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   713篇
  免费   20篇
  国内免费   1篇
林业   18篇
农学   12篇
基础科学   2篇
  82篇
综合类   109篇
农作物   32篇
水产渔业   29篇
畜牧兽医   395篇
园艺   25篇
植物保护   30篇
  2021年   5篇
  2020年   9篇
  2019年   16篇
  2018年   9篇
  2017年   8篇
  2016年   9篇
  2015年   7篇
  2014年   17篇
  2013年   42篇
  2012年   24篇
  2011年   38篇
  2010年   10篇
  2009年   18篇
  2008年   22篇
  2007年   27篇
  2006年   31篇
  2005年   33篇
  2004年   27篇
  2003年   26篇
  2002年   16篇
  2001年   19篇
  2000年   23篇
  1999年   17篇
  1998年   7篇
  1997年   6篇
  1992年   23篇
  1991年   16篇
  1990年   10篇
  1989年   10篇
  1988年   7篇
  1987年   11篇
  1986年   12篇
  1985年   11篇
  1984年   7篇
  1983年   6篇
  1979年   4篇
  1978年   11篇
  1977年   9篇
  1976年   4篇
  1974年   5篇
  1973年   5篇
  1972年   7篇
  1971年   6篇
  1970年   9篇
  1969年   10篇
  1968年   8篇
  1967年   5篇
  1966年   7篇
  1936年   4篇
  1935年   4篇
排序方式: 共有734条查询结果,搜索用时 703 毫秒
131.
Bovine tuberculosis (TB) is an important economic problem. The incidence of TB in cattle herds has steadily risen in the UK, and badgers are strongly implicated in spreading disease. Since the mid-1970s the UK government has adopted a number of badger culling strategies to attempt to reduce infection in cattle. In this report, an established model has been used to simulate TB in badgers, transmission to cattle, and control by badger culling. Costs were supplied by the UK Government's Department for Environment Food and Rural Affairs (Defra) for badger trapping and gassing. Regardless of culling intensity or area simulated, an overall reduction in the herd breakdown rate was seen. With a high culling efficacy and no social perturbation, the mean Net Present Value of a few simulated culling strategies in an "ideal world" was positive, meaning the economic benefits outweighed the costs. Further work is required before these results could be considered definitive, as it is necessary to evaluate uncertainties and simulate less than perfect conditions.  相似文献   
132.
Although 36 DLA-DRB1 and 10 DLA-DQA1 allele sequences have been published to date, no data on individual allele frequencies exists, either for specific breeds or cross breeds, and the full extent of the polymorphism at each of these loci is still not known. We have used sequence-specific oligonucleotide probing (SSOP) to characterise a series of 367 dogs for their DRB1 and DQA1 alleles. These included individual animals from over 60 different breeds, with numbers per breed ranging from 1 to 39. DLA types were generated from 218 dogs for DRB1 and from 330 dogs for DQA1, while 181 dogs were characterised for both these loci. The frequency of individual DRB1 and DQA1 alleles showed considerable interbreed variation, e.g. 83% of West Highland White Terriers were DRB1*01 as opposed to 9% of Collies. No breed had >9 of the 22 DRB1 types defined in this study; several breeds had only two DRB1 types. DLA-DQA1 showed less variation in allele numbers per breed, but also showed considerable interbreed frequency variation. Haplotype analysis revealed over 44 different DRB1/DQA1 combinations. Of these, 25 were in a number of animals, and also in an animal that was homozygous for one or both of these loci. Some DRB1 alleles could be found in combination with several different DQA1 alleles, while others were only present in one haplotypic combination. DLA allele frequency data in normal dogs will be critical for disease association studies. It may also be possible to use haplotype data to establish the genetic relationships between different dog breeds.  相似文献   
133.
