Eight single, six three-way and six double cross hybrids were evaluated with a standard control to assess their yield potential and agronomic performance. The study was conducted in the field and greenhouse using a randomized complete block design with three replications at the University of the Free State during 2002/03. The three categories of hybrids performed differently and showed high variation for the majority of characters studied. Three-way cross hybrids showed better performance for days to flowering (70·9 days after sowing), fruit diameter (3·2 cm) and fruit weight (30·3 g/fruit). As expected, single cross hybrids were the most uniform followed by three-way cross hybrids. Three-way cross hybrids showed 36·1 and 13·6% heterosis for fruit yield and fruit length, respectively. Higher mean heterosis for fruit yield (35·6%), fruit number (24·0%) and fruit weight (16·9%) was also observed in double cross hybrid. This study shows that three-way and double cross hybrids can be used in pepper hybrid breeding. Selection of inbred lines for crossing from a similar market group can decrease heterogeneity in these categories of hybrids.

The study reported examines the spectral specific responses of adult insect pests of mushroom (Agaricus bisporus (Lange) Imbach) cultivation to light traps and the potential for use of light trapping as a pest management tool within commercial mushroom production. Lycoriella ingenua (Dufour) (Diptera: Sciaridae) and Megaselia halterata (Wood) (Diptera: Phoridae) were the principal insect pests at the experimental site. Adult L. ingenua and M. halterata were positively phototactic and relatively high numbers were attracted to relatively low light emissions from light trap sources of differing wavelengths. Yellow sticky traps at a height of 1·7 m collected more flies of both species than traps located at crop height (0·6 m). Adult female L. ingenua were more numerous than males in all trap types suggesting that they were more active fliers than male counterparts. Numbers of females recorded in light traps greatly exceeded those in yellow sticky traps, indicating greater photo-responsiveness. Light wavelengths between 300–650 nm attracted greater numbers of adult L. ingenua than wavelengths exceeding 700 nm. This species was present throughout the year but was most abundant between May and December. Adult M. halterata were less numerous than adult L. ingenua in all trap types and particularly on yellow sticky traps at crop level. The lower disparity between numbers of adult M. halterata on light traps and yellow sticky traps suggests that adults of this species are not as photosensitive as adult L. ingenua. The observed response of adult M. halterata to light sources of different wavelengths was variable within the range 300–>700 nm. Adult M. halterata were most abundant between June and November.

Seeds of oilseed rape (Brassica napus L.) can persist in the soil over several years by becoming secondarily dormant and can then germinate to create volunteer plants in following crops. As well as agricultural impacts caused by volunteers, gene dispersal in time – particularly from genetically modified cultivars – can be another undesirable consequence. Conventionally bred and transgenic seeds were tested in 2001 and 2002 in laboratory experiments, and in a field experiment, by burying seeds in the soil to determine the variation in dormancy and persistence capacity.

In the conventional group of cultivars tested in the laboratory, the level of dormancy was 13–76% in 2001, and 3–76% with an extended group in 2002. The transgenic group of cultivars was 1–31% dormant. In the burial experiments the number of viable seeds recovered in the conventionally bred cultivars ranged from 34–90% in 2001, and 7–68% in 2002. In the same studies the transgenic cultivars developed persistence levels from 12–79% in 2001, and 46–67% in 2002. Since dormancy levels of conventionally bred cultivars from 2 harvest years in the laboratory tests correlated significantly (r=0·71), it appears that there is a genetic background to secondary dormancy. There was also a significant correlation (r=0·61 in 2001 and 0·80 in 2002) between the results from laboratory and burial experiments. This indicates that the laboratory approach can simulate the situation in the field. Ageing over 6 months decreased the capacity for seed persistence to about a fifth of the level shown when freshly harvested. As a consequence of ageing and environmental impacts on persistence, only seeds from the same location and harvest year should be used for testing genetic variability. The high genetic variability among currently available rape seed cultivars gives breeding strategies a good chance of ideotyping low persistence genotypes and minimizing the risk of gene dispersal in time.

