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   65610篇
  免费   2224篇
  国内免费   234篇
林业   2509篇
农学   2159篇
基础科学   492篇
  9044篇
综合类   8996篇
农作物   2524篇
水产渔业   2844篇
畜牧兽医   34500篇
园艺   759篇
植物保护   4241篇
  2021年   551篇
  2020年   650篇
  2019年   750篇
  2018年   1208篇
  2017年   1323篇
  2016年   1185篇
  2015年   921篇
  2014年   1239篇
  2013年   3216篇
  2012年   1999篇
  2011年   2434篇
  2010年   1711篇
  2009年   1709篇
  2008年   2357篇
  2007年   2191篇
  2006年   1978篇
  2005年   1695篇
  2004年   1652篇
  2003年   1593篇
  2002年   1416篇
  2001年   1877篇
  2000年   1870篇
  1999年   1385篇
  1998年   601篇
  1997年   608篇
  1995年   629篇
  1992年   1016篇
  1991年   1151篇
  1990年   1081篇
  1989年   1113篇
  1988年   983篇
  1987年   961篇
  1986年   1006篇
  1985年   909篇
  1984年   757篇
  1983年   690篇
  1979年   965篇
  1978年   720篇
  1977年   678篇
  1976年   771篇
  1975年   807篇
  1974年   924篇
  1973年   900篇
  1972年   853篇
  1971年   671篇
  1970年   715篇
  1969年   789篇
  1968年   746篇
  1967年   755篇
  1966年   724篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
112.
A diagnostic procedure, based on a polymerase chain reaction method (PCR) was developed to detect infection of crayfish with the Oomycete Aphanomyces astaci. A set of oligonucleotide primers was designed to specifically amplify A. astaci DNA in the ITS region surrounding the 5.8S rDNA gene. The PCR amplifies a 115 bp amplicon. The specificity of the primers was demonstrated by testing on 27 A. astaci strains and against 20 non-A. astaci Oomycetes and 5 fungal species. Most of the non-A. astaci Oomycete or fungal species included in the study are either known parasites of freshwater crayfish cuticle or can be found in their natural environment. Specificity was also tested against crayfish tissue and some known parasites and bacteria infecting crayfish.

A protocol for the extraction of A. astaci DNA from infected crayfish tissue was developed. The optimised method allows the detection of two genome equivalents of purified A. astaci genomic DNA.

The method was tested on noble crayfish (Astacus astacus), artificially infected with A. astaci. Detection of A. astaci was possible at the very first time of sampling, which was 2 days after the beginning of spore exposure.  相似文献   

113.
The Cornell Net Carbohydrate and Protein System (CNCPS), a mechanistic model that predicts nutrient requirements and biological values of feeds for cattle, was modified for use with sheep. Published equations were added for predicting the energy and protein requirements of sheep, with a special emphasis on dairy sheep, whose specific needs are not considered by most sheep-feeding systems. The CNCPS for cattle equations that are used to predict the supply of nutrients from each feed were modified to include new solid and liquid ruminal passage rates for sheep, and revised equations were inserted to predict metabolic fecal N. Equations were added to predict fluxes in body energy and protein reserves from BW and condition score. When evaluated with data from seven published studies (19 treatments), for which the CNCPS for sheep predicted positive ruminal N balance, the CNCPS for sheep predicted OM digestibility, which is used to predict feed ME values, with no mean bias (1.1 g/100 g of OM; P > 0.10) and a low root mean squared prediction error (RMSPE; 3.6 g/100 g of OM). Crude protein digestibility, which is used to predict N excretion, was evaluated with eight published studies (23 treatments). The model predicted CP digestibility with no mean bias (-1.9 g/100 g of CP; P > 0.10) but with a large RMSPE (7.2 g/100 g of CP). Evaluation with a data set of published studies in which the CNCPS for sheep predicted negative ruminal N balance indicated that the model tended to underpredict OM digestibility (mean bias of -3.3 g/100 g of OM, P > 0.10; RMSPE = 6.5 g/100 g of OM; n = 12) and to overpredict CP digestibility (mean bias of 2.7 g/100 g of CP, P > 0.10; RMSPE = 12.8 g/100 g of CP; n = 7). The ability of the CNCPS for sheep to predict gains and losses in shrunk BW was evaluated using data from six studies with adult sheep (13 treatments with lactating ewes and 16 with dry ewes). It accurately predicted variations in shrunk BW when diets had positive N balance (mean bias of 5.8 g/d; P > 0.10; RMSPE of 30.0 g/d; n = 15), whereas it markedly overpredicted the variations in shrunk BW when ruminal balance was negative (mean bias of 53.4 g/d, P < 0.05; RMSPE = 84.1 g/d; n = 14). These evaluations indicated that the Cornell Net Carbohydrate and Protein System for Sheep can be used to predict energy and protein requirements, feed biological values, and BW gains and losses in adult sheep.  相似文献   
114.
