A joint model for plot yield in response to fertility trends and interplot competition is described. The model combines the mixed model representation of a cubic smoothing spline to model fertility and a regression model with auto-regressive terms to model competition. Estimation is based on a generalization of residual maximum likelihood. The methods were applied to a series of 70 sugar beet trials conducted by the Plant Breeding Institute, Cambridge, UK, and the results summarized.
Twenty-two field experiments in England, done between 1986 and 2000, tested the effects of phosphorus (P) fertilizers on number of tubers and tuber yield in Solanum tuberosum. Applying P fertilizer resulted in statistically significant increases in tuber yield in six experiments and the optimal P application rate ranged from c. 90 to 180 kg P/ha. Statistically significant increases in yield in response to application of P fertilizers were found only in soils that contained 26 mg/l. The absence of yield responses on P Index 3 soils found in the current experiments was attributed to increased use of irrigation that may have increased the availability of soil P. Reinterpretation of data from long-term experiments showed that the agronomic benefits of increasing soil P status by applying more P than is removed by harvested crop parts, are small. Since large P residues, estimated by Olsen-P or degree of soil P saturation, are associated with desorption of P and consequent loss to drainage water it is inadvisable to increase soil P above Index 3. For these reasons, no P fertilizer is recommendod for Index 4 soils, an amount equivalent to replacement is recommended for Index 3 soils but up to 110-130 kg P/ha should be applied to Index 0 soils. Applications of foliar P had no effect on number of tubers or tuber yield and this practice cannot be recommended.
Incomplete block (IB) analysis of lentil yield trials in lattice block designs substantially reduced experimental error variability compared to randomized complete block (RCB) analysis. Spatial variability, which may exist in two dimensions in the field, can be modelled using various alternative covariance structures for the plot errors. To investigate the adequacy of the incomplete block analysis, we fitted a first order autocorrelation error structure (AR1) in both the column direction and the row direction after allowing for the variance model of the lattice design. We also considered random splines in columns. The best model was selected on the basis of the residual deviance of each of the 53 trials we examined. Gains in efficiency (over RCB) for pair-wise comparison of genotypes and selection gains were obtained for the selected models and for the lattice blocks (considered as a control model). Spatial models where the plot error was modelled as AR1 in columns or as AR1 x AR1 in rows and columns after allowing for random effects of lattice blocks were most frequently selected. Models with spatial errors were found best in 74% of the trials when used with random effects of lattice block and in the remaining trials when used without lattice blocks. The average gain in efficiency over RCB analysis by using the best models at the analysis stage was around 50%. The best models were also, on average, more efficient than the lattice model. Expected average genetic gain due to selection of the top five lines was approximately 20% for the best models. The predicted genotype means showed less change in rank when comparing RCB with lattice analysis than when comparing RCB with the best method. Use of spatial models resulted in different genotypes being selected, giving a higher genetic advance. Since the use of spatial models requires only a change in computation together with knowledge of the field layout, the use of spatial methods together with good experimental design is recommended as a cost-effective method for achieving improved genetic progress.
Rose production is limited by salinity and highly affected by the nitrogen source present in the nutrient solution. The influence of sodium on several aspects of nutrition has been investigated in 'Lambada' rose plants using different sources of nitrogen in the fertilization treatment. Experiments using a previously defined mono-shoot model plant and a simplified hydroponic culture allowed us to study the effects of salinity v. nitrogen on NPK uptake during the culture period. Mineral concentrations, nitrate reductase (NR) and glutamine synthetase (GS) activities were also analysed. This study showed that rose plants were more sensitive to saline conditions under NH4+ fertilization without detectable effects on growth or in NPK mineral contents in shoots. Parameters affected most were enzymatic activities analysed such as leaf nitrate reductase activity which was reduced under NH4+ nutrition. Leaf glutamine synthetase was also enhanced by saline conditions. The Na/K ratio showed that under NH4+ nutrition, the highest sodium accumulation occurred in roots. Nitrate uptake did not show a clear pattern related to nitrogen source, however, ammonium uptake was affected by salinity when NH4+ was the sole nitrogen source in the nutrient solution. Potassium and phosphate uptake were always lower when NH4+ was present in the nutrient solution.