Knowledge of nitrogen accumulation to the grains is of great importance for identifying breeding and growing strategies to improve cereal yield and quality. The aim of this study was to investigate post-anthesis N translocation in the spike and into different spikelets of two cultivars of spring wheat (Triticum aestivum L.) grown to maturity in solution culture. Sport (short grain filling time, low yield and high grain protein concentration) and WL (normal grain filling time, high yield and low protein concentration) were the cultivars used. Plants received stepwise declining rates of N on a daily basis, until 109 days after planting (DAP). Anthesis for cv. Sport was 92 DAP and for cv. WL 78 DAP. On seven occasions (from anthesis in cv. WL until maturity), 15N was given to selected groups of plants for 4 days. Then these plants were sampled and separated into plant parts. Spikelets and grains were separated according to spikelet position. Plant parts were weighed and analysed for N and 15N.

The N concentration in the grains was uniform for the spikelets in cv. WL but differed in cv. Sport. Final N concentration in a spikelet position was obtained at 126 DAP in cv. Sport and 112 DAP in cv. WL and was constant to the end of experiment at 154 DAP in cv. Sport and 147 DAP in cv. WL. A positive correlation between % atom excess 15N to spikelet position numbered from the base of the spike was exhibited in the lowermost part of the spike just after anthesis. Later, during grain filling, a negative correlation between % atom excess 15N to spikelet position was exhibited (−0·991 P=0·000). A positive correlation between % atom excess 15N to grain weight in the top of the spike was exhibited. Accumulation of 15N proceeded 14 days longer in the lowermost fertile spikelets than in the uppermost spikelets. It was concluded that the duration of N accumulation in the spikelets was dependent on spikelet position and uppermost spikelets had a shorter N accumulation period and contained lighter grains than lowermost spikelets.

Under rainfed Mediterranean conditions the dominant crops are cereals and legumes, with the most common crops being barley (Hordeum vulgare) and vetch (Vicia sativa). These two species are grown in rotation, where the cycle length depends on the productivity and the value of the products in the market. In order to study the productivity of cereals and legumes growing in various rotation combinations, an experiment was conducted during the 1982–2000 period, at Athalassa. The experimental site had a calcareous soil and is located in the central plane of Cyprus. The average rainfall is 250 mm per year. The rotation treatments tested were: (a) continuous barley, (b) continuous vetch, (c) vetch–barley, (d) vetch–barley–barley, (e) vetch–vetch–barley–barley and (f) vetch–vetch–vetch–barley–barley. Barley plots were equally divided to accommodate two rates of N fertilizer (0 and 60 kg N/ha) which were applied at seeding. Production of both vetch and barley was lower in monocultures than when the two species were grown in rotation. Vetch yield was similar in all rotations, irrespective of the position in the cycle. Productivity of barley was increased by nitrogen application only when grown as a second barley after vetch. However, in years with low productivity due to the rainfall amount and distribution, barley did not respond to nitrogen application. The first crop of barley after vetch gave maximum production without any nitrogen fertilizer application. Nitrogen fertilizer had a decreasing effect on the size and weight of barley grains. These results enable the farmers to select any of the rotation systems studied, based on the economics of the two species, without risking productivity. Nitrogen fertilizer should be applied only to the second crop after vetch in years with high rainfall.

The study reported presents the findings relating to commercial growing of genetically-modified Bt cotton in South Africa by a large sample of smallholder farmers over three seasons (1998/99, 1999/2000, 2000/01) following adoption. The analysis presents constructs and compares groupwise differences for key variables in Bt v. non-Bt technology and uses regressions to further analyse the production and profit impacts of Bt adoption. Analysis of the distribution of benefits between farmers due to the technology is also presented. In parallel with these socio-economic measures, the toxic loads being presented to the environment following the introduction of Bt cotton are monitored in terms of insecticide active ingredient (ai) and the Biocide Index. The latter adjusts ai to allow for differing persistence and toxicity of insecticides.

