The efficiency of ammonia-N in spring-applied pig slurry decreased by 0·4/100 kg N/ha when supplementary mineral nitrogen fertilizer was applied at the same time.

The use of environmentally induced genetic male sterile (EGMS) rice could alter the development of hybrid rice from a three-line system to a two-line system. It is critical for the utilization of EGMS rice to determine which are the main environmental factors influencing fertility changes. Fertility responses to photoperiod (P) and temperature (T) were studied in 101 EGMS rice lines under nine controlled regimes combining three photoperiods (15·0, 14·0 and 12·5 h)x three temperatures (30·1, 24·1 and 23·1 °C). According to the variance analysis of seed-setting data, 96% of the total EGMS lines studied could be divided into three types as follows: (1) photoperiod-sensitive genetic male sterility (PGMS) characterized statistically by significant (P < 0·05) P and P × T interaction effects but by a non-significant T effect on fertility, (2) thermosensitive genetic male sterility (TGMS) by a significant T effect, a non-significant P effect and by either a significant or a non-significant P × T interaction effect on fertility, and (3) photo-thermosensitive genetic male sterility (P-TGMS) by only a significant P × T interaction effect on fertility. Among the japonica EGMS lines studied, PGMS, TGMS and P-TGMS accounted for 32·3, 9·7 and 51·6%, respectively. However, among the indica EGMS lines, no PGMS lines were detected and most of them were TGMS or P-TGMS (61·4 and 35·7%, respectively). The results indicate that the selection of indica PGMS lines of rice might be very difficult. The availability of different types of EGMS rice in two-line system hybrid rice is evaluated and the selection of an ideal model of response to photoperiod and temperature for indica EGMS is discussed.

Soil surface moisture is a dominant factor influencing the establishment of surface sown seed, but its measurement is difficult. A cobalt chloride (CoCl2) saturated paper strip (20×5 mm) technique was developed as a cheap but sensitive indicator of soil surface moisture. The influence of soil surface moisture on the seedling survival of three oversown legume species, subterranean clover (Trifolium subterraneum L. cv. Karridale), strawberry clover (T.fragiferum L. cv. Grasslands Onward) and Caucasian clover (T. ambiguum Bieb. cv. Monaro) was investigated in a glasshouse and a field experiment at Palmerston North, New Zealand, between 1 October and 30 November 1993. Intact sods were sprayed with glyphosate, placed in plastic trays (420×300×50 mm) and transferred to the glasshouse or field. Three soil surface moisture treatments were imposed in the glasshouse. In the field trial, the plastic trays were buried flush with the soil surface in contact with the subsoil and exposed to natural wind and rainfall. Bare seed was oversown in a 20 × 20 mm grid and then pushed into the soil with a roller studded with metal rods to simulate treading by sheep. The low soil surface moisture treatment and the field trial had the lowest seedling survival. The main cause for this was low surface moisture caused by wind, which hindered radicle entry into the soil. Subterranean clover was less susceptible to low surface moisture and had better net seedling survival in all the treatments than the other two legume species.

The effects of varying the sowing date (early September-late October) and plant density (14–70 seeds/m2) on the establishment, overwinter survival, structure and yield of an autumn-sown, florallydeterminate line (CH304/70) of the white lupin (Lupinus albus) were examined in three contrasting growing seasons between 1991 and 1994. Crops established well when sown in early September and were sufficiently cold-hardy to survive prolonged and extremely severe early winter frosts, but crops sown in late October either lost many plants or were destroyed completely. There was a strong interaction between sowing date and autumn weather on crop structure and yield. Late sowing and cold autumn weather restricted the number of mainstem leaves and first-order lateral branches on the plant, and decreased plant height and yield potential.

Despite considerable differences between years in the weather during the summer and autumn, all crops were harvested in early September. Grain yields ranged from 0·3 to 4·5 t/ha depending on season, sowing date and plant density. Yields were strongly correlated with the number of podbearing axes and pods per m2 and, although actual yields differed depending on growing conditions, the same number of pod-bearing axes (100/m2) was required in each year to achieve maximum yield. The effects of sowing date and autumn weather on plant structure were well predicted by a simple developmental model that related vernalization and leaf development to post-sowing temperature.

The transmission of solar radiation within the crop canopies of oilseed rape is decreased both by the plant's yellow petals during flowering, and by the horizontal posture of the pods once they are formed. The significance of each of these morphologies was assessed, in the 1991/92 and 1992/93 growing seasons, by comparing the performances of an apetalous breeding line (N-o-112) and one with erectophile pods (N-5-130) with two conventional commercial genotypes (Falcon and Tapidor).

The apetalous floral layer of line N-o-112 reflected and absorbed significantly less radiation than those of the conventional, petalled genotypes. This resulted in a greater transmission of photosynthetically active radiation (PAR) to the leaf and bract canopy situated below. At peak flowering, 70% more PAR was transmitted through the apetalous floral layer.

