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   3974篇
  免费   247篇
  国内免费   2篇
林业   332篇
农学   78篇
基础科学   17篇
  676篇
综合类   648篇
农作物   172篇
水产渔业   359篇
畜牧兽医   1520篇
园艺   120篇
植物保护   301篇
  2023年   40篇
  2022年   65篇
  2021年   102篇
  2020年   126篇
  2019年   155篇
  2018年   150篇
  2017年   119篇
  2016年   134篇
  2015年   96篇
  2014年   142篇
  2013年   202篇
  2012年   262篇
  2011年   326篇
  2010年   171篇
  2009年   156篇
  2008年   269篇
  2007年   270篇
  2006年   227篇
  2005年   206篇
  2004年   191篇
  2003年   197篇
  2002年   150篇
  2001年   34篇
  2000年   23篇
  1999年   46篇
  1998年   29篇
  1997年   18篇
  1996年   18篇
  1995年   15篇
  1994年   18篇
  1993年   15篇
  1992年   20篇
  1991年   22篇
  1990年   19篇
  1989年   14篇
  1988年   18篇
  1986年   12篇
  1985年   10篇
  1984年   16篇
  1983年   7篇
  1982年   10篇
  1981年   8篇
  1980年   6篇
  1978年   6篇
  1976年   5篇
  1974年   6篇
  1971年   6篇
  1928年   5篇
  1925年   6篇
  1924年   6篇
排序方式: 共有4223条查询结果,搜索用时 171 毫秒
101.
Recent research shows that land use history is an important determinant of current ecosystem function. In the United States, characterization of land use change following European settlement requires reconstruction of the original property mosaic. However, this task is difficult in unsystematically surveyed areas east of the Appalachian Mountains. The Gwynns Falls watershed (Baltimore, MD) was originally surveyed in the 1600-1700s under a system of warrants and patents (commonly known as ‘metes and bounds’). A method for the reconstruction and mapping of warrant and patent properties is presented and used to map the original property mosaic in the Gwynns Falls watershed. Using the mapped mosaic, the persistence of properties and property lines in the current Gwynns Falls landscape is considered. The results of this research indicate that as in agricultural areas, the original property lines in the Gwynns Falls watershed are persistent. At the same time, the results suggest that the property mosaic in heavily urbanized/suburbanized areas is generally ‘reset.’ Further, trends in surveying technique, parcel size, and settlement patterns cause property line density and property shape complexity to increase in the less urbanized upper watershed. The persistence of original patterns may be damping expression of heterogeneity gradients in this urban landscape. This spatial pattern of complexity in the original mosaic is directly opposite of hypothesized patterns of landscape heterogeneity arising from urbanization. The technique reported here and the resulting observations are important for landscape pattern studies in areas settled under unsystematic survey systems, especially the heavily urbanized areas of the eastern United States. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
102.
A survey was conducted to define the fungi population contaminating onion bulbs in Israel during three growing seasons. Significant rots were found to be caused byBotrytis allii, B. cinerea andAspergillus niger. All the onion stocks tested showed infections, but the severity and identity of the pathogens varied between seasons.B. cinerea andB. allii were the most prevalent fungi in winter-harvested onions. Species ofAspergillus, Penicillium, Fusarium, Rhizopus andTrichoderma were also isolated. The first two were the only species found on summer onions. High fungal contamination had been the main factor affecting an attempt to improve the storage quality and shelf life of the harvested bulbs. Chemical disinfection using a reduced-volume application (RVA) technique was efficient in controlling the major storage rots (caused byA. niger andB. cinerea) of the bulbs, without the major disadvantages of the dipping method. The rate of control was directly correlated with the cover density of the deposited fungicide, but not with the amount deposited. The RVA technique should enable prolongation of postharvest shelf life and storability of onion bulbs.  相似文献   
103.
