耕作对土壤有机物和土壤团聚体稳定性的影响

Agricultural sustainability relates directly to maintaining or enhancing soil quality. Soil quality studies in Canada during the 1980‘s showed that loss of soil organic matter (SOM) and soil aggregate stability was standard features of non-sustainable land management in agroecosystems. In this study total soil organic carbon (SOC), particulate organic matter (POM), POM-C as a percentage of total SOC, and aggregate stability were determined for three cultivated fields and three adjacent grassland fields to assess the impact of conventional agricultural management on soil quality. POM was investigated using solid-state ^13C nuclear magnetic resonance (NMR) to determine any qualitative differences that may be attributed to cultivation. Results show a highly significant loss in total SOC, POM and aggregate stability in the cultivated fields as compared to the grassland fields and a significant loss of POM-C as a percentage of total SOC.Integrated results of the NMR spectra of the POM show a loss in carbohydrate-C and an increase in aromatic-C in the cultivated fields, which translates to a loss of biological lability in the organic matter. Conventional cultivation decreased the quantity and quality of SOM and caused a loss in aggregate stability resulting in an overall decline in soil quality.     

The nitrogen content of particle size fractions separated from peat and its rate of mineralization during incubation

Particle size fractions, varying from 5 to 0.005 mm, were separated from samples of several peat types using a wet sieving technique. In all types, nitrogen content of fractions increased as particle size decreased, the fine fractions (0.15–0.005 mm) accounting for around 43–64% of the total nitrogen. During incubation, fractions from two blanket peats showed a net release of mineral nitrogen, whereas those from raised bog peats were characterized, for the most part, by net immobilization. Amounts of mineral nitrogen released in fractions from any one peat after 28 days incubation at 30°C did not always correlate with either total nitrogen content or C:N ratio. However, better correlations between mineral nitrogen production and total N were obtained for fractions of similar particle size. The proportion of the total nitrogen that was mineralized tended to be higher in coarse fractions > 0.15 mm, blanket peats giving much higher values than samples from raised bogs. Carbon dioxide release also varied with particle size, being highest in large particles and fibres and least in the size range 0.15–0.5 mm. Smaller particles < 0.15 mm, gave intermediate values that were higher in blanket than in raised bog peat.     

A DUNE CATENA ON THE CLAY PLAINS OF THE WEST CENTRAL GEZIRA, REPUBLIC OF THE SUDAN

Fixed dunes occupy 400 000 acres in the west central Gezira. Isolated dunes occur sporadically through the central Gezira, and trend NW from Sennar on the Blue Nile to El Geteina on the White Nile. They probably originated as channel deposits laid down by former distributaries of the Blue Nile. Minor aeolian re-sorting took place, and was followed by plant colonization and dune stabilization. Erosion of the dunes is locally active, resulting in exposure of the underlying clays. The catena comprises leached sands (Psammustents) on the dune crests and illuvial loams and clays (Orthustents) in the swales. Swale soils contain pedogenetic and relict lacustrine carbonate. The catena is often superimposed upon former soils, resulting in polygenetic profiles.     

Derivation, safety and efficacy of a Marek's disease vaccine developed from an Australian isolate of very virulent Marek's disease virus

Objective To develop a serotype 1 Marek's disease (MD) vaccine from a very virulent MDV (vvMDV) pathotype and demonstrate safety and efficacy against early challenge with very virulent field strains in the presence of maternal antibody.
Study design Strain BH 16 was isolated and attenuated by serial cell culture passage. One of two cloned passages was selected for vaccine development following early laboratory-scale protection trials in commercial birds. Comparative protection trials were carried out on the BH 16 vaccine and on a CVI 988 Rispens vaccine using commercial and SPF chickens. Challenge viruses used were either a low passage strain BH 16 virus, the Woodlands No. 1 strain or MPF 57 strain of MDV. The BH 16 vaccine was back-passaged in SPF chickens six times and virus recovered from the final passage and the original vaccine virus were tested for safety. The immunosuppressive potential of the BH 16 and Rispens vaccines was also assessed in parallel.
Results The BH 16 and Rispens vaccines induced comparable levels of protection when used as monovalent or multi-valent vaccines, although protection achieved with the mono-valent vaccines was lower. No gross tumour formation was evident in any birds receiving the BH 16 vaccine or bird-passaged virus, although microscopic lesions were present in 2/12 birds that received the bird-passaged virus. In tests for immunosuppression, there was no histological evidence of damage to either the bursa of Fabricius or the thymus.
Conclusion The BH 16 vaccine was shown to be safe and at least as protective as the Rispens vaccine against three highly virulent MD challenge viruses.     

