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   579篇
  免费   28篇
林业   9篇
农学   16篇
基础科学   2篇
  50篇
综合类   93篇
农作物   15篇
水产渔业   30篇
畜牧兽医   371篇
园艺   11篇
植物保护   10篇
  2020年   11篇
  2019年   10篇
  2018年   11篇
  2017年   9篇
  2016年   5篇
  2015年   10篇
  2014年   13篇
  2013年   71篇
  2012年   11篇
  2011年   22篇
  2010年   14篇
  2009年   19篇
  2008年   12篇
  2007年   17篇
  2006年   16篇
  2005年   10篇
  2004年   11篇
  2003年   7篇
  2002年   14篇
  2001年   11篇
  2000年   8篇
  1999年   14篇
  1998年   9篇
  1997年   9篇
  1996年   6篇
  1995年   8篇
  1994年   6篇
  1993年   13篇
  1992年   12篇
  1991年   18篇
  1990年   12篇
  1989年   19篇
  1988年   8篇
  1987年   6篇
  1986年   11篇
  1985年   8篇
  1984年   5篇
  1983年   8篇
  1982年   4篇
  1979年   7篇
  1977年   5篇
  1974年   13篇
  1973年   6篇
  1972年   9篇
  1971年   4篇
  1970年   11篇
  1968年   4篇
  1967年   5篇
  1966年   6篇
  1881年   4篇
排序方式: 共有607条查询结果,搜索用时 15 毫秒
111.
112.
Phenobarbital has been the primary antiepileptic drug used in primates, but the dosage required for seizure control is frequently associated with significant side effects. Newer antiepileptic drugs and adjunctive therapies currently being used in human medicine provide additional options for treatment of nonhuman primates. This report describes different drug regimes used for control of epileptic seizures in apes at the Milwaukee County Zoo (Milwaukee, Wisconsin, U.S.A.), including the addition of acetazolamide to phenobarbital, levetiracetam, carbamazepine, and the use of extended cycle oral contraceptives to assist seizure control in female apes with catamenial epilepsy.  相似文献   
113.
Intrauterine swabs were obtained from cows after calving on two commercial dairy herds with contrasting hygienic environments and incidence of leucorrhea, and cultured aerobically and anaerobically. Of 26 cows with a normal calving and puerperium, eight of 14 (57 per cent) were sterile on farm B where hygiene was poor, compared with five of 12 (42 per cent) on farm A where hygiene was better. Two cows on farm B retained their placentas and subsequently developed metritis/endometritis. Actinomyces pyogenes was the commonest bacterial isolate and Fusobacterium nucleatum, Proteus mirabilis and Bacteroides melaninogenicus were also frequently observed. Similar isolates were obtained from cows on farm B with parturient or puerperal disorders. The contrasting hygienic environments had no influence on either the quantitative or qualitative uterine bacterial flora. Thus, the difference in the incidence of endometritis must have been due to factors other than hygiene.  相似文献   
114.
OBJECTIVE: To estimate seroprevalence of bluetongue virus (BTV) and the geographic distribution of seropositive cattle herds in Illinois and western Indiana. SAMPLE POPULATION: 10,585 serum samples obtained from cattle in 60 herds during 3 transmission seasons (2000 through 2002). PROCEDURES: In a longitudinal study, serum samples were tested for BTV antibodies by use of a competitive ELISA. Four geographic zones were created by use of mean minimum January temperature. A multivariable mixed-effects logistic regression model with a random effect for herd was used to estimate seropositive risk for zone, age of cattle, herd type, and transmission season. RESULTS: Overall, BTV antibodies were detected in 156 (1.5%) samples. Estimated seroprevalence in 2000, 2001, and 2002 was 1.49%, 0.97%, and 2.18%, respectively. Risk of being seropositive for BTV was associated with geographic zone and age. Seroprevalence increased progressively from northern to southern zones, with no evidence of BTV infection in the northernmost zone. In the southernmost zone, annual seroprevalence ranged from 8.65% to 11.00%. Adult cattle were 2.35 times as likely as juvenile cattle to be seropositive. CONCLUSIONS AND CLINICAL RELEVANCE: Overall seroprevalence was lower than has been reported for Illinois cattle. Bluetongue virus antibodies were distributed heterogeneously in this region. Only in the southernmost zone was seroprevalence consistently > 2%. Regionalization of BTV risk based on state borders does not account for such variability. Serologic data could be combined with landscape, climate, and vector data to develop predictive models of BTV risk within transitional regions of the United States.  相似文献   
115.
