首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   432篇
  免费   11篇
  国内免费   1篇
林业   18篇
农学   13篇
  46篇
综合类   47篇
农作物   9篇
水产渔业   66篇
畜牧兽医   224篇
园艺   4篇
植物保护   17篇
  2023年   2篇
  2022年   3篇
  2021年   12篇
  2020年   9篇
  2019年   15篇
  2018年   5篇
  2017年   9篇
  2016年   6篇
  2015年   10篇
  2014年   13篇
  2013年   28篇
  2012年   28篇
  2011年   43篇
  2010年   14篇
  2009年   15篇
  2008年   28篇
  2007年   28篇
  2006年   30篇
  2005年   21篇
  2004年   12篇
  2003年   25篇
  2002年   10篇
  2001年   7篇
  2000年   6篇
  1999年   5篇
  1998年   3篇
  1997年   4篇
  1993年   1篇
  1992年   4篇
  1991年   5篇
  1990年   4篇
  1989年   4篇
  1988年   2篇
  1987年   1篇
  1986年   6篇
  1985年   3篇
  1983年   1篇
  1982年   1篇
  1980年   1篇
  1979年   1篇
  1978年   1篇
  1977年   1篇
  1976年   1篇
  1974年   1篇
  1968年   3篇
  1967年   5篇
  1966年   1篇
  1965年   1篇
  1960年   1篇
  1956年   1篇
排序方式: 共有444条查询结果,搜索用时 31 毫秒
1.
Environmental conditions influence phenology and physiological processes of plants. It is common for maize and sorghum to be sown at two different periods: the first cropping (spring/summer) and the second cropping (autumn/winter). The phenological cycle of these crops varies greatly according to the planting season, and it is necessary to characterize the growth and development to facilitate the selection of the species best adapted to the environment. The aim of this study was to characterize phenological phases and physiological parameters in sorghum and maize plants as a function of environmental conditions from the first cropping and second cropping periods. Two parallel experiments were conducted with both crops. The phenological characterization was based on growth analyses (plant height, leaf area and photoassimilate partitioning) and gas exchange evaluations (net assimilation rate, stomatal conductance, transpiration and water-use efficiency). It was found that the vegetative stage (VS) for sorghum and maize plants was 7 and 21 days, respectively, longer when cultivated during the second cropping. In the first cropping, the plants were taller than in the second cropping, regardless of the crop. The stomatal conductance of sorghum plants fluctuated in the second cropping during the development period, while maize plants showed decreasing linear behaviour. Water-use efficiency in sorghum plants was higher during the second cropping compared with the first cropping. In maize plants, in the second cropping, the water-use efficiency showed a slight variation in relation to the first cropping. It was concluded that the environmental conditions as degree-days, temperature, photoperiod and pluvial precipitation influence the phenology and physiology of both crops during the first and the second cropping periods, specifically cycle duration, plant height, leaf area, net assimilation rate, stomatal conductance and water-use efficiency, indicating that both crops respond differentially to environmental changes during the growing season.  相似文献   
2.
A method developed to evaluate the cumulative effect of wetland mosaics on water quality was applied to 33 lake watersheds in the seven-county region surrounding Minneapolis-St. Paul, Minnesota. A geographic information system (GIS) was used to record and measure landscape variables derived from aerial photos. Twenty-seven watershed land-use and land-cover variables were reduced to eight principal components which described 85% of the variance among watersheds. Relationships between lake water quality variables and the first six principal components plus an index of lake mixis were analyzed through stepwise multiple regression analysis. A combination of three landscape components (wetland/watershed area, agriculture/wetlands, and forest/soils components) explained 49% of the variance in a trophic state index, even though most of the lakes examined were already highly eutrophic, and thus were influenced by internal loading. The regression equations explained a range of 14 to 76% of the variation in individual water quality variables. Forested land-use was associated with lower lake trophic state, chloride, and lead. High lake trophic state was associated with agricultural land-use and with wetland distance from the lake of interest. The extent of wetlands was associated with low total lead and high color in lakes downstream. Wet meadows or herbaceous, seasonally-flooded wetlands contributed more to lake water color than did cattail marshes.  相似文献   
3.