Results show substantial and significant financial benefits to smallholder cotton growers of adopting Bt cotton over three seasons in terms of increased yields, lower insecticide spray costs and higher gross margins. This includes one particularly wet, poor growing season. In addition, those with the smaller holdings appeared to benefit proportionately more from the technology (in terms of higher gross margins) than those with larger holdings. Analysis using the Gini-coefficient suggests that the Bt technology has helped to reduce inequality amongst smallholder cotton growers in Makhathini compared to what may have been the position if they had grown conventional cotton. However, while Bt growers applied lower amounts of insecticide and had lower Biocide Indices (per ha) than growers of non-Bt cotton, some of this advantage was due to a reduction in non-bollworm insecticide. Indeed, the Biocide Index for all farmers in the population actually increased with the introduction of Bt cotton.

The results indicate the complexity of such studies on the socio-economic and environmental impacts of GM varieties in the developing world.

Six S5 lines of maize, previously identified as having contrasting responses to nitrogen (N) supply, were used to carry out a complete series of diallel crosses. The resulting 15 hybrids were grown in a field at two N levels. The general combining ability (GCA) and specific combining ability (SCA) were estimated using the method 4, model I analysis of Griffing for grain yield kernel weight, grain number, harvest index, green leaf number, ear leaf area, N utilization efficiency, total plant N, grain N content, anthesis-silking interval, chlorophyll content and prolificacy. For the majority of the traits, GCA was more important than SCA and there was an increase of non-additive effects under low N. Significant interactions were observed between GCA and N levels for grain yield, grain number and chlorophyll content, indicating the selection of different lines for each N level. The lines with the largest effects of N supply per se on grain yield, were those with the largest effects of GCA for this trait. This association indicates that the response of the lines to N supply, should be considered in breeding programmes in order to achieve acceptable hybrids for environments with both high and low N.

‘Mal de Río Cuarto’ (MRC) disease, caused by a member of the family Reoviridae belonging to the genus Fijivirus, is considered to be the most damaging viral disease of maize (Zea mays L.) in Argentina. Resistance to MRC disease is a quantitative trait with moderate heritability ranging from 0·44 to 0·56. The objective of this study was to identify simple sequence repeats (SSR) loci linked to quantitative trait loci (QTL) contributing to MRC disease resistance. Two hundred and twenty-seven F3 derived-lines from a cross between a susceptible inbred line, Mo17, and a partially resistant inbred line, BLS14, were evaluated across four Río Cuarto environments. A disease severity index (DSI) based on disease grades was calculated and used to rate F3 derived-lines for their resistance to MRC disease. A subset of parental F2 plants belonging to susceptible and resistant F3 derived-lines from field assessments was assayed for 180 SSR primer pairs to map resistance genes. Fifty-six maize SSR were employed for the testing of linkage among DNA markers and the mapping of QTL through composite interval mapping. Resistance to MRC disease was affected by two QTL on chromosomes 1 and 8 which showed overdominance and dominant gene action, respectively. A simultaneous fit with these QTL in the joint analyses explained 36·2% of the phenotypic variance. In spite of the fact that relative efficiency of marker-assisted selection (MAS) in comparison to phenotypic selection was close to 1, the mapped QTL could improve the efficiency of efforts in breeding for resistance to MRC disease.