It is estimated that half the soil carbon globally is in the subsoil, but data are scarce. We updated estimates of subsoil organic carbon (OC) in England and Wales made by Bradley et al. (2005) using soil and land‐use databases and compared the results with other published data. We estimated that the soils of England and Wales contained 1633, 1143 and 506 Tg of OC at 0–30, 30–100 and 100–150 cm depths, respectively. Thus, half of the soil OC was found below 30 cm depth. Peat soils accounted for the largest proportion, containing 44% of all the OC below 30 cm despite their small areal extent, followed by brown soils, surface‐water gley soils, ground‐water gley soils and podzolic soils. Peat soils had more than 25% of their profile OC per unit area in the 100–150 cm depth, whereas most other soils had <8% at this depth. The differences between soil types were consistent with differences in soil formation processes. Differences in depth distributions between land uses were small, but subsoil OC stocks in cultivated soils were generally smaller than in soils under grassland or other land uses. Data on subsoil OC stocks in the literature were scarce, but what there was broadly agreed with the findings of the above database exercise. There was little evidence by which to assess how subsoil OC stocks were changing over time.  相似文献   
116.
Cellular immune responses of peripheral blood lymphocytes to canine distemper virus and measles virus were determined in vaccinated or infected gnotobiotic dogs, using the technique of syncytia inhibition. Cross-reactivity between viruses was detected in both groups of dogs. Peak responses in vaccinated dogs occurred 11 days after vaccination and declined to base-line levels by 3 weeks, whereas responses in infected dogs were present 30 days after inoculation. Fractionation experiments with peripheral blood lymphocytes indicated that synctia inhibition is probably mediated by T lymphocytes.  相似文献   
117.
Coliform myositis was diagnosed in a young calf with signs of pain, swelling, and edema of the right hind limb. Diagnostic methods included bacteriologic culture and antimicrobial susceptibility testing. The organism isolated was a gas-producing Escherichia coli. The infection responded to administration of trimethoprim/sulfadiazine, to which the organism was susceptible in vitro.  相似文献   
118.
Cattle, ethanol, and biogas: Does closing the loop make economic sense?   总被引:1,自引:0,他引:1  
The recent surge in US ethanol production has led to an increase in the amount of byproduct feed available to cattle producers. The impetus behind the increase in ethanol production is US policy to increase the production and use of renewable fuels. Manure from cattle production can also be used to produce a renewable fuel: methane. By co-locating a cattle feedlot with an existing ethanol plant, there may be synergies between ethanol, methane and cattle production. Byproduct from the ethanol plant can be fed to feeder cattle, manure from the cattle used in methane production, and the methane used as an energy source in the ethanol plant. Alternatively, methane can be used to generate electricity. We investigate the economic advisability of these systems. Using cost estimates for construction and operating a feedlot and anaerobic digester for methane production, we project revenues and costs over a 20-year time period. Our findings are consistent with studies that have considered just cattle production and methane production. The investment required to build a slatted-floor feedlot and concrete anaerobic digester cannot be justified under existing economic conditions. An increase in renewable fuel/electricity subsidies of at least $0.053 per kW h or $72 per 1000 m3 of methane are required to just break even when methane is used to produce electricity or is burned for thermal energy, respectively.  相似文献   
119.
The ring‐tailed coati (Nasua nasua) is a procyonid whose population is in sharp decline. Therefore, studies are needed to better understand the reproduction of this animal. For this reason, this study aimed to evaluate the morphology, morphometry and sperm ultrastructure of ring‐tailed coati sperm. Four captive adult males were used for this study. Slides stained with Bengal Rose were used for the morphometric and morphologic analyses. The length and width of the head were measured, as well as the length of the midpiece and tail and the total length of the sperm. Scanning electron microscopy and transmission electron microscopy were used for the ultrastructural analyses. The most obvious morphological abnormalities observed were coiled tails (6.1 ± 8.7%) and the lack of acrosomes (5.4 ± 4.4%). Regarding the morphometry, the measurements of the head (length × width), midpiece (length) and tail (length) were (mean ± SD) 6.2 ± 0.4 × 8.1 ± 0.6 μm, 14.1 ± 0.5 and 63.9 ± 4.1 μm, respectively, and the total length of the sperm was 86.1 ± 4.3 μm. Through electron microscopy, the presence of electron‐lucent points in the nucleus and the presence of approximately 55 mitochondrial spirals in the midpiece were identified. The data obtained in this study provide detailed information on the sperm characteristics of coatis and may inform future research on germplasm conservation, both for this species and other threatened procyonids.  相似文献   
120.
A series of asymmetrical thio-bis-N-methylcarbamate derivatives were synthesised by combining 3,4-methylenedioxyphenyl N-methylcarbamate with different N-methylcarbamates. Some of these derivatives combined good insecticidal activity against the housefly with reduced acute oral toxicity to mice. The thio-bis-N-methylcarbamates obtained exhibited a lower anti-cholinesterase activity than the more toxic of the two carbamate moieties contained in each combination. The weak synergism of piperonyl butoxide on the insecticidal activity of the derivatives suggests the possibility of a self synergism phenomenon caused by the 3,4-methylenedioxyphenyl group.  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