ObjectiveTo evaluate the efficacy of combined femoral and sciatic nerve blocks as an alternative to epidural anesthesia and analgesia in dogs undergoing stifle surgery under general anesthesia.Study designProspective, blinded, randomized, clinical comparison.AnimalsTwenty dogs weighing 37 ± 11 (mean ± SD) kg, aged 3 (1–8) [median (minimum–maximum)] years undergoing elective unilateral tibial-plateau leveling osteotomy.MethodsDogs were assigned randomly to receive either epidural anesthesia (bupivacaine 0.5%, 0.5 mg kg?1 + morphine 0.1%, 0.1 mg kg?1, in 0.2 mL kg?1; EPID) or femoral and sciatic nerve blocks (Bupivacaine 0.5%, 0.1 mL kg?1, was administered at each site; F + S) guided by electrolocation. All patients received a standard general anesthesia technique. Pain and sedation were scored (on scales of 0–10 and 0–3, respectively) pre-operatively, at extubation, and at 1, 4 and then every 4 hours thereafter up to 24 hours. Postoperatively, hydromorphone was administered to any patient with a pain score of >5 or whenever the blinded caregiver determined that more hydromorphone was necessary. Intraoperative heart rate (HR), mean arterial pressure (MAP), end tidal isoflurane (FE′ISO), body temperature, post-operative pain scores, time to first hydromorphone dose after surgery, time to first feeding, time to first drinking, time to first urination, time to first ambulation (walk on a lead) and cumulative dose of hydromorphone were recorded.ResultsIntra-operatively, FE′ISO and MAP were significantly lower in the EPID group (p = 0.05 and p = 0.04, respectively). Postoperatively, the cumulative hydromorphone consumption (p = 0.04) and the incidence of urinary retention (p = 0.03) were higher in the EPID group.Conclusion and clinical relevance F + S is a practical alternative to EPID that produces less urine retention and reduces opioid consumption in the 24 hours after surgery. EPID might be associated with a lower isoflurane requirement and lower systemic blood pressure.  相似文献   
115.