The study investigated the effects of lamb genotype produced from hill flocks on feed efficiency, carcass characteristics and meat quality in animals finished on either a high forage: concentrate ratio diet (HFC) or a low forage: concentrate ratio diet (LFC). Purebred Scottish Blackface (BXB) lambs were compared with Blue-Faced Leicester x Scottish Blackface (BLXB) and Texel x Scottish Blackface (TXB) lambs. Purebred Cheviot (CXC), Suffolk x Cheviot (SXC) and Texel x Cheviot (TXC) lambs were also investigated. Lambs on the LFC diet were offered grass silage and concentrates mixed in the proportion of 0.80 silage and 0.20 concentrates on a dry matter basis. The LFC diet consisted of 0.20 grass silage and 0.80 concentrates on a dry matter basis. Representative lambs were slaughtered at the start of the experiment and on reaching 38 and 46 kg live weight. Averaged over the two slaughter weights, within the Blackface cross lambs, BXB had a lower liveweight gain (P < 0.01) than either BLXB or TXB (138,207 and 203 (S.E. 11.1) g/day, respectively). Within the Cheviot cross lambs, CXC had a lower liveweight gain (P < 0.05) than SXC (188 v. 220 (S.E. 11.1) g/day), while SXC and TXC (204 g/day) had similar liveweight gains. Lambs on the LFC diet had a higher dressing proportion (P < 0.001) compared with lambs finished on the HFC diet (0.466 v. 0.434 (S.E. 0.0018) kg carcass per kg live weight). Averaged over the two slaughter weights BXB lambs had a lower (P < 0.05) dressing proportion than the other genotypes. Carcass conformation classification (assessed on a five-point scale) was higher (P < 0.001) in lambs finished on the LFC diet (3.0 v. 2.4 (S.E. 0.04)). Conformation classification was higher in lambs produced from Cheviot compared with Blackface ewes (P < 0.05). Within the Blackface cross lambs, BXB and BLXB lambs had a lower conformation classification (P < 0.001) than TXB lambs (2.4, 2.4 and 2.9 (S.E. 0.08) respectively). Within the Cheviot ewes, TXC lambs had a higher (P < 0.001) classification than either CXC or SXC (3.1, 2.7 and 2.7 (S.E. 0.08) respectively). Lambs finished on the LFC diet had a higher fat classification score compared with lambs on the HFC diet (P < 0.001) (3.4 v. 2.8 (S.E. 0.04)). Fat classification was similar across all genotypes. Lambs finished on the LFC diet had a lower ash content in the carcass than lambs finished on a HFC diet (P < 0.01) (44 v. 47 (S.E. 0.8) g/kg). Lamb genotype and dietary forage: concentrate ratio had no significant effect on cooking loss, sarcomere length, Warner-Bratzler shear force and L* values. Ultimate pH (pH(U)) was not influenced by dietary forage: concentrate ratio, but was significantly (P < 0.01) higher in BXB lambs compared with BLXB, TXB, CXC and TXC lambs. Lambs offered the LFC diet during the finishing period had significantly higher values for a* (P < 0.05), b* (P < 0.001), C* (P < 0.001) and HI (P < 0.05) than lambs offered a HFC diet. Of these parameters only b* was significantly affected by lamb genotype, with BXB lambs having a lower value (P < 0.05) than the other genotypes.
