全文获取类型
收费全文 | 5220篇 |
免费 | 225篇 |
国内免费 | 8篇 |
专业分类
林业 | 311篇 |
农学 | 160篇 |
基础科学 | 52篇 |
782篇 | |
综合类 | 829篇 |
农作物 | 243篇 |
水产渔业 | 437篇 |
畜牧兽医 | 2068篇 |
园艺 | 152篇 |
植物保护 | 419篇 |
出版年
2023年 | 31篇 |
2022年 | 45篇 |
2021年 | 100篇 |
2020年 | 117篇 |
2019年 | 126篇 |
2018年 | 87篇 |
2017年 | 101篇 |
2016年 | 97篇 |
2015年 | 105篇 |
2014年 | 118篇 |
2013年 | 244篇 |
2012年 | 283篇 |
2011年 | 350篇 |
2010年 | 192篇 |
2009年 | 154篇 |
2008年 | 343篇 |
2007年 | 292篇 |
2006年 | 296篇 |
2005年 | 300篇 |
2004年 | 268篇 |
2003年 | 277篇 |
2002年 | 264篇 |
2001年 | 63篇 |
2000年 | 65篇 |
1999年 | 70篇 |
1998年 | 52篇 |
1997年 | 44篇 |
1996年 | 43篇 |
1995年 | 38篇 |
1994年 | 46篇 |
1993年 | 55篇 |
1992年 | 45篇 |
1991年 | 34篇 |
1990年 | 44篇 |
1989年 | 35篇 |
1988年 | 33篇 |
1987年 | 40篇 |
1986年 | 29篇 |
1985年 | 39篇 |
1984年 | 40篇 |
1983年 | 39篇 |
1982年 | 20篇 |
1981年 | 33篇 |
1980年 | 23篇 |
1979年 | 24篇 |
1978年 | 22篇 |
1977年 | 18篇 |
1975年 | 18篇 |
1974年 | 24篇 |
1973年 | 25篇 |
排序方式: 共有5453条查询结果,搜索用时 15 毫秒
1.
Lampreys have a complex life cycle which includes a multi‐year infaunal larval stage (ammocoete). Gut content analysis has generally identified detritus (i.e., unidentifiable organic matter) as the major dietary component to ammocoetes, though algae can also be important. However, gut content preserves only a snapshot of the animal's diet and does not reflect assimilated material. In order to better characterise the nutritional sources supporting ammocoete growth, we analysed ammocoete body tissue and potential dietary sources at two streams using natural Δ14C and δ15N to estimate time‐integrated nutritional support. Bayesian isotope mixing models revealed differences in the importance of sources supporting ammocoetes between sites. Ammocoetes from a stream in a mixed land usage area (~50% agriculture, ~40% forest and ~10% developed) were primarily supported (mean: ~50%) by fresh terrestrial organic matter but were also supported by substantial contributions (mean: ~30%) by aged organic matter (AOM) and autochthonous material (algae; mean ~20%). In a predominantly forested (~90%) headwater stream, different modelling scenarios (uninformed or informed priors) suggested that algal support of ammocoete nutrition ranged from 7% to 45%. However, the model relying on informed priors developed from gut content analysis produced the low estimates, suggesting these were more reliable. When algae were a minor component of the nutrition at the forested site, ammocoetes were highly dependent on AOM (83 ± 26%; mean ± SD). Based on these findings, ammocoete growth and development are predicted to be strongly influenced by both land use and the availability of allochthonous and autochthonous materials of varying ages within streams. 相似文献
2.
3.
Jeffrey R. Crass MD Ronald L. Genovese VMD James A. Render DVM Errol M. Bellon MD 《Veterinary radiology & ultrasound》1992,33(4):206-216
Ultrasonography and MRI have become valuable tools for imaging of tendon injuries. The current study examines the histopathologic basis for the imaging abnormalities. Five injured equine forelimbs and two normal contralateral limbs were studied with high resolution real time ultrasound and MRI. Histologic sections were made and correlated with the diagnostic images. All lesions were readily seen by both modalities. Lesions characterized by hemorrhage, edma, and cellular infiltration were sonolucent on ultrasound and bright on MRI images. MRI returned to normal as fibrogenesis ensued. Ultrasound images remained abnormal until fibrillar reallgnment occured with completion of the healing process. High resolution real time ultrasound and MRI both accurately reflect the tissue abnormalities in acute tendon injuries. The injured horse does appear to be an effective model for correlative imaging studies of tendon injuries. 相似文献
4.
