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   9280篇
  免费   417篇
  国内免费   14篇
林业   588篇
农学   229篇
基础科学   84篇
  1590篇
综合类   1613篇
农作物   365篇
水产渔业   822篇
畜牧兽医   3373篇
园艺   278篇
植物保护   769篇
  2024年   19篇
  2023年   59篇
  2022年   90篇
  2021年   155篇
  2020年   211篇
  2019年   208篇
  2018年   206篇
  2017年   218篇
  2016年   210篇
  2015年   184篇
  2014年   253篇
  2013年   449篇
  2012年   542篇
  2011年   708篇
  2010年   368篇
  2009年   352篇
  2008年   635篇
  2007年   624篇
  2006年   592篇
  2005年   574篇
  2004年   608篇
  2003年   571篇
  2002年   504篇
  2001年   98篇
  2000年   90篇
  1999年   93篇
  1998年   81篇
  1997年   70篇
  1996年   67篇
  1995年   62篇
  1994年   61篇
  1993年   50篇
  1992年   57篇
  1991年   49篇
  1990年   58篇
  1989年   49篇
  1988年   42篇
  1987年   45篇
  1986年   34篇
  1985年   34篇
  1984年   34篇
  1983年   40篇
  1982年   26篇
  1981年   25篇
  1980年   29篇
  1979年   24篇
  1978年   20篇
  1976年   20篇
  1975年   22篇
  1974年   15篇
排序方式: 共有9711条查询结果,搜索用时 0 毫秒
41.
Spontaneous endometriosis is an estrogen-dependent, progressive and painful disease that affects a variety of nonhuman primates, including several species of baboons (Papio sp.). This case documents multimodal management of severe endometriosis in a captive female baboon within a zoological institution. An 18-yr-old, intact female Guinea baboon (Papio papio) was found to have an enlarged uterus. Fifteen months post ovariohysterctomy, scarring associated with endometrial tissue resulted in ureteral strictures, bilateral hydronephrosis, and azotemia. Cystoscopic placement of bilateral ureteral stents with fluoroscopy was performed and resulted in short-term clinical improvement. The animal's condition declined and euthanasia was elected 4 mo after ureteral stent placement. Severe endometriosis with secondary inflammation resulting in bilateral hydroureter and hydronephrosis, as well as concurrent cystitis, ureteritis, and pyelonephritis were confirmed at necropsy. Despite possible complications, ureteral stents can be considered a useful therapeutic option in patients with ureteral disease.  相似文献   
42.
A pilot study was initiated to determine the seroprevalence of bovine viral diarrhea virus (BVDV) neutralizing antibodies in finisher hogs in Ontario swine herds, including 2 swine herds with clinical syndromes suspicious of BVDV. No herds were positive for BVDV antibodies by virus neutralization. The 2 swine herds with clinical disease suggestive of pestivirus infection were also negative for antibodies to BVDV in indirect fluorescent antibody assays. Prevalence of BVDV in Ontario swine farms is negligible.  相似文献   
43.
We conducted an on-line questionnaire to assess stakeholder perceptions about damage to silo bags by vertebrate species in Argentina. We received 190 responses reporting damage to silo bags by vertebrates from the Pampas, Espinal, and Chaco regions. More than two-thirds of respondents indicated armadillos were the main harmful species, followed by rodents and dogs. Frequency of damage to silo bags was higher for armadillos (79%) than for the other species together (64%), and was higher in the Pampas (79%) than in the other regions (45–66%), suggesting a latitudinal gradient of damage. Economic losses were reported as “medium” or “high” by 37% of respondents. According to these stakeholders, damage to silo bags by vertebrates is a regionally extended phenomenon in most of the Argentine agricultural region and it is recognized as economically relevant by these stakeholders. Management of this conflict should focus on environmental aspects.  相似文献   
44.