OBJECTIVES: (1) To review the signalment, clinical, and histological features of canine limbal melanoma; (2) to perform pedigree analysis on breeds predisposed to limbal melanoma to establish if common ancestry exists; and (3) to investigate if any ancestral relationship exists between canine limbal melanoma and canine anterior uveal melanoma (CAUM). DESIGN: Retrospective study. ANIMALS STUDIED: Thirty dogs with limbal melanoma. METHODS: Medical records of patients were reviewed. Follow-up information was obtained by re-examination of patients or telecommunications with the referring veterinary surgeons or the owners. Pedigrees were analyzed for common ancestry amongst affected dogs. RESULTS The mean age (+/- SD) at diagnosis was 6.2 (+/- 2.75) years with a range from 1 to 11 years. There was a bimodal distribution of ages with a peak at 3-4 years and a peak at 7-10 years. There was no eye predilection or predisposition for sex or coat color. Twenty-five (83%) of the limbal melanomas occurred within a dorsal arc from the dorsomedial to the ventrolateral limbus. Golden retrievers were four times more common in the melanoma group compared to the Animal Health Trust population (P < 0.0001). Labrador retrievers were three times more common in the melanoma group (P=0.01). Pedigree analysis on eight Golden retrievers [limbal melanoma (n=5), CAUM (n=2) and diffuse ocular melanosis (n=1)], revealed a pattern of inter-relatedness consistent with the condition(s) being caused, at least in part, by a genetic mutation(s). A similar level of inter relatedness was evident in six Labrador retrievers (limbal melanoma (n=2) and CAUM (n=4)). In 5/22 cases (23%), histological features suggestive of malignancy were present including intratumor necrosis in 4/22 cases (18%) and cellular atypia in 1/22 cases (5%). CONCLUSIONS: In Golden and Labrador retrievers there is evidence that limbal melanomas, CAUM and ocular melanosis are at least in part heritable and that the same genetic mutation(s) may be causally associated with melanocytic disease at different ocular sites. The same genetic mutation(s) may be present in these two breeds. Histology should be performed on all cases to identify those with greater malignant potential.  相似文献   
116.
Three domestic shorthair cats, one male and two females, 17.6+/-6.5 months old and 3.5+/-0.4 kg body weight, were admitted with a 6.6+/-1.1 (range 6-8) month history of recurrent bouts of obstipation. Defecatory difficulties had started within a month of conservatively managed pelvic fractures. Clinical and radiographic examination revealed severe colonic distension with faeces and a narrow pelvic canal caused by malunion of the pelvic fractures. A pelvic symphyseal distraction-osteotomy (SDO) technique was performed, using a novel metal spacer of spirally fashioned orthopaedic wire. Pelvic canal enlargement allowed the insertion of an index finger into the rectum. Following this, no further episodes of obstipation occurred during a follow-up period of 1-3 years. The SDO technique may be successful for the treatment of obstipation secondary to post-traumatic pelvic canal stenosis in the cat, if the colon motility is not already permanently affected.  相似文献   
117.
Intracranial neoplasia is commonly diagnosed in dogs and can be treated by a variety of methods, but formal comparisons of treatment efficacy are currently unavailable. This review was undertaken to summarize the current state of knowledge regarding outcome after the treatment of intracranial masses in dogs, with the aim of defining optimal recommendations for owners. This review summarizes data from 794 cases in 22 previously published reports and follows PRISMA guidelines for systematic review. A Pubmed search was used to identify suitable articles. These then were analyzed for quality and interstudy variability of inclusion and exclusion criteria and the outcome data extracted for summary in graphs and tables. There was a high degree of heterogeneity among studies with respect to inclusion and exclusion criteria, definition of survival periods, and cases lost to follow‐up making comparisons among modalities troublesome. There is a need for standardized design and reporting of outcomes of treatment for brain tumors in dogs. The available data do not support lomustine as an effective treatment, but also do not show a clear difference in outcome between radiotherapy and surgery for those cases in which the choice is available.  相似文献   
118.
119.
Rats were successfully infected with Japanese Fasciola sp. by transplantation of juvenile worms (JW) or metacercariae (MC) into the abdominal cavity. Moreover, the rat was investigated on its suitability for different experiments with liver flukes. JWs or MCs transplanted intraperitoneally (IP) matured in the bile duct of rats. Moreover, more stable infections were established by inoculation of JWs than MCs. About 3 of 10-15 JWs transplanted into the abdominal cavity of a rat matured and laid eggs in the bile duct. The mean prepatent period was 63.5 days in the JW inoculated group. EPG values were kept constant at a level of 10(2)-10(3) about 100 to 230 days after the transplantation of JWs. The life span of Japanese liver flukes was estimated to be about 400 days in rats. From these results, it was concluded that the rat is suitable for various experiments with Fasciola sp.  相似文献   
120.
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