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   135632篇
  免费   7787篇
  国内免费   70篇
林业   5577篇
农学   4644篇
基础科学   862篇
  17483篇
综合类   18955篇
农作物   5087篇
水产渔业   7089篇
畜牧兽医   72969篇
园艺   1798篇
植物保护   9025篇
  2020年   1171篇
  2019年   1410篇
  2018年   2229篇
  2017年   2489篇
  2016年   2295篇
  2015年   1999篇
  2014年   2451篇
  2013年   5731篇
  2012年   4350篇
  2011年   5163篇
  2010年   3502篇
  2009年   3547篇
  2008年   5152篇
  2007年   4970篇
  2006年   4641篇
  2005年   4262篇
  2004年   4108篇
  2003年   4175篇
  2002年   3808篇
  2001年   4773篇
  2000年   4641篇
  1999年   3747篇
  1998年   1471篇
  1997年   1452篇
  1996年   1341篇
  1995年   1599篇
  1994年   1327篇
  1993年   1359篇
  1992年   2762篇
  1991年   2901篇
  1990年   2866篇
  1989年   2897篇
  1988年   2610篇
  1987年   2592篇
  1986年   2539篇
  1985年   2400篇
  1984年   1956篇
  1983年   1715篇
  1982年   1154篇
  1979年   1649篇
  1978年   1284篇
  1977年   1135篇
  1976年   1100篇
  1975年   1164篇
  1974年   1357篇
  1973年   1448篇
  1972年   1378篇
  1971年   1195篇
  1970年   1247篇
  1969年   1146篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
RFamide‐related peptide‐3 (RFRP‐3), the mammalian ortholog of gonadotropin‐inhibiting hormone, has been implicated as a mediator between reproduction and energy balance. This study aimed to investigate the physiological effects of RFRP‐3 on the process of ovarian development in food‐restricted pre‐pubertal ewes. The results showed that food restriction significantly inhibited the ovarian development and follicular growth. The data of qPCR in the hypothalamic–pituitary–ovarian (HPO) axis showed that food restriction not only upregulated RFRP‐3 mRNA expression but also downregulated the mRNA expression of gonadotropin‐releasing‐hormone receptor, follicle‐stimulating hormone receptor and luteinizing hormone receptor (LHR). Immunohistochemistry of RFRP‐3 in the ovaries suggested that RFRP‐3 may regulate the follicular development. These results suggested that the changes of RFRP‐3 in response to food restriction might influence the HPO axis and inhibit ovarian development.  相似文献   
942.
Conjugated linoleic acids (CLA), 9-cis:11-trans and 10-trans:12-cis, have been shown to be able to modify some immune cells parameters and plasma lipids in a variety of experiment models. Since lymphocytes and polymorphonuclear cells (PMNC) have a large spectrum functions in the immune response, the knowledge in this field has to be expanded. Beagle dogs were fed a control diet or a CLA supplemented diet for nine months. Blood was collected for biochemical analysis and lymphocyte and PMNC isolation. PMNC were assayed for lysosome content, phagocytic activity and superoxide anion production. A lymphocyte proliferation capacity assay was done. The CLA fed dogs had a 34% reduction in total cholesterol (P < 0.05), 28% in LDL (P < 0.05) and 28% non-HDL-cholesterol (P < 0.05). Neither of the PMNC parameters evaluated demonstrated significant alteration. Lymphocytes from CLA group increased by 45% their mitotic capacity (P < 0.05). Our study demonstrates that CLA can successfully modify the lipid profile of dogs (monogastrics) when fed at reasonable levels, but did not significantly alter inflammatory function as would generally predicted. Further, we had some indication that CLA modulated T cell responsiveness.  相似文献   
943.
The objectives of the present study were to compare Mycoplasma hyopneumoniae (Mh) colonization and serologic status on Mh vaccinated and non-vaccinated sows and to assess the effect of sow vaccination on colonization and serologic status of their piglets at weaning as well as presence of enzootic pneumonia (EP) lung lesions at slaughter. Fifty sows (25 vaccinated and 25 unvaccinated) as well as five of their piglets were included in the study. Blood samples and nasal swabs from sows at 7 weeks pre-farrowing and 1 week post-farrowing and from piglets at 3-4 weeks of age were taken. Nasal swabs and sera were tested by a nested polymerase chain reaction (nPCR) to detect Mh DNA and by an enzyme-linked immunosorbent assay (ELISA) test to detect antibodies to the pathogen, respectively. Finally, at 23 weeks of age, pigs were sent to the slaughter where the extension of EP-compatible gross lesions was assessed. Vaccination with two doses of Mh vaccine resulted in a significantly higher (p<0.05) percentage of seropositive sows than in the non-vaccinated group at 1 week post-farrowing. On the contrary, no statistical significant differences were found in the number of nasal nPCR positive sows among different treatments (p>0.05). At 3-4 weeks of age, a significantly higher percentage (p<0.001) of seropositive piglets came from vaccinated than from non-vaccinated sows. Although the number of Mh infected piglets coming from non-vaccinated sows was higher than the one from vaccinated sows, the difference was not statistically significant (p>0.05). Overall, piglets from vaccinated sows had a significant lower (p<0.05) mean of EP-compatible lung lesions (1.83+/-2.8) than piglets from non-vaccinated sows (3.02+/-3.6). Under the conditions described in this study, sow vaccination did not affect sow or piglet colonization but increased the percentage of seropositive sows and piglets at weaning and reduced significantly the mean EP-compatible lung lesion scoring at slaughter.  相似文献   
944.