The erectophile pods of line N-5-130 were angled 20–25° further from the horizontal than the conventional genotypes. The extinction coefficient (K) of its pod layer during late pod growth ranged from 0·35 to 0·44 as compared to 0·45 to 0·55 in the commercial varieties. Line N-5-130 carried a greater proportion of its total fertile pod number in the lower regions of the pod canopy compared to the two conventional genotypes. It also had a greater overall mean number of seeds per pod, and the number decreased proportionately less with depth in the canopy. The line was thus better able to maintain a higher photosynthetic area and to sustain pod and seed growth at the lower levels of the crop canopy than the conventional, commercial varieties. The potential physiological effects of apetalous flowers and erectophile pods are considered to be sufficiently beneficial for their introgression into near-isogenic lines to be pursued.

Traditionally, only one groundnut (Arachis hypogaea) crop is grown in the uplands during the rainy season (May-October) in Sierra Leone, with sowing starting at the beginning of the rains. However, a recent trend among groundnut farmers is to sow a second crop in September after harvesting the first crop. Thus a system comprising a first (major) and a second (minor) cropping season is being adopted, with the second season crop having to depend largely on moisture stored in the soil. Experiments were conducted in 1993 at the Institute of Agricultural Research Experimental Farm, Njala, to assess the agronomic potential of growing two crops a year. Crop phenology, growth and productivity were strongly influenced by cropping season. Pod yield ranged from 1·63 to 2·58 and from 1·16 to 1·95 t ha-1 in the major season, declining to 0·72–1·29 and 0·57—1·07 t ha-1 in the minor season. Haulm yield, number of mature pods, crop growth rate and partitioning were lower in the minor season by 58, 57, 40 and 13%, respectively. However, kernel quality was higher in the minor season. The results suggest that the growing of two groundnut crops in the uplands is a potentially viable management option in Sierra Leone, and could represent a more efficient utilization of the growing period.

The effect of foliar applications of different formulations of manganese on pea yield and ‘marsh spot’, a seed defect which reduces quality of peas (Pisum sativum) grown for human consumption or seed was evaluated in nine experiments in the East of England from 1992 to 1994. Sprays, at dose rates recommended by the manufacturer, were applied to field peas, cvs Maro or Bunting, on three occasions at the four node pea growth stage, first pod and 14 days later, or on two occasions at first pod stage and 14 days later. Although the sites selected had a previous history of marsh spot in pea crops, there were no visual foliar symptoms of manganese deficiency in peas grown in any year at any site. Applications of manganese did not increase yields compared with untreated peas and there was no effect on maturity. Marsh spot was negligible in untreated peas at sites where soils had the lowest levels of manganese (22 and 44 mg/kg), thus soil analysis for manganese may be inappropriate as a guide to prediction of marsh spot problems. The three spray programmes, which included an early application of manganese at the 4–5 node stage, gave no statistically significant improvements in marsh spot control. There was a good correlation between amounts of manganese applied and reductions in marsh spot, and manganese sulphate, 31% w/w at 3·1 kg/ha applied as a split dose, achieved the best control. An exception was the manganese carbonate 50% w/v formulation which may have been poorly assimilated by the plant. Chelated manganese as MnEDTA 6·4% w/v at the rates recommended by the manufacturer was inadequate for control of marsh spot in all years.

Agronomic practices can be modified to decrease autumn soil nitrate and nitrate leaching. This experiment aimed to measure the effectiveness of such practices when integrated into a farming system under UK conditions. The experiment started in autumn 1988 on a sandy soil in Nottinghamshire, UK, and comprised a four-course rotation of potatoes–cereal–sugarbeet–cereal. Three husbandry systems were superimposed, ranging from current commercial practice to most nitrate retentive. Plots were split further to receive either half or full recommended rates of nitrogen (N) fertilizer. Soil mineral N (Nmin) and nitrate leaching (using porous ceramic cups) were measured on selected treatments; this paper presents the findings after five winters.

Autumn Nmin and N leached were strongly influenced by the previous crop, consistently following the order potatoes > cereal > sugarbeet. Pre-harvest management (chiefly N fertilizer input) affected Nmin, and post-harvest management also modified N loss. Cover crops (winter rye or forage rape) after cereals removed 10–40 kg/ha N, depending on previous N management, time and method of establishment. They decreased leaching and were particularly effective if they were able to establish fully before significant drainage occurred. Nmin following sugarbeet, which had received 125 kg/ha N, was less after November lifting than after October lifting (16 and 28 kg/ha N, respectively, as amean of autumns 1989–92). Potatoes left most Nmin (a mean of 60 kg/ha for autumns 1989–92, receiving 220 kg/ha fertilizer N), and their late harvest gave little scope for decreasing leaching losses by establishing green cover before the start of winter. After late harvested root crops (both beet and potatoes), it was often preferable to leave the land fallow over winter, rather than ploughing and drilling a winter cereal.