Selection for specific adaptation in a breeding programme may be required when significant genotype×environment (G×E) interaction occurs within a target production region. Knowledge of the key stress factors underlying the G×E interaction permits delineating a production region into homogeneous zones for the purpose of recommending specific varieties. Twenty-eight cowpea genotypes were evaluated in 2002 and 2003 at four locations in the Guinea and Sudan savannah zones of Ghana, to assess grain yield stability and its relationship with intensity of stress in each production environment. Data were analysed in a year-location cross-classification, producing eight environments. Significant variations were observed between the genotypes for grain yield and grain yield components, biomass production, days to maturity, and resistance to Striga, pod-sucking bugs, and bacterial blight severity. Across environments, stress intensity (in terms of grain yield) was negatively correlated with the genotypic and error variances but positively correlated with the phenotypic, genotypic and experimental coefficients of variation. Stress intensity was also negatively correlated with heritability for grain yield. Genotypic correlations for grain yield between locations were moderate to high. The b-value as a measure of yield stability showed positive correlations with mean yield over all environments, and also with average yield in each of the three most favourable locations. In the least favourable location, the b-value showed a weak inverse relationship (P=0·10) with grain yield. Although direct selection for grain yield was considered superior in the target environments, simultaneous selection for grain yield and tolerance to stress in the less favourable environments will identify genotypes broadly adapted to the major cowpea growing regions.

The Japanese rice cultivar, Akenohoshi, has numerous spikelets in a panicle (extra-heavy panicle type) and achieves a large yield sink capacity. However, this cultivar, as well as other extra-heavy panicle types, does not always produce higher yields because of poor grain filling of the spikelets on the secondary branches in a panicle. To determine whether the poor grain filling found in Akenohoshi was due to source-limited or sink-limited conditions, the present study examined the responses of grain-filling characteristics to several spikelet-removal treatments immediately after heading. Only when the spikelets on secondary branches remained was a significant increase in filled grain percentage in this spikelet position observed. This increase seemed to be associated with the increases in rate and duration of the grain-filling process and in single grain weight in this spikelet position. These results clearly indicate that the poor grain filling in the spikelet on secondary branches of Akenohoshi could mainly be attributed not to sink-limited conditions, but to source-limited conditions probably at specific stages of grain filling.

Canopy development, radiation absorption and its utilization for yield was studied in four grain legume species Cicer arietinum, Lens culinaris, Lupinus angustifolius and Pisum sativum. The grain legumes were grown at different plant populations and sowing depths over two seasons in Canterbury, New Zealand. The green area index (GAI), intercepted radiation, radiation use efficiency (RUE) and total intercepted photosynthetically active radiation (PAR) increased significantly (P<0·001) with increased plant population. Narrow-leafed lupin produced the highest maximum biomass (878 and 972 g/m2, averaged over all populations during 1998/99 and 1999/2000, respectively) and intercepted more radiation (600 and 714 MJ/m2, averaged over all populations during 1998/99 and 1999/2000, respectively) than the other three legumes. In all four species, in both trials, the highest plant populations reached their peak GAI about 7–10 days earlier than legumes sown at low populations. Cumulative intercepted PAR was strongly associated with seed yield and crop harvest index (CHI).

The RUE increased (from 1·10 to 1·46 and from 1·04 to 1·34 g/MJ during 1998/99 and 1999/2000, respectively) as plant population increased and was highest in the highest yielding species (e.g. 146 and 1·36 g/MJ for narrow-leafed lupin in both experiments). The larger leaf canopies produced at the higher plant populations reduced the extinction coefficient (k).

The results suggest that in the subhumid temperate environment of Canterbury, grain legume species should be selected for the development of a large GAI. This should maximize PAR interception, DM production and, consequently, seed yield.

North Carolina Design I (NCD I) progenies derived from Pool 16 DT, a white, dent, early maturing, drought-tolerant and streak resistant tropical maize (Zea mays L.) population, were evaluated in the 1995/96 and 1997/98 dry seasons, using two sites each season in Côte d'Ivoire. In all sites, the crop was irrigated from planting to about 2 weeks before anthesis; irrigation was discontinued thereafter for the rest of the season in one site (Soumis) in 1995 and both sites (Ferkessédougou and Sinematialli) in 1997. Irrigation was continued until maturity in the second site (Temoin) in 1995. In 1995, the additive component of genetic variance was much larger than the other components. The narrow-sense heritability estimate (h2) was 73% for grain yield and 43–71% for seven other traits. In 1997, only grain moisture at harvest, ear height, and days to anthesis and silking had positive additive genetic variances but the h2 were much lower (1·5–41%) than in 1995. Ears/plant, plant height and ear height had consistent positive correlation with grain yield; correlation of days to anthesis and silking date with yield were negative. Because of negative variances, additive genetic correlation could not be computed for most traits in 1997, but in 1995 grain moisture at harvest, plant height and ear height showed positive additive genetic correlation with yield. Other additive genetic correlation coefficients were low and/or not consistent. There is sufficient genetic variance to warrant continued selection for drought tolerance in Pool 16 DT. The induced stress, however, appears to be too severe to properly elicit the true differences among families and may, therefore, need to be modified.