4.
As dogs experience oestrus only once or twice a year, it is necessary to establish an effective method of oestrous induction for efficient breeding. In the present study, we evaluated inhibin antiserum (IAS) on oestrous induction in anoestrous females. Bitches were administered 0.5 ml/kg IAS or a mixture of 50 IU/kg equine chorionic gonadotropin (eCG) and 0.5 ml/kg IAS and 500 IU human chorionic gonadotropin (hCG) administered 7 days after the mixture injection. As a control, bitches received 50 IU/kg eCG, with 500 IU hCG administered 7 days after eCG injection. Blood-tinged vaginal discharge, vulvar swelling, plasma progesterone concentrations and ovarian follicular development were assessed from day 0 to day 14. IAS alone injection did not induce oestrus in bitches at the anoestrous stage. Conversely, vulvar swelling, blood-tinged vaginal discharge and an estimated luteinizing hormone (LH) surge appeared on days 3–7, days 3–6 and days 7–9 after the IAS+eCG mixture injection, respectively, in all five bitches at the anoestrous stage. The average number of developing and ovulated follicles in bitches administered IAS+eCG was 8.8 and 9.6 respectively. A single eCG injection followed by hCG induced oestrous signs, with an average of 8.3 developing follicles and 4.5 ovulated follicles. This study revealed that IAS alone did not induce oestrus, but when IAS was used in combination with eCG, it induced oestrus and promoted a considerable number of ovulations in anoestrous dogs.  相似文献   
5.
OBJECTIVE: To evaluate use of electroacupuncture combined with standard Western medical treatment versus Western medical treatment alone for treatment of thoracolumbar intervertebral disk disease in dogs. DESIGN: Prospective controlled study. ANIMALS: 50 dogs with signs of thoracolumbar intervertebral disk disease. PROCEDURES: Dogs were randomly allocated to 1 of 2 treatment groups and classified as having grade 1 to 5 neurologic dysfunction. Dogs in group 1 received electroacupuncture stimulation combined with standard Western medical treatment; those in group 2 received only standard Western medical treatment. A numeric score for neurologic function was evaluated at 4 time points to evaluate effects of treatments. RESULTS: Time (mean +/- SD) to recover ambulation in dogs with grade 3 and 4 dysfunction in group 1 (10.10 +/- 6.49 days) was significantly lower than in group 2 (20.83 +/- 11.99 days). Success (able to walk without assistance) rate for dogs with grade 3 and 4 dysfunction in group 1 (10/10 dogs) was significantly higher than that of similarly affected dogs in group 2 (6/9 dogs). Dogs without deep pain perception (grade 5 dysfunction) had a success (recovery of pain sensation) rate of 3 of 6 and 1 of 8 in groups 1 and 2, respectively, but the difference was not significant. Overall success rate (all dysfunction grades) for group 1 (23/26; 88.5%) was significantly higher than for group 2 (14/24; 58.3%). CONCLUSIONS AND CLINICAL RELEVANCE: Electroacupuncture combined with standard Western medical treatment was effective and resulted in shorter time to recover ambulation and deep pain perception than did use of Western treatment alone in dogs with signs of thoracolumbar intervertebral disk disease.  相似文献   
6.
ABSTRACT:   The effects of silvering state of wild female Japanese eels Anguilla japonica on the success of induced maturation and the following spawning were examined. Thirty-eight females, collected in Mikawa Bay, were divided into four stages based on their silvering state: yellow (Y1), late-yellow (Y2), silver (S1) and late silver eels (S2). Despite injections of salmon pituitary extract (SPE) through the standard technique, Y1 and Y2 eels did not respond to the treatment with undeveloped gonad (gonad-somatic index [GSI]: 0.3–0.9), and all these females died by 5 weeks, probably due to an abnormal physiological condition. Most S1 (81%) and S2 eels (100%) matured completely (GSI: 17.8–51.4), and finally spawned successfully (69% for S1, 89% for S2). S2 eels fully matured with oocytes of over 750 μm in diameter by significantly smaller number of injections of SPE (5–6 times) than the case of S1 eels (6–8 times). The amount of eggs released by S2 eels (0.65 ± 0.11 g/fish per body weight [BW]) was significantly larger than those by S1 eels (0.54 ± 0.09 g/fish per BW). There was no difference in fertilization and hatching rates between eggs released by S1 eels and those of S2 eels. These results indicate that the success of induced maturation and spawning in wild female Japanese eels depends on their silvering state, and matured eggs can be obtained efficiently through the use of S2 eels rather than other stages.  相似文献   
7.