Data are presented on the effects of modern farming practices on soil as an agricultural resource in England and Wales (E&W), namely: nutrient status, pH, soil organic carbon (SOC), heavy metal concentrations, and on soil erosion risk. Fertilizer-N inputs to tillage (arable) and grassland soils increased from 84 and 65 kg/ha N in 1969 to 151 and 120 kg/ha N, respectively, in 1997. The estimated N surplus (inputs less outputs) increased from 84 and 96 kg/ha N to 102 and 154 kg/ha N, respectively, over the same time. Phosphorus inputs to tillage (mean 37 kg/ha P) and grassland (mean 21 kg/ha P) changed little over the period. However, P surpluses decreased from 25 kg/ha P in 1969 to 15 kg/ha P in 1997 on tillage land (largely as a result of greater P offtakes), and from 20 to 17 kg/ha P in 1997 on grassland (largely because of a small decrease in fertilizer P). The cumulative tillage land soil P surplus was e. 580 kg/ha P, and was estimated to increase topsoil total P concentrations by c. 170 mg/kg P and Olsen-extractable soil P by c. 26 mg/1 P. The mean annual P surplus for grassland was 18 kg/ha P which, over the study period, added c. 427 kg/ha P to the soil, an increase in topsoil total P of e. 214 mg/kg P, and in Olsen-extractable P of 19 mg/1 P. Concentrations of SOC in some soils have decreased between 1980 and 1995, especially where soils have been ploughed out of grassland and on lowland organic and peaty soils in tillage. The mean SOC of soils in arable/ley cultivation in 1980 was 3.4 % and 2.8 % in 1995. The proportion of arable soils with pH < 6.0 decreased from 10 % in 1969-73 to 4 % in 1990-93, reflecting the better targeting of lime inputs. In contrast, the proportion of grassland soils with pH < 6.0 increased from 39 % in 1969-73 to 56 % in 1990-93. Although there were statistically significant changes in the mean soil concentrations of Cd, Cu, Ni, Pb and Zn between 1980 and 1995, many of the changes were small in absolute terms. Hence, there was little evidence of marked or systematic changes in topsoil total heavy metal concentrations that could not be explained by factors other than increased pollutant loadings. Over the next 50 years or so, the threat from soil erosion to crop productivity will be greatest on shallow soils (less than or equal to c. 0.3 m depth), mostly over chalk and sandstone as further removal of soil will lead to increased drought stress. Provided that nutrient supplies are maintained, the evidence is that losses in arable crops on eroded soils would range between 2 % and 8 % of current yields. Thus, within the limitations of the data available, both in time and space, we found little evidence that most soils in E&W cannot continue to support modern farming practices, and the associated crop and animal outputs, given appropriate inputs of nutrients and an adequate degree of crop protection.
The possibility of increasing seed yield in chickpeas was studied by changing sowing time from spring to autumn in an experiment conducted in central Greece over 4 years. Six chickpea varieties, two susceptible to Ascochyta blight and Sclerotinia diseases and four resistant, were evaluated in two sowing seasons (autumn and spring). Results showed that autumn sowing in comparison with spring influenced reproductive and growth periods of the varieties as follows: advanced initiation of flowering (April instead of May), increased flowering duration of the varieties from 7 to 13 days and advanced time for harvest, on average up to 4 days when an exceptionally rainy spring prevailed and up to 30 days when exceptionally dry. Due to the above changes induced by autumn sowing, varieties yielded on average from 23-188% (655-1015 kg/ha) more than if they were sown in spring. The largest per cent increase was observed during the year with the driest spring and the smallest during the year with the wettest spring. The two susceptible varieties 'Thiva' and 'Gravia' when infected by Ascochyta blight and Sclerotinia diseases in autumn sowing, had yields similar to the spring sowing. However, during the years without infection these two varieties yielded more in autumn sowing.
Forty-three near-isogenic lines (NILs) of white clover (Trifolium repens), derived from four parental self-compatible genotypes containing the rare self-fertility allele, were inoculated with the arbuscular mycorrhizal fungus Glomus mossac. Plant growth response (shoot and root weight and root length), shoot P uptake and mycorrhizal root infection rates were recorded 12 weeks after inoculation. There was generally a high degree of variation between individual lines in all recorded parameters. The most sensitive indicator of plant response to mycorrhizal infection was root length with almost half of all lines showing significant responses (in most cases a decrease in root length). Shoot weight was significantly different between mycorrhizal and nonmycorrhizal plants in nine lines. Parental genotype significantly affected both plant response to mycorrhiza as well as mycorrhizal infection rates. The results suggest that the NILs will prove useful for further studies to elucidate the molecular genetic control of the symbiosis and inform plant breeding strategies of this agronomically important species.