Assessment of the stability of micronutrients is important in breeding for the enhanced nutritional quality of staple food crops as a means to alleviate malnutrition. Twenty early-maturing elite tropical maize (Zea mays L.) genotypes were evaluated over 2 years at three locations representing three distinct agroecologies in West and Central Africa (WCA). The objectives were to analyse the pattern of genotype×environment interactions (GEI) and environmental stability of iron and zinc concentrations in grain using the Additive Main Effects and Multiplicative Interaction (AMMI) statistical model. Results indicated that the effects of genotypes, environments and GEI were significant (P<0·05) for both micronutrients. The effect of GEI was about double the contribution of the genotypes for grain iron and more than double the effect of genotypes for grain zinc. Partitioning of GEI indicated that variety×location was the dominant source of a significant amount of GEI for both micronutrients. Scores of the first two interaction principal component axes (IPCA1 and IPCA2) from the AMMI were significant and accounted for 0·68–0·75 of the pattern of GEI for both micronutrients. About half of the genotypes evaluated were stable for grain iron and zinc concentration over the set of environments. The AMMI model identified ACR98TZEMSR-W as the most stable genotype for grain iron and MAKA-SRBC5 was the most stable for grain zinc. However, the yellow genotype, AK94-DMR-ESR-Y was the most promising, with high and moderately stable concentrations of iron and zinc in the grain. Because it is yellow, with beta-carotene content and high concentrations of iron and zinc in the grain, it might significantly contribute to an improved intake of these micronutrients in populations who rely on maize for a major portion of their daily diet.

Growth environment and plant nutrition are two important factors influencing growth, yield and quality of aromatic rice (Oryza sativa L.). The present study was conducted at Kalyani, India to determine the effect of two planting dates and four fertilizer levels on different aromatic rice cultivars during the dry seasons of 1995/96 and 1996/97, while nine cultivars were evaluated during the wet seasons of 1996 and 1997. Thermal and photoperiodic conditions significantly influenced the vegetative (leaf area index and light extinction co-efficient) and reproductive (filled spikelets/panicle) growth of the crop. Delayed planting (23 February) significantly reduced the grain yield by 0·88 t/ha, amylose content by 0·5% and duration by 10 days; but increased the summed heliothermal units (17806 v. 18505). Thus, the cultivars became less efficient (27%) in heat use with delay in planting from 2 to 23 February. Relative availability of NH4+-N from urea and Azolla influenced the crop growth (leaf area index [LAI], tiller production and leaf chlorophyll content) and nutrient uptake. Supply of inorganic N either alone or in conjunction with Azolla significantly increased grain yield (18–41%) and protein content (0·1–0·7%) over 15 t/ha of Azolla alone. However, combined application of Azolla and urea lowered the amylose content below that achieved by application of either substance alone. Correlation studies among quality attributes indicated that long-grained varieties had lower head rice recovery (r=−0·69) due to more breakage during milling and greater test weight (r=0·93).