Contact neodymium:yttrium aluminum garnet (Nd:YAG) laser transscleral cyclophotocoagulation (TSCP) was performed on 23 eyes of 16 horses for treatment of glaucoma. The mean highest preoperative IOP was 51 ± 17 mmHg. Follow-up evaluation was available for 19 eyes 1 day after surgery, 14 eyes from 1 to 2 weeks, 16 eyes from 4 to 6 weeks, 9 eyes from 12 to 16 weeks, and 10 eyes greater than 20 weeks after laser treatment. The mean intraocular pressure (IOP) the day following surgery was 34 ± 13 mmHg. The mean IOP for each follow-up period was: one to two weeks postoperative, 23 ± 9 mmHg; four to six weeks, 24 ± 7 mmHg; 12–16 weeks, 28 ± 10 mmHg; and  20 weeks, 22 ± 9 mmHg. IOP measurements were significantly different from pretreatment values for all follow-up intervals except for weeks 12–16 ( P < 0.05). Treatment success was defined as maintenance of IOP < 30 mmHg. Treatment success was achieved in 93%, 88%, 78%, and 70% of the treated eyes at the 1–2 weeks, 4–6 weeks, 12–16 weeks, and the  20 weeks re-evaluation, respectively. No significant difference was found between the number of eyes visual at presentation (52.2%) and visual at 20 weeks (60%). The most common laser complications were conjunctival hyperemia (21.7%) and corneal ulcers (13.0%). Results of this study indicate that Nd:YAG TSCP is an effective method of controlling IOP and preserving vision in horses with glaucoma. An effective Nd:YAG laser protocol for TSCP in the equine glaucomatous eye is a power setting of 11 W, duration of 0.4 s, applied 5 mm posterior to the limbus at 60 sites, resulting in a total energy dose of 264 J.  相似文献   
104.
105.
A mean reduction of plasma calcium level to 51 per cent of normal infusion of 4.7 per cent Na2EDTA solution into four cows was associated with a mean reduction of 48 per cent in cardiac output. Hypocalcaemia sufficient to produce sternal recumbency in a further four cows resulted in a highly significant fall in the mean arterial blood pressure, which returned to normal immediately after treatment with an intravenous infusion of 350 ml of 32.5 per cent calcium borogluconate.  相似文献   
106.
ATrichoderma-selective agar medium (TSM) was developed for quantitative isolation ofTrichoderma spp. from soil. Selectivity was obtained by using chloramphenicol as a bacterial inhibitor, and pentachloronitrobenzene, p-dimethylaminobenzenediazo sodium sulfonate and rose-bengal as selective fungal inhibitors. The TSM also contains a low concentration of glucose which still allows relatively rapid growth and sporulation ofTrichoderma, enabling convenient and rapid identification ofTrichoderma colonies. All the 15Trichoderma isolates tested formed colonies and grew well on this medium. Recovery ofTrichoderma from artificially inoculated soils was high and was not affected by soil type or by other microorganisms. A positive correlation was observed betweenTrichoderma added to soil and counts ofTrichoderma colonies on TSM plates. When combined with a soil pellet sampler, the selective medium was also used successfully for recovery of the indigenousTrichoderma population of natural soils.  相似文献   
107.
Furosemide is a potent loop diuretic used for the prevention of exercise-induced pulmonary hemorrhage in horses. This drug may interfere with the detection of other substances by reducing urinary concentrations, so its use is strictly regulated. The regulation of furosemide in many racing jurisdictions is based on paired limits of urinary SG (<1.010) and serum furosemide concentrations (>100 ng/ml). To validate this regulatory mechanism, a liquid chromatography/mass spectrometry/mass spectrometry method employing a solid-phase extraction procedure and furosemide-d5 as an internal standard was developed. The method was used to determine the pharmacokinetic parameters of furosemide in equine serum samples and its effects on urinary SG after IV administration (250 mg) to 10 horses. Pharmacokinetic analysis showed that serum concentrations of furosemide were well described by a two-compartmental open model. Based on results in this study, it is very unlikely for horses to have serum furosemide concentrations greater than 100 ng/ml or urine SG less than 1.010 at 4 hours after administration (250 mg IV). However, it should be remembered that urine SG is a highly variable measurement in horses, and even without furosemide administration, some horses might naturally have urine SG values less than 1.010.  相似文献   
108.
