黄鳝精巢发育的周年变化

对黄鳝精巢显微和超微结构的观察结果表明 ,其精原细胞可分两种类型 :初级精原细胞和次级精原细胞 ;精子鞭毛轴丝由外周 9组微管和两根中央微管构成 ;间质细胞分布有广泛的滑面内质网和具小管的线粒体。据对10 6尾 (体重为 17～ 32 0 g)黄鳝精巢切片观察 ,黄鳝由雌性完全转变为雄性后 ,精巢已接近发育成熟 ;经 χ2 检验表明 ,其性腺成熟度与体重关系不大 (P >0 0 5 )。性成熟后 ,精巢所处发育阶段 (Ⅰ -Ⅵ期 )主要随不同季节而变化     

多年定位施肥对紫色土钾素形态变化的影响

在西南农业大学国家紫色土肥力与肥料效益监测基地上 ,研究多年定位施肥对紫色土钾素形态变化的影响。研究结果显示 :施钾各处理 0～ 2 0cm和 2 0～ 40cm土层土壤水溶性钾、非特殊吸附态钾、特殊吸附态钾下降不明显 ,基本能保持在试验初期 ( 1991年 )的水平上 ,不施钾处理则呈现明显下降趋势 ,这表明施钾能维持紫色土速效钾 (水溶性钾 +非特殊吸附态钾 +特殊吸附态钾 )的平衡。不论施钾与否 ,各处理 0～ 2 0cm和 2 0～ 40cm土层土壤非交换性钾、矿物钾呈下降趋势 ,其中不施钾处理土壤非交换性钾下降更明显     

Pasture research in South Africa

The “systems approach” means that decision‐rich parts of any given system are considered quantitatively and in their proper context. This is done by using models of various kinds. A model can be built if an adequate information base exists for the system in question, and if decisions have been taken about the scope and focal point(s) of the model, its application, and its form. The systems approach is illustrated by outlining a general procedure for livestock enterprise planning. Some of the research and extension problems revealed, and new problems created, by this and other models, are mentioned.

Two matters of general concern are discussed. It is argued that the quality of leadership in agricultural research leaves something to be desired. Secondly, the class of agricultural scientists seems to be regrettably indifferent to the long‐term effects of its work, and blind to the needs of primitive food producers. The systems approach can contribute to improvement in both areas, as a result of forcing people to define clear objectives and order their priorities. There is a need for grassland and other research workers to state the objectives of their disciplines for the next 30 years.     

Impacts of non-selective grazing on cover,composition, and productivity of Nama-karoo grassy shrubland

The non-selective rotational grazing system has undergone a long and controversial development. The merits of this grazing system, where relatively large numbers of livestock are herded into numerous small paddocks for short timespans with long rests between grazings, have not been formally evaluated in a long-term monitoring experiment in the Karoo. In this study we used exclosures (controls) on a 7 000 ha farm in the Central Lower Karoo, camped into approximately 50-ha paddocks, to evaluate the impact of this grazing system on certain vegetation parameters. We report on the first four years of monitoring, after each of four replicate paddocks had received four treatments (one treatment = 40 - 60 Large Stock Unit grazing days per hectare over a period of 2 - 16 days). Concentrated defoliation with concomitant trampling, dunging, and urinating did not influence the perennial species composition, and cover of this grassy, semi-arid shrubland. Changes over time in plant composition, and cover are explained by annual, and short-term (e.g. quarterly) rainfall rather than by grazing impacts. Ephemerals were not favoured by this grazing system, but litter was more abundant in the treatment than the control areas. The dominant grass, Eragrostis lebmanniana, and shrub, Pentzia incana, are resilient to this defoliation regime, and show signs of compensatory growth. As yet there is no evidence that non-selective grazing increases diversity, but the severe defoliation and trampling may enhance the biomass turnover rate, resulting in more vigorous, and productive plants in the grazed areas. Diversity is unlkely to change rapidly in response to grazing, largely because of the persistence of grazing-tolerant perennials. Concentrating mixed herds of livestock onto small areas with lengthy rests can be a useful tool for 'kick-starting' moribund karoo veld into greater productivity. Several of the impacts hold potential for restoring the rangeland quality of degraded areas, but this needs to be tested.     

五氯酚钠中毒鱼的残毒分析与病理变化研究

五氯酚钠灭螺因撒布不匀使局部地段浓度过大，雨后造成某鱼塘一万多条鱼死亡。采用石油醚──乙醚抽提法检测24份死亡鲤鱼、鲫鱼、白鲢鱼组织中五氯酚钠残毒（mg／kg）：肌肉18．5－28．4，内脏25．2─35.4。观察200条中毒活鱼和剖检60条中毒死鱼：活鱼以运动无力、张口呼吸、神经紊乱、肌肉痉挛为特征；死鱼以全身肌肉内脏鳃丝严重淤血出血、鳃盖张开、尾鳍收缩为主要变化。检测18份沟水和塘水中五氯酚钠含量（mg／L）：灭螺沟水193.5－233.9，鱼塘入口水114.5－123．8，塘中水15．8－98.5。仅塘中水含量为鲤鱼致死量的551倍。中毒鱼不能上市销售，应加强肉品卫生检验和正确使用五氯酸钠灭螺。