Evapotranspiration (ET) is a key component limiting groundwater recharge past the root zone in semiarid regions. Vegetation management may alter groundwater recharge if ET is altered due to changes in vegetation type or cover. This study quantifies changes in groundwater recharge following vegetation cover change from native woodland to pasture in a semiarid region of southwest Texas. The Carrizo–Wilcox aquifer is a valuable groundwater resource in this area, where overuse by dependent farming practices has lowered aquifer levels significantly in the last 85 yr. Combining data from short-term (30 mo) monitoring of the changes in soil moisture and long-term (5–30 yr) changes in total soil chloride indicated deep drainage increased slightly where land had been cleared of vegetation. Annual recharge rates below rooting depths (standardized to 155 cm) averaged only 0.72 ±  mm · yr-1 (mean ± SE) in areas not cleared of woody vegetation, as estimated by chloride mass balance. Upon clearing, 72% of the total chloride naturally occurring in the soil profile was flushed away within 30 yr, leading to an estimated 2.59 ±  mm · yr-1 additional recharge. Deep soil moisture in recently cleared land increased by up to 17% during the growing season of wet years (double the average rainfall) but did not increase in dry or normal precipitation years, providing supporting evidence that more water penetrated below the roots under certain environmental conditions. These results demonstrate that brush management can increase recharge by modest, but measurable, amounts depending on site-specific soil characteristics and degree of reduction in vegetation.  相似文献   
45.
46.
Despite the early notion that canine oral malignant melanoma is radioresistant, recent data suggest that external beam radiotherapy is effective in local tumor control. However, optimal fractionation schedules have not been established. The high rate of regional and distant metastasis is another problem that has hindered long-term control. The role of chemotherapy in the management of canine oral melanoma has also not been determined. In this study, data from 140 dogs irradiated at North Carolina State University were evaluated with the following objectives: (1) to compare the efficacy of three radiation therapy protocols (36 Gy, 9 Gy x 4 fractions; 30 Gy, 10 Gy x 3 fractions; or >45 Gy, 2-4 Gy x 12-19 fractions) for the treatment of dogs with oral malignant melanoma, (2) to identify any host or tumor factors influencing prognosis, and (3) to determine the impact of systemic chemotherapy on treatment outcome. Information regarding response to therapy, disease progression, and survival were determined from the medical records or from information obtained by telephone or mail survey. Relationships between host, tumor, and treatment variables and outcome measures (response, time to first event, and survival) were evaluated using Fisher's exact test (response) and the Cox regression model (time to first event and survival). The median time to first event for the 140 dogs was 5.0 months (95% C.I., 4-6 months) and the median survival was 7.0 months (95% C.I., 6-9 months). In the univariate analysis, the following variables were associated with increased time to first event and survival: (1) rostral tumor sublocation; (2) lack of bone lysis observed on skull imaging, and (3) microscopic tumor burden. In a multivariate analysis of 111 dogs with complete data for these variables, tumor sublocation, bone lysis, and tumor volume were identified as joint predictors of time to first event (p < .001, p < .001, and p = .04, respectively) and survival (p < .001, p < .001, and p = .05, respectively). There were no differences in response, time to first event and survival between the three radiation therapy protocols used. Systemic chemotherapy had no impact on the development of metastatic disease, time to first event, or survival, although the dosages used in this study were suboptimal. External beam radiation therapy is effective in local disease control of canine oral malignant melanoma; however, the optimal fractionation scheme has yet to be determined. The high metastatic rate observed with this disease and the inefficacy of systemic chemotherapy indicate that further investigation into novel therapies is warranted.  相似文献   
47.