Grass pea (Lathyrussativus) is a potentially valuable feed and food crop in semi-arid regions. Much work has been done on lowering toxicity and on selection of low toxicity varieties, while research on the eco-physiological characteristics of grass pea is very rare. Stomatal character, photosynthetic character and seed chemical composition were measured in four varieties of L. sativus to investigate their relationships at different water availabilities. For L. sativus cv. Yongshou (YS), L. sativus cv. Dingxi (DX), L. sativus cv. Heilongjiang (HLJ) and L. sativus cv. Xide (XD), stomatal density was in the order XD>HLJ>DX>YS under both control and drought conditions. Stomatal aperture, photosynthetic rate (Pn), transpiration rate (E), and the concentrations of seed β-N-oxalyl-L-α, β-diaminopropionic acid (ODAP), protein and starch were in the order YS>HLJ>DX>XD, while the opposite order was found for water use efficiency (WUE). Under drought conditions, stomatal aperture, Pn and E were lower than those under the control, while the other parameters were higher. A significant positive correlation was observed between stomatal density and WUE, while negative correlations were found between stomatal density and the remaining parameters. Obvious positive correlations were also observed between stomatal aperture and Pn, E, the concentrations of seed ODAP, protein and starch, while a negative correlation appeared between stomatal aperture and WUE. Under drought conditions, R2 values were more comparable with the control. Intriguingly, the R2 values of stomatal aperture were higher than of stomatal density, especially under drought conditions. These results indicate that stomatal aperture may be more closely related to photosynthetic character and seed chemical composition in grass pea, and water deficit may enhance the correlations.

Experiments to evaluate the optimum sowing date for okra (Abelmoschus esculentus) sown in monoculture or in mixed stands with cassava (Manihot esculenta) were sited on free-draining sandy loam soils in southwestern Nigeria. Okra was sown at the end of July, in mid-August and early September as a late-season crop in 1999 and at the end of May, in early June and at the end of June as an early-season crop in 2000. It was sown at seed rates sufficient to achieve final population densities of 33000 and 40000 plants/ha in late-season crops (1999) and early-season crops (2000), respectively. The late-season crops had shorter growth duration, received less rainfall, and experienced cooler temperatures during establishment and the early vegetative stage, and warmer temperatures during the reproductive phase than the early-season crops. Intercropping had no significant effect on the growth and tuber yields of cassava, or on phenology (i.e. time to vegetative growth, flowering and fruiting) and pod yield of okra in both seasons. However, it reduced weed growth by 35–57%, and kept both the soil and canopy environments of cassava cooler by 2·3–5·8 °C and more moist by 15–30 g/kg, compared with monoculture. The phenology and pod yields of the early- and late-season okra in both cropping systems were dependent on sowing date, indicating that okra production is only suitable at particular sowing dates in both seasons. July-sown okra in the 1999 late-season and May-sown crop in the 2000 early-season took progressively the longest time (i.e. 3–10 and 2–5 days, respectively) to flower and fruit, but these crops controlled weeds and modified the cassava environment better than the rest, and gave the highest fresh pod yields and economic returns. It took okra pods longer to reach marketable size in the late season than early season (i.e. 5–9 v. 2–6 days). It is concluded that the optimal sowing date to attain maximum pod yield and economic returns from late-season okra is July or August and from early-season crop is May or early June. Bearing in mind financial constraints and production costs, the optimal season target for maximum edible pods is the early season and for maximum economic returns is the late season.

A methodology was developed for large area yield forecast using a crop simulation model and a discrete technology trend, and was applied to the coherent wheat yield variability zones of Eastern Uttar Pradesh, India. The approach consisted of three major steps: (a) prediction of technology trend yield using historical yield series of the region; (b) prediction of weather-induced deviation in wheat yield using CERES-Wheat simulation model and relating weather-induced deviation in simulated yield to deviation in observed yield deviations from technology trend; and (c) final yield forecast by incorporating predicted yield deviation in trend predicted yield. The regression coefficients for step (b) were generated using 10 years' data (1984/85–1994/95) and the reliability of the approach was tested on a data set of 5 years' independent data (1995/96–1999/2000). The results showed that this approach could capture year-to-year variability in large area wheat yield with reasonable accuracy. The Root Mean Square Error (RMSE) between observed and predicted yield was reported as 0·098 t/ha for the mean yield of 2·072 t/ha (4·72%). However, the RMSE was slightly higher in the forecasting period in comparison to the calibration period. The use of this methodology for issuing the pre-harvest forecast and the effect of upgrading the technology trend were also studied. The pre-harvest forecasts were made using in-season weather data up to the end of February and climatic-normal for the rest of the wheat-growing season, which showed good agreement with observed wheat yields. The forecasts of wheat yield for the season 1999/2000 were made using the technology trend up to 1994/95 and the updated technology trend up to 1998/99, which showed that the RMSE fell in the latter case, from 4·10 to 2·50%.