干草原区植被保护支持技术研究绿色添加剂—大蒜(Allium sativum)对肉牛增重影响的研究

大蒜是一种植物鳞茎 ,作为肉牛饲料添加剂尚未见报道。本文研究的目的是 :研究大蒜对肉牛增重的影响。试验选用西门塔尔杂交后代公牛 3 8头其中 3 5头牛随机分为七组 ,大蒜组 A( 50 g/ d.头 )、D( 1 0 0 g/ d.头 )、F( 1 50 g/ d.头 ) ;大蒜素组 C( 0 .2 g/ d.头 )、E( 0 .4g/ d.头 )、G( 0 .6g/ d.头 ) ;对照组。试验采用完全拉丁方设计。试验结果表明 :1 .大蒜在添加 74.6g/ d.头左右 ,对肉牛增重有明显的促进作用 ,Y=-0 .0 0 2 2 x2 +0 .3 2 8x+2 8.3 2 ,R2 =0 .972 8( Y为增重 ,X大蒜用量 )。 2 .当大蒜素添加量为 0 .3 5g/ d.头时肉牛体增重最高 ,y=-92 .5x2 +65.4x+2 7.3 3 R2 =0 .91 96( Y为增重 ,X大蒜素用量 )。 3 .大蒜促进肉牛增重的主要成分是大蒜素     

Slot-seeding investigations. 6. A comparison of the spread and productivity of different white clover varieties introduced into permanent pasture

Seeds of six cultivars of white clover were sown in 1983 in slots in tares of permanent pasture in soil-filled tanks in spring and in small plots in the field during midsummer. The clover cultivars investigated were the large-leaved Olwin and Milk nova, the medium-leaved Grasslands Huia and Aberystwyth S100, and the small-leaved Kent Wild White and Aberystwyth S184. Seeds from an indigenous clover population were also sown in the tanks. All herbage was defoliated at three- to six-week intervals to simulate rotational grazing and assessments were made until autumn 1984.
Leaf and stolon production and spread of stolons from the slots were more rapid in Milkanova, S184, Huia and Kent than in Olwen and S100 In the tanks, harvested yields of clover leaf and petiole were largest for Milkanova and Olwen and least for the indigenous population; both Olwen and S100 had a smaller percentage of their total weight outside the slot area than had the other varieties. In the field, Milkanova greatly out yielded all other varieties and S100 yielded the least, contributing 48 and 27%, resf actively, to total herbage yield. The yield of grass differed little between varieties in tanks but in the field it tended to be higher with the smaller than with the larger-leaved varieties. Total herbage yield was largest for Olwen and Milkanova in tanks and for Milkanova in the field. The greatest weight of stolons in tanks was produced by Olwen and in the field by Olwen and Kent, but the greatest length of stolons and number of nodes in both environments occurred in the smalt-leaved varieties, especially Kent. Least length of stolons and fewest nodes in the tanks were produced by Olwen and in the field by Milkanova.
It was concluded that all the varieties investigated could be successfully slot-seeded into permanent pastures but that the choice of variety will depend on subsequent use and management of the resulting swards.     

欧拉羊与山谷型藏羊杂交F_1代羔羊补饲兼放牧育肥试验效果观察

用欧拉羊杂交改良山谷型藏羊,对杂交F1代羔羊补饲兼放牧育肥试验,补饲阶段试验结果表明：经过90 d补饲,试验组增重10.88kg,比对照组高4.32 kg（P〈0.01）,平均日增重0.121 kg,比对照组高0.048 kg;放牧育肥阶段试验结果表明：经过3个月放牧育肥,试验组增重9.09 kg,比对照组高2.67 kg（P〈0.05）,平均日增重0.101 kg,比对照组高0.03 kg。