OBJECTIVE: To determine clinical, radiographic, and scintigraphic abnormalities in and treatment and outcome of horses with trauma-induced osteomyelitis of the proximal aspect of the radius. DESIGN: Retrospective study. ANIMALS: 5 horses. PROCEDURE: Data collected from the medical records included signalment; history; horse use; degree of lameness; radiographic, ultrasonographic, and scintigraphic findings; treatment; and outcome. RESULTS: Duration of lameness prior to referral ranged from 14 to 60 days. Mean severity of lameness was grade 3 of 5, and all horses had a single limb affected. All horses had signs of pain during elbow joint manipulation and digital palpation over the lateral aspect of the proximal end of the radius. Radiographic lesions consisted of periosteal proliferation, osteolysis, and subchondral bone lysis. Scintigraphy in 3 horses revealed intense pharmaceutical uptake diffusely involving the proximal end of the radius. Two horses had sepsis of the elbow joint. All horses were treated with antimicrobials long-term; 1 horse was also treated by local perfusion of the radial medullary cavity through an indwelling cannulated screw. At follow-up, all horses had returned to their previous function. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that osteomyelitis of the proximal end of the radius can result from a traumatic injury to the antebrachium. Because lesions may be an extension of septic arthritis, a thorough examination of the wound area and elbow joint is recommended. Prolonged systemic antimicrobial treatment can result in a successful outcome.  相似文献   
48.
A group discussion on the theme of "leadership" has been a central event in the annual Cornell Leadership Program for Veterinary Students since 1990. However, these discussions were often unfocused and did not readily demonstrate the leadership skills of distinguished guests who were invited to participate. Since 1998, a new format for this session has been developed in which students and guests are assigned individual roles in a scenario that is unfolded by a moderator over two to three hours. This role-playing exercise ensures that every student is obliged to participate and has an opportunity to practice such leadership skills as critical thinking, verbal communication, and decision making under pressure and with inadequate information. The distinguished guests, in their assigned roles, are able to interact freely with the student fellows and thus demonstrate their expertise as experienced leaders. This challenging experience has become an enjoyable part of the 10-week Leadership Program and one that shows the importance of leadership skills for those who aspire to careers in the biomedical sciences.  相似文献   
49.
The control of ovine Johne's disease (OJD) is important for domestic trade, the viability of farming units and possibly also for public health. Current strategies in Australia have included quarantine and pasture spelling to decrease prevalence and shedding rates and reduce numbers of Mycobacterium paratuberculosis (Mptb) ingested by susceptible sheep. However, alternative procedures are needed and vaccination with Gudair has recently commenced. This review examines prospects for the control of OJD by chemotherapy, vaccination and mycophages. Current chemotherapeutic regimes for treatment of M. paratuberculosis in ruminants are prohibitively expensive and of dubious efficacy, and apart from environmental concerns, mycophage therapy lacks a track record of success against intracellular bacteria. There is substantial evidence that live and killed mycobacterial vaccines reduce the incidence of clinical disease and shedding rates in OJD. An appraisal of recent experimental results suggests that neonatal vaccination with a defined dose of M. paratuberculosis offers the best prospects for the induction of protective Th1-type immunity.  相似文献   
50.
OBJECTIVE: To measure changes in the modulus of elasticity (E) and describe histologic findings after extracorporeal shock wave therapy and radial pressure wave therapy on equine cortical bone specimens. SAMPLE POPULATION: 16 bone specimens from the proximodorsal cortex of an equine third metacarpal or metatarsal bone. PROCEDURE: Baseline E was determined by the density (p) and unidirectional ultrasound transmission velocity (C) of each specimen according to the equation E = pC2. Eight specimens were treated with 500 pulses of 0.15 mJ/mm2 of extracorporeal shock wave therapy, and 8 specimens were treated with 500 pulses of 0.16 mJ/mm2of radial pressure wave therapy. After treatment, C was determined again. Four treatment sessions resulted in 2,000 pulses and 5 C measurements. The p of each sample was measured again. Mean post-treatment E was calculated for each group. Nondecalcified sections of all specimens were stained with toluidine blue or basic fuchsin for histologic evaluation. RESULTS: Overall treatment group effect was not significant for C or E. Final E was not different from baseline values for extracorporeal shock wave therapy and radial pressure wave therapy. No histologic changes could be attributed to either treatment modality. CONCLUSIONS AND CLINICAL RELEVANCE: Extracorporeal shock wave therapy and radial pressure wave therapy did not affect the material properties of equine bone at the energy and pulse values used in this study.  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