James Mintert 《Veterinary Clinics of North America: Food Animal Practice》2003,19(2):387-395
The United States cattle feeding sector has changed dramatically over time. Movement from an industry largely populated by farmer feeders using livestock to market their grain production to an industry composed of large, specialized, commercial cattle feeding firms has occurred since the end of World War II. Moreover, the industry also shifted geographically, moving away from the Corn Belt to the Plains as farmer feeders exited the industry. Today, the 10 largest firms in the industry likely market 22% to as much as 29% of all fed cattle produced in the United States. The shift in industry structure was at least partially motivated by the advent of improved technology. Successful implementation of rapidly evolving technology often required more intense, specialized management (and in some cases, larger operations) than was available in many farmer feeder operations. The ongoing shift away from live-weight pricing of fed cattle toward grid-based pricing could increase the need for specialized management yet again, encouraging even more industry consolidation in the future. 相似文献
5.
6.
A stripspawning methodology was evaluated for tilapia (oreochromid) species. This technique achieved an average hatching success of 68.6 ± 3.6% (N= 31). Female fecundity and spawning frequency were dependant on both genetical and husbandry factors. Egg yields for Oreochromis niloticus, O. mossambicus, and O. niloticus±O. mossambicus hybrids averaged 4.54, 10.86 and 10.36 eggs/g female/spawn, respectively. Female broodstock that were adapted to an intensive spawning regime exhibited a significant increase in fecundity. Additionally, egg survival was not affected by hydration for up to 15 minutes prior to fertilization. Results suggest that the strip spawning of tilapia species may be an efficient method of providing viable gametes for hatchery purposes. 相似文献
7.
Ronald D. Hodges DVM Russell L. Tucker DVM James J. Brace DVM 《Veterinary radiology & ultrasound》1993,34(4):249-252
8.
David A. Detweiler David S. Biller DVM James J. Hoskinson DVM Kenneth R. Harkin DVM 《Veterinary radiology & ultrasound》2001,42(2):108-112
Canine dysautonomia is an idiopathic condition resulting in loss of autonomic nervous system function. Recently, the prevalence of dogs diagnosed with dysautonomia in the mid-western United States has increased. In this study the medical records and radiographic findings in 24 dogs with dysautonomia were reviewed. A diagnosis of dysautonomia was made in 17 (71%) of the dogs in this study by postmortem examination, the remaining 7 (29%) dogs were diagnosed pharmacologically. The radiographic findings supportive of dysautonomia include aspiration pneumonia, megaesophagus, or a distended stomach, small bowel, or urinary bladder. In some instances, the disease radiographically mimicked other disorders of the gastrointestinal tract, including mechanical obstruction. 相似文献
9.
10.
Factors associated with past and potential future use of veterinarians by Ontario trout farmers
下载免费PDF全文
![点击此处可从《The Canadian veterinary journal. La revue veterinaire canadienne》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Thorburn MA 《The Canadian veterinary journal. La revue veterinaire canadienne》1993,34(10):611-618
This study investigated whether certain variables were associated with Ontario trout farmers' use of the Fish Pathology Laboratory (FPL), Ontario Veterinary College, or their willingness to pay for private veterinary services. Sixty-two farmers, representing 91% of Ontario's 1990 farmed trout production, were personally interviewed. Logistic regression was used to compare the 26 farmers who had submitted fish to the FPL during the previous two years to those who had not. Submitters tended to have been fish farming for fewer years, and to produce more fish, use more workers, treat prophylactically less frequently, keep records of drug treatments, and not disinfect holding units (p<0.05). Most farmers stated they would occasionally hire a private veterinarian. Only 13 farmers anticipated hiring one more than once per year. Polychotomous regression showed that farmers who purchased, instead of hatched, their seed stock, kept records, treated therapeutically often, and used several workers would hire private veterinarians more frequently (p<0.05). Strategies are discussed by which the veterinary profession might better understand and meet the needs of the aquaculture industry. 相似文献