A study was carried out on six hill farms in Northern Ireland over 2 years (1996-1998) to investigate the effect of ewe and ram breed on ewe prolificacy, lamb viability and weaned lamb output. On each farm, groups of 40 Scottish Blackface ewes (mature weight of 53.8 kg) were mated to Scottish Blackface, Blue-Faced Leicester and Texel rams. Similarly, groups of 40 Wicklow Cheviot ewes (mature weight 63.7 kg) were mated to Cheviot, Suffolk and Texel rams. All ewe x ram breed combinations were present on each farm. Overall, ewe prolificacy was similar in Blackface and Cheviot ewes (1.52 and 1.55 (S.E,. 0.026) lambs born/ewe lambed). However, there was a farm x ewe breed interaction (P < 0.001) indicating that, whilst prolificacy was similar in Blackface and Cheviot ewes on the majority of farms (4 out of 6), on one farm prolificacy was higher in Blackface and on another lower, compared with Cheviot ewes. There were no farm x breed interactions for any of the other main production traits. The proportion of ewes lambing without assistance was higher in Cheviot compared with Blackface ewes when crossed with Texel sires (P < 0.001). Cheviot ewes produced heavier Texel-sired lambs compared with Blackface ewes (4.76 versus 4.51 (S.E. 0.076) kg; P < 0.05). Mortality levels were similar in lambs produced from Blackface and Cheviot ewes. The weight of lamb weaned per ewe was higher in Cheviot compared with Blackface ewes (41.5 versus 38.8 (S.E. 1.01) kg/ewe lambed; P < 0.05). However, the weight of lamb weaned per kg of ewe metabolic weight did not differ significantly between the breeds. With Blackface ewes, the proportion of ewes lambing without assistance was lower for Blue-Faced Leicester compared with Blackface sires (P < 0.001). In addition, the proportion of ewes lambing without assistance was lower (P < 0.001) for Texel compared with both Blackface and Blue-Faced Leicester-sired lambs. Lamb birth weights were higher in Blue-Faced Leicester (P < 0.05) and Texel (P < 0.001) compared with Blackface-sired lambs (4.38, 4.51 and 4.09 (S.E. 0.076) kg, respectively). Similarly, the weight of lamb weaned per ewe lambed was higher (P < 0.001) with Blue-Faced Leicester and Texel compared with Blackface sires (39.8, 38.8 and 33.8 (S.E. 1.01) kg, respectively). The carcass weight of the male lambs 3 weeks post-weaning was significantly higher (P < 0.001) in Blue-Faced Leicester and Texel compared with Blackface-sired lambs (12.5, 12.0 and 10.2 (S.E. 0.20) kg, respectively). Carcass conformation classification was higher in Texel compared with Blue-Faced Leicester and Blackface-sired lambs (P < 0.001). Fat classification was higher in Texel (P < 0.01) and Blue-Faced Leicester (P < 0.05) compared with Blackface-sired lambs. With Cheviot ewes, a greater number of ewes lambed unaided (P < 0.05) with Cheviot and Texel compared with Suffolk-sired lambs. The number of lambs born dead was higher (P < 0.01) with Suffolk compared with Cheviot and Texel-sired lambs (0.14, 0.08 and 0.07 (S.E. 0.016) lambs born dead/ewe lambed, respectively). Growth rates were higher in Suffolk compared with Cheviot-sired lambs (P < 0.05). Overall, Suffolk (P = 0.06) and Texel (P < 0.001) sires produced a greater weight of lamb at weaning compared with Cheviot sires (40.0, 41.5 and 36.9 (S.E. 1.01) kg, respectively). Carcass weight of lambs 3 weeks postweaning was higher for Suffolk (P < 0.05) and Texel (P < 0.01) compared with Cheviot-sired male lambs. Carcass conformation classification was higher in Texel and Suffolk compared with Cheviot-sired (P < 0.001) lambs. Fat classification was also higher in Texel compared with Cheviot-sired lambs (P < 0.05). Carcass chemical composition was not significantly affected by lamb genotype.
The effect of 27 years of continuous cropping, fertilization and manuring on potassium (K) supplying capacity of a Typic Ustochrept soil profile from Delhi, India under a maize-wheat-cowpea (fodder) cropping system was investigated by employing the quantity/intensity (Q/I) approach. The predominant mineral suite of the 100 % NPK > control (no fertilizer) > 100 % N > 100 % NP. The AR(e)(K) value, a measure of availability or intensity of labile K in soil decreased with profile depth due to greater K fixation by specific sites in the lower layers. The quantity of specifically sorbed K (K-x) and the potential buffering capacity of soil (PBCK) showed a increasing trend with soil depth. In soil without K fertilizer treatments (control, 100 % N and 100 % NP) about 100% of the total K uptake by crops was from non-exchangeable soil K reserve as compared to 49.5 and 32.2 % when annually 84 kg K/ha and 84 kg K/ha + FYM at the rate of 15 t/ha were applied. The results showed the greatest depletion of non-exchangeable K reserves in the plots which did not receive K fertilization. To ensure sustained crop production under intensive cropping, application of recommended dose of NPK plus FYM is required.