Beef production systems that increase use of unharvested forages and use animals with greater potential for gain affect age and size of animals placed on a finishing regimen. This experiment was conducted to evaluate effects of genetic potential for gain, age at the start of a finishing period, and time on feed on composition, quantity, and quality of beef produced and efficiency of production during finishing. Crossbred cows were bred by AI to Charolais or Line 1 Hereford bulls that represented potentially high (HG) or moderate growth (MG) rates, respectively, to produce spring- or fall-born calves. Steer calves from these matings were placed on an individually fed finishing diet at three ages (A). Spring-born steers were started at 6 or 18 mo of age (A6 and A18), and fall-born steers were started at 12 mo of age (A12). Slaughter times (T) were at 0, 90, 180, and 270 d for A6; 68, 136, and 204 d for A12; and 0, 45, 90, and 135 d for A18. Data collected on each animal included feed intake, growth, chemical composition of the complete body and carcass, and quantitative and qualitative assessment of the meat produced. Four steers of each sire group were slaughtered in each of the 11 A-T treatment groups, and the experiment was repeated for 2 yr in the A12 groups and 3 yr in the A6 and A18 groups (n = 237). Steers sired by HG bulls were larger and produced larger carcasses and more carcass protein than MG-sired steers (S, P < .05 or .01). Steers sired by MG bulls were fatter, had higher quality grades, and accumulated fat at a faster rate than HG-sired steers, and this effect was greater in older steers (G and GA, P < .05 or .01). Sire growth potential did not affect gain, intake, live weight efficiency, tenderness, or taste panel scores (P > .2). Steers sired by HG bulls were more efficient at producing carcass weight and carcass protein at A12 and A18 than were MG-sired steers. At the end of the finishing period, older (A18), HG-sired steers were too large with insufficient fat by current industry standards, and younger (A6), MG-sired steers were too small. Our conclusions are that both HG- and MG-sired steers can produce acceptable carcasses for current market standards with comparable efficiencies of live-weight gain, but the growing and finishing strategy must be adapted to the genotype.  相似文献   
134.
The role of inland fisheries in livelihoods, food security and sustainable development is often overshadowed by the higher profile interest in ocean issues. Whilst inland fisheries' catch and contribution to global nutrition, food security and the economy, are less than that of marine fisheries, global‐level comparisons of fish production obscure considerable livelihood impacts in certain countries and sub‐national areas. To highlight these contributions, this paper synthesizes recent data and innovative approaches for assessing such livelihood contributions and their importance in countries with limited access to ocean resources and aquaculture. Inland fisheries are crucial for many socially, economically and nutritionally vulnerable groups of people around the world, but the challenges in monitoring inland fisheries preclude a complete understanding of the magnitude of their contributions. This situation is rapidly improving with increasing recognition of inland fisheries in development discourses, which has also encouraged research to enhance knowledge on the importance of inland fisheries. We review this work, including collated information published in a recent Food and Agriculture Organization report, to provide an up to date characterization of the state of knowledge on the role of inland fisheries.  相似文献   
135.
This article describes the management of otitis media in a domestic rabbit (Oryctolagus cuniculus) that presented with a history of chronic upper respiratory disease. Deep nasal culture yielded a pure growth of Bordetella bronchiseptica susceptible to chloramphenicol. To further evaluate recurring clinical signs after treatment with chloramphenicol, skull radiographs were obtained and showed an increased density in the right tympanic bulla. A ventral bulla osteotomy was performed, and the success of treatment was determined by the resolution of respiratory signs and the absence of increased radiographic density in the right tympanic bulla 5 months postoperatively.  相似文献   
136.
In this paper, we analyzed a very large field data set on intramammary infections (IMI) and the associated somatic cell count (SCC) in dairy cows. The objective of the study was to analyze the impact of coagulase-negative staphylococci (CNS) IMI on cow SCC, both mean and variability, and on the potential of these infections to have a major impact on the bulk milk SCC (BMSCC). Data and milk samples for bacterial culture were collected by Quality Milk Production Services (QMPS) between 1992 and March of 2007. The QMPS program services dairy farms in New York State and other states in the Northeastern USA and operates in conjunction with Cornell University. Only records from cows where SCC and milk production data were available, and where only one organism was isolated from bacterial cultures of milk samples (or where culture was negative) were used for this analysis. A total of 352,614 records from 4200 whole herd mastitis screening sampling qualified for this study. Within herds an average of 15% (S.D. 12%) of cows sampled were infected with CNS, ranging between 0 and 100%. Average within herd prevalence of cows with a CNS IMI and an SCC over 200,000 cells/ml was 2% (S.D. 4%) with a minimum of 0% and a maximum of 50%. Results of linear mixed models showed three distinct populations of IMI statuses: negative cultures with the lowest SCC; CNS and Corynebacterium bovis with a moderate increase in SCC, and Streptococcus agalactiae, Streptococcus spp. and Staphylococcus aureus showing an important increase in SCC. Surprisingly, milk production was slightly but significantly higher in CNS infected cows compared to culture-negative cows, whereas it was strongly reduced in cows with a major pathogen IMI. The percentage contribution of CNS infections to the BMSCC was 17.9% in herds with a BMSCC less than 200,000 cells/ml. This value decreased to 11.9 and 7.9% in herds with bulk milk SCC between 200,000 and 400,000 and over 400,000 cells/ml, respectively. We concluded that very few herds with milk quality problems would have an important increase in BMSCC that could be mostly attributed to CNS infections. On the other hand, in herds with low BMSCC, CNS infections may be an important contributor to the total number of somatic cells in the bulk milk.  相似文献   
137.