OBJECTIVES: To study whether natural short tail in adult Pembroke Welsh corgi is associated with congenital spinal defects. To report anatomical defects in two newborn tailless puppies from short-tailed parents, and to check whether they were homozygous for the dominant mutation in the T-gene (C295G). METHODS: The vertebral column of 19 adult dogs with natural short tail, from short-tail x long-tail crossings, was radiographically examined. Two tailless puppies were radiographed and submitted for necropsy. Samples from the puppies, their parents and five siblings were analysed for the mutation of the T-gene. RESULTS: No congenital spinal defects were diagnosed in any of the short-tailed dogs. The tailless puppies had anorectal atresia, had multiple spinal defects and were homozygous for the mutation in the T-gene. CLINICAL SIGNIFICANCE: As tail docking is forbidden in many countries, breeding Pembroke Welsh corgis with natural short tail is becoming increasingly common. Previous studies indicated that the mutation in homozygotes is lethal in early fetal life. It is of clinical significance that natural short tail is probably not associated with congenital spinal defects, as is known from studies of other species, and that homozygotes for this mutation with serious anatomical defects may be born.  相似文献   
945.
Hyperadrenocorticism in ferrets is associated with increased circulating concentrations of adrenal androgens, whereas plasma concentrations of cortisol and ACTH are usually not affected. Here, we report on a 5-year-old castrated male pet ferret (Mustela putorius furo) in which the major presenting signs were polyuria and polyphagia. Routine biochemistry values were within their reference ranges. The urinary corticoid:creatinine ratio (UCCR) was increased and the plasma ACTH concentration was suppressed. Abdominal ultrasonography revealed an enlarged right adrenal gland and atrophy of the left adrenal gland. Administration of hCG resulted in an increase of plasma cortisol and androstenedione concentrations. Based on these findings LH/hCG-dependent hypercortisolism and hyperandrogenism were suspected and treatment was started with a depot GnRH-agonist implant containing 9.4mg deslorelin. Within 3 weeks after placement of the implant all clinical signs had disappeared. Three months later the endocrine parameters had normalized, while abdominal ultrasonography revealed that the right adrenal gland had diminished in size and the left adrenal gland was considered of normal size. No recurrences of clinical signs were seen within 2 years after placement of the deslorelin implant. At that time urinary corticoid and plasma hormone concentrations were within their reference ranges, and no further change in the size of the adrenal glands was seen. In conclusion, this is the first confirmed case of LH-dependent hypercortisolism in a ferret that was treated successfully with a depot GnRH-agonist.  相似文献   
946.
The mitochondrial cytochrome oxidase I gene was partially sequenced for 164 Ancylostoma caninum individuals, originating from five different localities in Brazil, with the aim of describing the genetic diversity and genetic structure of Brazilian hookworm populations. Allelic and nucleotide diversity were moderate (overall h=0.88 and pi=0.016) and were similar among cities. There was moderate genetic differentiation among the populations sampled (approximately Phi(ST)=0.12) and a weak but nonsignificant correlation between geographical and genetic distance. This genetic structure was similar to that observed among populations of the human hookworm, Necator americanus, but distinct from that typically found in trichostrongylid nematode parasites of livestock. Thus, a pattern of different genetic structures among different groups of nematodes is emerging. We also observed a few individuals that had a highly divergent mtDNA sequence (almost 7% sequence divergence from the other sequences). These results in combination with data from other studies suggest that A. caninum populations worldwide consist of a mix of previously differentiated populations, or perhaps even cryptic species. This study contributes to the knowledge of genetic structure and diversity of hookworms, which in turn will be useful in developing methods for their control.  相似文献   
947.
The management of a suspected index case of highly pathogenic avian influenza (AI) is crucial to the subsequent actions aimed at limiting the spread of infection and ultimately in prompt eradication of the virus. In this study, the legislative basis, basic concepts and actions behind a successful disease management programme are reviewed. These include actions at local veterinary headquarters and at farm level which must be coordinated centrally to ensure the flow of information essential to decision making. The success of any emergency intervention strategy is dependent upon the level of preparedness, including action plans to source manpower, equipment and on the degree of communication between relevant parties. Availability of information on the successes and failures in field outbreak management, will inevitably result in improved AI control worldwide.  相似文献   
948.
949.
Fifty faecal samples from diarrheic calves between 1 and 6 months old were collected per rectum from 5 farms around Petaling District in Selangor, Malaysia for Cryptosporidium species detection and genotyping investigation. Oocysts were purified using sedimentation and gradient centrifugation, then examined by immunofluorescence assay (IFAT). Genomic DNA was extracted from all samples and nested PCR was performed to amplify the SSU rRNA gene. Eighteen samples (36%) were positive for Cryptosporidium species by PCR. The sequence and phylogenetic analysis of 14 isolates indicated that Cryptosporidium parvum was most common (11 isolates) followed by Cryptosporidium deer-like genotype (3 isolates). The present work reports the first data on Cryptosporidium genotyping from cattle in Malaysia.  相似文献   
950.
This article examines the job expectations of applicants as reported by recruiters interviewing food-supply veterinary medicine (FSVM) candidates and the career-choice decision factors used by year 3 and 4 veterinary students pursuing careers in FSVM. The responses of 1,047 veterinary recruiters and 270 year 3 and 4 students with a food-supply focus from 32 colleges of veterinary medicine in the United States and Canada were examined. Recruiters were asked to report the two most important job factors applicants took into account when deciding to accept an offer; students were asked the two most important reasons for choosing a career in FSVM and the two most important benefits of working as a food-supply veterinarian. Recruiters reported that high salaries and good benefits are the two most important decision factors. Interest in the food-animal career area and a desire for a rural, outdoor lifestyle were the top reasons students gave for choosing an FSVM career. Students saw the enjoyment of working with and helping producers and food animals as the most important benefits of a career in FSVM.  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