Nitrogen-15 enriched ammonium sulphate was applied to micro-plots in a field in which two leguminous (white clover and peas) and two non-leguminous (ryegrass and winter wheat) crops were grown to produce N-15-labelled crop residues and roots during 1993/94. Nitrogen benefits and recovery of crop residue-N. root-N and residual fertilizer-N by three succeeding winter wheat crops were studied. Each crop residue was subjected to four different residue management treatments (ploughed, rotary heed. mulched or burned) before the first sequential wheat crop (1994/95) was sown, followed by the second (1995/963 and third wheat crops (1996/97), in each of which residues of the previous wheat crop were removed and all plots were ploughed uniformly before sowing. Grain yields of the first sequential wheat crop followed the order: white clover > peas > ryegrass > wheat. The mulched treatment produced significantly lower grain yield than those of other treatments. In the first sequential wheat crop, leguminous and non-leguminuus residues supplied between 29-57% and 6-10% of wheat N accumulated respectively and these decreased with successive sequential crops. Rotary heed treatment reduced N benefits of white clover residue-N while no significant differences in N benefits occurred between residue management treatments in non-leguminous residues. On average, the first wheat crop recovered between 29-37% of leguminous and 11-13% of nonleguminous crop residues-N. Corresponding values for root plus residual fertilizer-N were between 5-19% and 2-3%, respectively. Management treatments produced similar effects to those of N benefits. On average. between 5 to 8% of crop residue-N plus root and residual fertilizer-N was recovered by each of the second and third sequential wheat crops from leguminous residues compared to 2 to 4% from non-leguminous residues. The N recoveries tended to be higher under mulched treatments especially under leguminous than non-leguminous residues for the second sequential wheat crop but were variable for the third sequential wheat crop. Relatively higher proportions of leguminous residue-N were unaccounted in ploughed and rotary heed treatments compared with those of mulched and burned treatments. In non-leguminous residue-N, higher unaccounted residue N occurred under burned (33-44%) compared with other treatments (20-27%).
Naked oat grain, which is free from lemma and palea, has high nutritional quality, but the unprotected grain is prone to mechanical damages caused by combine harvesting. Naked oats were grown for 3 years in southern Finland, at Viikki Experimental Farm, University of Helsinki (60 degrees 13'N) to produce seed material for laboratory tests that evaluated: (1) genotypic differences of naked oat in sensitivity to damage during harvesting at grain moisture varying from c. 10 % up to 50 % (2) the effect of mechanical damage on germination and grain vigour, and (3) grain characteristics contributing to susceptibility to reduced grain viability. In 1997, one naked (Rhiannon) and husked oat cultivar (Salo) were harvested, and in 1998-1999 additional four naked cultivars (Bullion, Lisbeth, Neon, SW 95926) were included. One large plot (14 m x 10 m) was sown per cultivar. Two sowing times were used. Fully ripened grains were combine harvested on several occasions for each plot to obtain differences in grain moisture at harvest. Simultaneously, panicle samples were collected, dried and threshed by hand (controls). Grain moisture at each sampling and harvesting was monitored. About 3 months after harvesting, germination tests on blotting paper were carried out. Proportions of normally developed seedlings, seedlings lacking either radicle or hypocotyl, damaged coleoptiles, dead grains and lethally fungus-infected grains were recorded from combine harvested and hand threshed samples on different cultivars and harvest moistures. Tests on seedling elongation, seedling emergence through sand (2 cm and 5 cm. depth), and ion leakage were applied to evaluate grain vigour. Groat weight, diameter, length, roundness, hardness and protrusion of embryo were determined. Our results indicated that naked cultivars were far more prone to mechanical damages than husked Salo, but differences among naked cultivars in susceptibility occurred. When targeting germination of greater than or equal to 75 %, grain moisture at harvest should not exceed 19-26 % depending on cultivar. Abnormal seedlings appeared irrespective of grain moisture at harvest, but the higher the grain moisture, more dead grains were found in harvested grains after storage. Seed vigour did not alter parallel to germination ability. High proportion of small grains in harvested yield and softer groats contributed to decreased sensitivity to mechanical damages.