Lameness in the dairy industry in New Zealand causes a problem in lost production, animal welfare and associated costs. To understand what bacteria may be present on the hooves of lame dairy cattle in this grass-fed system, samples were scraped from lame dairy cows and examined for the presence of Fusobacterium necrophorum (F. necrophorum) and Dichelobacter nodosus (D. nodosus) using the polymerase chain reaction (PCR). The PCR primers were designed to detect the presence of the lktA gene, which encodes a leukotoxin unique to F. necrophorum, and the fimA gene of D. nodosus. A total of 148 hoof scrapings were collected by farm staff over the period September 2005 to May 2006. F. necrophorum was detected in 79/148 of the samples, while D. nodosus was detected in 7/148 of the samples. The frequent finding of F. necrophorum within dairy herds in New Zealand is noteworthy and the occasional finding of D. nodosus on some dairy cattle suggests a possible role in both ovine and bovine hoof pathology.  相似文献   
138.
Bergamot peel is an underutilized byproduct of the essential oil and juice-processing industry. As with other Citrus peels, it still contains exploitable components, such as pectins and flavonoids. Commercial glycoside hydrolases, specifically a combination of pectolytic and cellulolytic enzymes, solubilized a high percentage of the material (81.94%). The flavonoid profile of the peel consisted of characteristic Citrus species flavanone rutinosides and neohesperosides derived from naringenin, eriodictyol, and hesperetin. In addition, a number of minor flavanone and flavone glycosides, not found in orange and lemon peels, were identified. The majority of flavonoids were extracted in the two 70% v/v EtOH extractions. Processing this material clearly has economic potential leading to low environmental impact.  相似文献   
139.
水稻β-淀粉酶基因密码子使用特性分析   总被引:1,自引:0,他引:1  
本研究分析了水稻10个β-淀粉酶基因(OsBmy1~OsBmy10)在cDNA和蛋白质上的同源性,分析了每个基因反映密码子使用特性的参数——GC含量、同义密码子第1~3位置中的GC含量、有效密码子数和相对同义密码子使用度。结果表明:水稻β-淀粉酶家族基因序列间的同源性差异较大,在cDNA和蛋白质序列上的基因同源性分别为39.2~79.6和25.9~74.0,其中,OsBmy3和OsBmy9间的同源性最高;OsBmy3、OsBmy4、OsBmy5、OsBmy7、OsBmy9和OsBmy10等为高GC含量基因,并表现出极强的密码子使用偏好。可见,水稻β-淀粉酶基因在进化过程中不仅氨基酸组成发生了分化,而且其在氨基酸密码子使用偏好上也发生了分化。  相似文献   
140.
BACKGROUND: Field studies of diuron and its metabolites 3-(3,4-dichlorophenyl)-1-methylurea (DCPMU), 3,4-dichlorophenylurea (DCPU) and 3,4-dichloroaniline (DCA) were conducted in a farm soil and in stream sediments in coastal Queensland, Australia. RESULTS: During a 38 week period after a 1.6 kg ha(-1) diuron application, 70-100% of detected compounds were within 0-15 cm of the farm soil, and 3-10% reached the 30-45 cm depth. First-order t(1/2) degradation averaged 49+/-0.9 days for the 0-15, 0-30 and 0-45 cm soil depths. Farm runoff was collected in the first 13-50 min of episodes lasting 55-90 min. Average concentrations of diuron, DCPU and DCPMU in runoff were 93, 30 and 83-825 microg L(-1) respectively. Their total loading in all runoff was >0.6% of applied diuron. Diuron and DCPMU concentrations in stream sediments were between 3-22 and 4-31 microg kg(-1) soil respectively. The DCPMU/diuron sediment ratio was >1. CONCLUSION: Retention of diuron and its metabolites in farm topsoil indicated their negligible potential for groundwater contamination. Minimal amounts of diuron and DCMPU escaped in farm runoff. This may entail a significant loading into the wider environment at annual amounts of application. The concentrations and ratio of diuron and DCPMU in stream sediments indicated that they had prolonged residence times and potential for accumulation in sediments. The higher ecotoxicity of DCPMU compared with diuron and the combined presence of both compounds in stream sediments suggest that together they would have a greater impact on sensitive aquatic species than as currently apportioned by assessments that are based upon diuron alone.  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