Papaya seed represents a rich source of biologically active isothiocyanate   总被引:1,自引:0,他引:1  
In the present study, papaya (Carica papaya) seed and edible pulp were carefully separated and then the contents of benzyl isothiocyanate and the corresponding glucosinolate (benzyl glucosinolate, glucotropaeolin) quantified in each part. The papaya seed with myrosinase inactivation contained >1 mmol of benzyl glucosinolate in 100 g of fresh seed. This content is equivalent to that of Karami daikon (the hottest Japanese white radish) or that of cress. The papaya seed extract also showed a very high activity of myrosinase and, without myrosinase inactivation, produced 460 micromol of benzyl isothiocyanate in 100 g of seed. In contrast, papaya pulp contained an undetectable amount of benzyl glucosinolate and showed no significant myrosinase activity. The n-hexane extract of the papaya seed homogenate was highly effective in inhibiting superoxide generation and apoptosis induction in HL-60 cells, the activities of which are comparable to those of authentic benzyl isothiocyanate.  相似文献   
8.
The gonadal sex differentiation in red sea bream, Pagrus major, which is one of the most important species for aquaculture in Japan, was revealed histologically. The suitable conditions for induction of all-male groups in the fish were investigated, and functional males were induced by the conditions of oral administration of 17α-methyltestosterone. The sex determination of this fish was also discussed.  相似文献   
9.
SUMMARY: The aim of this study was to investigate the cause of green liver in red sea bream fed substitute protein diets. Red sea bream Pagrus major was given either of the following diets for 28 weeks: (1) control diet (50% fish meal), (2) low level fish meal diet (15% fish meal), and (3) non-fish meal diet (0% fish meal). The green liver was observed in all groups tested, but the incidence was much higher in the experimental diet groups. The feeding of substitute protein diets reduced plasma triglyceride and cholesterol levels. However, there was no significant difference in plasma hepatic enzyme activities and plasma bile salts concentration among the treatments. Fish fed the substitute protein diets showed low hepatic taurine levels with an appearance of a biliverdin in the liver. Moreover, the proportion of ditaurobilirubin to total biliary bile pigments was significantly lower in fish fed the substitute protein diets. These data indicate that feeding of substitute protein diets did not induce any cholestatic hepatobiliary obstructions and that the low hepatic taurine level was one of the probable factors responsible for the occurrence of green liver in red sea bream fed substitute protein diets.  相似文献   
10.
The timing of primordial germ‐cell (PGC) migration with regard to the gonadal anlagen, gonad formation and sex differentiation was examined histologically in the chub mackerel (Scomber japonicus) at 5–190 days post hatching (dph). At 5 dph, PGCs appeared on the peritoneal epithelium surface or in the mesentery, on the dorsal side of the abdominal cavity. By 10 dph, stromal cells around the PGCs proliferated. The gonadal primordium was formed by 15 dph. The gonadosomatic index was 0.01% at 30 dph and increased thereafter (0.32% in females and 0.04% in males at 160 dph). Ovarian differentiation occurred at 30–40 dph, indicated by ovarian cavity formation (elongation and fusion of the upper and lower ovarian edges). Meiosis was subsequently initiated. A few meiotic oocytes surrounded the cavity at 50 dph; most were in the perinucleolus stage at 60 dph and attained a diameter of 60–70 μm at 190 dph. Testicular differentiation occurred at 30 dph, indicated by the formation of the sperm duct primordium. Spermatogonia gradually proliferated, developing into spermatocytes at the chromatin–nucleolus stage (after 90 dph) and subsequently into spermatids and spermatozoa (160 dph). These data could aid the development of seeding and cell‐engineering technologies for scombrid fish.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号