We show that nitrate leaching can be decreased by simple and inexpensive modifications to an existing crop rotation. Averaged over five winters, adopting such practices decreased the mean N concentration in drainage from 22·3 to 14·5 mg/1.

A dynamic model of digestion and absorption of nutrients in cattle fed sugarcane-based diets is described. There are 11 rumen state variables, and four zero pools representing absorbed nutrients. The rumen state variables represent nitrogen, carbohydrate, long chain fatty acid, microbial and volatile fatty acid pools. The zero pools relate to absorbed amino acids, glucose, long chain fatty acids, and volatile fatty acids. The flux equations are described by mass-action and Michaelis-Menten forms. Wherever possible, data derived from trials with cattle fed sugarcane-based diets were used to parameterize the model. Sensitivity analyses revealed that stability and behaviour of the model was generally satisfactory. The model was most sensitive to changes in fractional passage and substratehydrolysis rates and to the microbial maintenance requirement. Within the limited information available for comparison, the simulations agreed with observations of fibre flows and ammonia and volatile fatty acid concentrations in rumen fluid. Duodenal non-ammonia nitrogen flow was consistently underpredicted and reasons for this underprediction are suggested.

A previously described model of digestion in cattle fed sugarcane diets was applied to predict nutrient supply to the host animal from dietary intake, to indicate pre-experimentally the suitability of various supplements to enhance the milk production of dairy cattle in the tropics fed sugarcane–based diets. Potential milk production was calculated according to simulated energy and lipogenic, glucogenic and aminogenic nutrient availability. The addition of small amounts of urea (10 g/kg sugarcane fresh weight) improved microbial efficiency and degradation of substrates in the rumen, but further increases could only be achieved by supplements containing rumen degradable protein. The simulated level of absorbed amino acids was low in comparison with the level of absorbed energy and glucogenic substrates. This low amino acid availability was the factor most limiting milk production on sugarcane diets and supplements which increase the amino acid availability in the gastrointestinal tract are recommended. At increasing supplementation or intake levels, and once amino acid requirements were met, an increased absorption of energy and long chain fatty acids could enhance milk production. These suggestions should be evaluated experimentally in order to improve the model and ultimately, to result in an improved system to evaluate feeds, based on the supply and utilization of individual substrates.

The effect of sward structure and species composition on dietary quality in cattle and sheep grazing a mesic, low quality grassland (sourveld) in South Africa was investigated over the 1990/91 and 1991/92 grazing seasons. Species composition had no measurable effect on dietary quality in either cattle or sheep but this variable could affect quality over grazing periods longer than those used in the study. In contrast, sward structure, and in particular sward greenness, had a major effect on dietary quality in both animals. Dietary in vitro digestibility (IVD) was positively correlated (P < 0·001) with sward greenness but negatively correlated (P < 0·05) with herbage mass and sward height. Dietary IVD in sheep was consistently higher (P < 0·001) then in cattle but both species reacted in a similar manner to changes in sward structure and season. Dietary quality declined over the grazing season, with IVD being highest in spring (> 60%) and lowest in winter (< 45%). Marked intra-seasonal fluctuations in quality also occurred in response to sward structure and apparent variation in soil moisture. Fire also appeared to have a significant effect on quality, with IVD being higher (P < 0·001) in the first year following the burn than in the subsequent year. It is suggested that on sourveld, dietary quality in the growing season is determined by a combination of environmental and management factors. Conversely, in the dormant season, dietary quality is largely determined by environmental factors beyond the control of the grazier.

The effects of species composition and sward structure on the ingestive behaviour of cattle and sheep grazing a mesic, low quality grassland in South Africa were investigated over the 1990–93 grazing seasons. Species composition had a significant (P < 0·05) effect on sheep bite size and on cattle and sheep bite rates but had no effect on dry matter intake rate (IR). Species composition could, however, affect IR over longer grazing periods than those used in the experiment.

Sward structure had a major effect on ingestive behaviour. Cattle and sheep bite rates and cattle grazing time, were negatively correlated (P < 0·05) with plant height but positively correlated (P < 0·05) with sward greenness. Bite size and hence IR in cattle and sheep were strongly correlated (P < 0·001) with plant height. Cattle IR increased from 6 to 20g/min over the range of heights encountered and appeared to reach an asymptote at a plant height of 20–25 cm. Sheep IR, expressed per unit of body mass, increased from 0·01 to an asymptote or maximum of 0·13 g/min/kg at plant heights of 10–15 cm. For sheep there was evidence of a non-asymptotic functional response at some sites with IR being maximized at certain sward heights but declining thereafter. This suggests the possible existence of a third, quality dimension to the functional response on these low-quality grasslands.