OBJECTIVE: To determine the effects of endotoxin administration on thyroid function test results and serum tumor necrosis factor-alpha (TNF-alpha) activity in healthy dogs. ANIMALS: 6 healthy adult male dogs. PROCEDURES: Serum concentrations of thyroxine (T4), 3,5,3'-triiodothyronine (T3), 3,3'5'-triiodothyronine (rT3), free T4 (fT4), and endogenous canine thyroid stimulating hormone (TSH), and TNF-alpha activity were measured before (day-1; baseline), during (days 0 to 3), and after (days 4 to 24) IV administration of endotoxin every 12 hours for 84 hours. RESULTS: Compared with baseline values, serum T3 concentration decreased significantly, whereas rT3 concentration increased significantly 8 hours after initial endotoxin administration. Serum T4 concentration decreased significantly at 8 and 12 hours after initiating endotoxin administration. Serum T4 concentration returned to reference range limits, then decreased significantly on days 6 to 12 and 16 to 20. Serum fT4 concentration increased significantly at 12, 24, and 48 hours after cessation of endotoxin treatment, compared with baseline values. Serum rT3 concentration returned to reference range, then decreased significantly days 5 and 7 after stopping endotoxin treatment. Serum TNF-alpha activity was significantly increased only 4 hours after initial endotoxin treatment, compared with baseline activity. CONCLUSIONS AND CLINICAL RELEVANCE: Endotoxin administration modeled alterations in thyroid function test results found in dogs with spontaneous nonthyroidal illness syndrome. A decrease in serum T4 andT3 concentrations and increase in serum rT3 concentration indicate impaired secretion and metabolism of thyroid hormones. The persistent decrease in serum T4 concentration indicates that caution should be used in interpreting serum T4 concentrations after resolution of an illness in dogs.  相似文献   
109.
OBJECTIVE: To determine the functionality of canine anti-Malassezia IgE via the passive transfer of immediate hypersensitivity localized to the skin (ie, cutaneous anaphylaxis) from atopic dogs with dermatitis attributable to overgrowth of Malassezia pachydermatis (Malassezia dermatitis [MD]) to healthy recipient dogs by use of the Prausnitz-Küstner (P-K) technique. ANIMALS: 7 clinically normal dogs, 32 atopic dogs with MD, serum from 11 atopic dogs with MD, and 3 healthy dogs without prior sensitization to M pachydermatis. PROCEDURE: Serum from atopic dogs with MD was used for P-K tests in 3 clinically normal recipient dogs. Serial dilutions of untreated, heat-inactivated, IgE-absorbed, and bovine serum albumin (BSA)-absorbed (control) aliquots of serum were injected ID in triplicate for dermal sensitization. Twenty-four, 48, and 72 hours later, a crude extract of M pachydermatis was injected ID into the sites used for sensitization injections, and immediate hypersensitivity reactions were graded on a 4-point scale. RESULTS: Untreated serum caused P-K reactivity beginning 24 hours after passive sensitization and persisting through 72 hours (titers, 1:32 to 1:64). Heat inactivation and IgE-absorption of serum eliminated P-K reactivity, whereas treatment of serum with BSA did not. CONCLUSIONS AND CLINICAL RELEVANCE: Analysis of P-K test results supports the passive transfer of cutaneous anaphylaxis by anti-Malassezia IgE and indicates it is functional in type-1 hypersensitivity reactions of atopic dogs with MD. Reduction or blockade of anti-Malassezia IgE in atopic dogs with MD may provide better clinical control of the disease.  相似文献   
110.
OBJECTIVE: To quantify dimensions of the small intestine of dogs and describe changes in histologic characteristics of the mucosa during postnatal development. SAMPLE POPULATION: Gastrointestinal tract tissues obtained from 110 Beagles (15 adult females and 95 puppies of both sexes). PROCEDURE: Several variables (length, total weight, mucosal weight, and nominal surface area) of the small intestine were measured in puppies at birth but before suckling; 1 day after birth and subsequent suckling, 21, 42, and 63 days after birth, and in the adult dams of the puppies. Tissue structure was examined and quantified at each time point by use of routine histologic examination and ocular micrometry of formalin-fixed specimens stained with H&E. RESULTS: Small intestinal dimensions increased throughout development with the greatest proportional changes during the first day after birth and onset of suckling. Villus height decreased during suckling but had consistent values from 42 days after birth to maturity, whereas crypt depth increased from birth to maturity. Vacuolated enterocytes were evident from birth to 21 days but not thereafter. CONCLUSIONS AND CLINICAL RELEVANCE: Increases in intestinal dimensions provide growing dogs with a greater capacity for digestion and absorption. Changes in mucosal architecture and cell populations coincided with shifts in dietary inputs. These findings may assist in the diagnosis of small intestinal diseases and nutritional responses during growth and development of dogs.  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