Experiments were conducted in Nigeria to evaluate the optimum plant population density for okra (Abelmoschus esculentus) sown in monoculture or intercropped with cassava (Manilhot esculenta). The crops were sown as late and early season crops in 1997 and 1998. Okra was sown at varied spacings to achieve final plant densities of 25000, 35000 and 50000 plants/ha in both monoculture and mixed stands. Intercropping had no significant effect on okra phenology (time to vegetative growth, flowering and fruiting). However, it significantly reduced weed growth by 25-45 % and nematode infection in okra by 36 %, and kept the canopy environment of cassava cooler by 3.0-4.4 degreesC and more moist by 5-10 g/kg, compared with monoculture. The durations of vegetative and reproductive growth and the weed-control ability of okra in both cropping systems were influenced by population density. Okra sown at 50000 plants/ha took the longest time to reach specific phenological stages, controlled weeds best and gave the highest fresh pod yields in both seasons. There was little increase in pod yields between 35000 and 50000 plants/ha in mixed stands in late season sown crops. The growth and tuber yield of cassava were not significantly affected by intercropping and okra population density. The growth of okra and cassava were better with early than late season sowing, irrespective of okra population density or cropping system. This appears to have been due mainly to the effect of temperature and rainfall differences between the seasons. The substantial variation in the weather during the two seasons also had greater effect in monoculture than mixed stands. It is concluded that when rainfall is limiting, okra could be intercropped with cassava using population density up to 35 000 plants/ha to allow the vegetable to develop and to maximize pod yield under the relatively dry conditions. The optimum target suggested is 50000-60000 plants/ha during the wet periods.
dThis paper reports an analysis of the yield of sugar beet crops grown under experimental conditions between 1993 and 1995 in the UK. Crops were either healthy (unstressed) or subjected to drought, infection with Beet yellows virus (BW) or a combination of both. The study investigated whether the large differences in yield between the crops grown in different seasons and subjected to different stresses could be accounted for by simple relationships between total biomass and radiation interception (epsilon (s)), transpiration (epsilon (w)) or epsilon (s) and epsilon (w) adjusted for mean saturation deficit (Omega (s) and Omega (w) respectively). Mean values of epsilon (s), epsilon (w), Omega (s) and Omega (w) in healthy crops were 1.42 g/MJ, 0.89 g/kg, 6.76 g/kPa/MJ and 4.29 g/kPa/kg respectively. Variations in the dry matter yield between seasons were best accounted for by Omega (w) and less well by epsilon (w) or epsilon (s).Omega (s) accounted for least variation in yield between seasons. None of these relationships remained constant in stressed plants; both drought and BW-infection decreased epsilon (s) (and Omega (s)) but Omega (w) was increased by drought and decreased by BYV-infection. However, in common with healthy crops, seasonal variation in yields was best accounted for by Omega (w). Mean values of epsilon (s), Omega (s), epsilon (w) and Omega (w) for all healthy, infected and droughted crops accounted for 61, 50, 88 and 97% of the Variation in dry matter yield between experiments respectively. Accurate prediction of the yield of stressed plants requires a knowledge of their infection and drought status. If this information is unavailable then the mean value of Omega (w) for healthy, infected and droughted crops will provide a reasonable prediction of the yield of such crops.
A laboratory experiment was designed to determine the fate of N-15-labelled slurry ammonium ((NH4)-N-15-N) and compare soil inorganic-N distribution following surface applied or injected pig slurry. A system of cylindrical volatilization chambers equipped to allow continuous trapping of ammonia (NH3) was used. Undisturbed soil columns were placed in the chambers prior to the application of slurry. A nitrogen balance including soil, air and plant analysis was established for both treatments, 8 days after application. Average cumulative emissions of NH3, were 15% and 11% of the total ammoniacal-N added with the surface and injected treatments, respectively. After 8 days 55% of the (NH4)-N-15-N applied through slurry injection was recovered in the soil inorganic-N pool: 37% as (NH4)-N-15-N and 18% as (NO4)-N-15-N. These figures compare with only 25% N-15-NH4 recovered with the surface applied slurry treatment: 7% as (NH4)-N-15-N and 17% as (NO4)-N-15-N. Immobilization into soil organic-N accounted for 8% of the (NH4)-N-15-N applied for the injected treatment and 6% of the surface applied slurry-N-15. N-15 uptake by the grass was 2% and 7% for the injected and surface applied treatments, respectively. The percentage of added N-15 accounted for was 76% for the injected treatment and 53 % for the surface applied slurry treatment.