An algorithm to fit boundary lines, using cubic smoothing splines, was written and used to identify yield responses to changes in soil properties. This method involves fitting a curve that represents the maximum yield response to each predictor value, which represents the yield potential at each soil property value. Boundary-line yield responses to individual soil properties were found to differ from responses found by fitting curves through the data scatter. The effects of correlated variables appeared to be lessened using the boundary line approach. Multivariate boundary-line models, based on the Law of the Minimum, were found to be useful for the identification of site-specific causes of yield variation and yield potentials. The boundary line was found to be a useful complement to more traditional data analysis techniques.

The partitioning of above ground nitrogen (N) in chickpea (Cicer arietinum), lentil (Lens culinaris), lupin (Lupinus angustifolius) and pea (Pisum sativum) at crop maturity was investigated. The plants were grown at different plant populations and sowing depths in 1998/99 and 1999/2000 in Canterbury, New Zealand. In all four legumes the N concentration was highest in seed (29–36 mg/g N) followed by senescent leaves. The lowest N concentration was in stems. Lupin had the highest seed N yield at 16·82 and 19·29 g/m2 followed by chickpea at 10·26 and 13·10 g/m2, in 1998/99 and 1999/2000, respectively. Lentil had the lowest N concentrations and yield. The distribution of N to all legume plant parts increased as population increased up to twice the optimum and 400 plants/m2 in 1998/99 and 1999/2000, respectively. Over all plant populations, the nitrogen harvest index (NHI) was stable in each of the four species across the two seasons. However, the NHI changed with changes in plant population and sowing depth. The N accumulation efficiency (EN) was highest in lentil at 0·024 and 0·027 g N/g DM and lowest in chickpea at 0·018 and 0·021 g N/g DM in 1998/99 and 1999/2000, respectively. The NHI and the crop harvest index (CHI) were correlated and both were strongly associated with seed yield. Thus, NHI might be useful as a selection criterion to improve seed yield in grain legumes.

Crop rotation serves as a mechanism for developing sustainable crop production systems. Crop-rotation trials are used to identify agronomic input factors suitable for use in a cropping system. In crop-rotation trials, experimental errors within the same plot over time are correlated. The form of the covariance structure of the plot errors may be specific to the data from a rotation trial, but is unknown and is generally assumed. Statistical analyses are usually based on the assumption that plot errors are independent, or have constant covariance. An experiment was conducted using wheat-based, three-course rotations containing tillage treatment subplots over 12 years at ICARDA's experimental station at Tel Hadya, a moderately dry area in northern Syria. This study examined several covariance structures for temporal errors arising over the rotation plots and tillage subplots, in order to model wheat yield data. Eighteen covariance structures were examined, and the best pair was selected using the Akaike Information Criterion. The best pair comprised first-order autocorrelation and homogeneous variance for temporal errors in rotation plots, and uniform correlation with heterogeneous variances for temporal errors in tillage subplots. Using the 12 years of data obtained for wheat yield and the best pair of covariance structures, the tillage and rotation effects were found to be statistically significant and to have significant interactions with the cycle of rotation. The precision of the means calculated differed from those calculated using a control structure based on homogeneous error variances and constant correlation. The cumulative yield build-up over time differed significantly over the rotations and the tillage methods. An increasing yield trend was observed for the bread wheat rotation, while a yield decline was observed in durum wheat when the rotation was repeated. When evaluating the effects of input factors in crop rotations, we therefore recommend that the covariance structures be examined and that a suitably chosen structure be used.