Honeycomb selection in the F-2 generation of maize (Zea mays L.) hybrid PR 3183, based on line performance per se in the absence of competition, led to recycled hybrids with improved potential yield per plant (mean yield per plant in the absence of competition). In the present study six S-6 X S-6 recycled hybrids and two commercial single-cross hybrids (PR 3183 and B73 x Mol7) were tested at three plant densities (25 000, 41 667 and 83 333 plants/ha), in two locations (Technological Education Institute farm of Florina, Greece and University farm of Thessaloniki, Greece), for 2 years (1998, 1999). The study was undertaken to assess indirectly the potential yield per plant (p), the crop yield potential (Y-max), and the optimum plant density (D-opt) of the hybrids. Estimate of p and Y-max were obtained through linear regression analysis of yield per plant (Y-p) on plant density (D), expressed by the equation Y-p = p - qD, with Y-max being equal to (1/4)p(2)q(-1). Optimum plant density was assessed through linear regression analysis of natural logarithm of yield per plant on plant density, expressed by the equation ln(Y-p) = alpha -bD, with D-opt being equal to 1/b. The recycled hybrids had higher estimated potential yield per plant (p), than the two check hybrids, with p values being positively correlated with yield per plant of hybrids obtained experimentally in the absence of competition. Results indicated that the higher potential yield per plant decreases the optimum plant density, and renders the hybrids less density-dependent.
The effectiveness of alternative fertilizer strategies to control nitrate leaching was investigated in a field experiment at IGER, Aberystwyth using simulated pastures to represent swards grazed by dairy cattle between May and October 1997, following an initial silage cut. Cut-plots with and without applications of artificial urine were used to represent the separate components of grazed pastures managed with the following nitrogen (N) fertilizer strategies: standard (fertilizer applied uniformly to all areas of the sward at a total rate of 180 kg N/ha between May and October), tactical (fertilizer rates adjusted to match the average soil mineral-N content of the pasture to that of ungrazed reference plots receiving the standard rate) and patch-avoidance (fertilizer applied at the standard rate but withheld from areas that had received urine). Calculated stocking rates derived from herbage yields indicated that 12 % of the pasture would have been affected by urine up to the end of 'grazing' in October. The presence of urine patches increased the nitrate-N content of the 0-90 cm soil profile in October from 61 kg N/ha for ungrazed pasture to 104 kg/ha for 'grazed' pasture receiving the standard fertilizer rate. Although the patch-avoidance strategy was more effective than the tactical in reducing the accumulation of nitrate in soil under urine patches, they were both of limited effectiveness in reducing the content over the pasture as a whole. Profile contents in October for the simulated pastures managed with the tactical and patch-avoidance strategies were equivalent to 99 and 97 kg nitrate-N/ha, respectively. The tactical strategy achieved a 26 % saving in overall fertilizer use. Under the conditions of the experiment this did not significantly reduce herbage yields. The patch-avoidance strategy reduced fertilizer use by only 3 %.
The aim of the experiment was to determine the importance of liquid digested sewage sludge ('sewage sludge') as a source of phosphorus leaching from sandy soils in the UK. The sewage sludge was applied to uncropped loamy sand and sandy loam monolith lysimeters (1.2 m deep) annually for 3 years. The application rate was sufficient to supply approximately 60 kgP/ha annually. An application equivalent to four times this amount was also applied to two other loamy sand lysimeters to test the effects of excessive amounts. Total P (TP), total dissolved P (TDP) and molybdate-reactive P (MRP) concentrations were measured in the drainage water and were compared with leaching from untreated control soils. After two sewage sludge applications, Olsen's extractable P content of the topsoil had increased: a P surplus of c. 100 kg/ha was required to raise the topsoil concentration by 5 mg/kg. Sewage sludge applications at the lower rate did not increase P concentrations in the drainage. Phosphorus concentrations were generally small and were, as an average of both soil types and nil and lower rate sludge applications, 12, 20 and 34 mug/l for MRP, TDP and TP respectively. Phosphorus leaching losses from the higher rate (excessively large sewage sludge applications) were more variable but not significantly (P > 0.05) different from the other treatments. Average concentrations were 9, 23 and 50 mug/l for MRP, TDP and TP respectively. Leaching from sludge biosolid applications at operational rates to sandy soils moderately well supplied with P is not a major P loss pathway.