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   157131篇
  免费   8707篇
  国内免费   368篇
林业   6474篇
农学   5219篇
基础科学   1080篇
  20461篇
综合类   25418篇
农作物   6433篇
水产渔业   7915篇
畜牧兽医   80322篇
园艺   2127篇
植物保护   10757篇
  2020年   1425篇
  2019年   1668篇
  2018年   2451篇
  2017年   2803篇
  2016年   2529篇
  2015年   2122篇
  2014年   2682篇
  2013年   6453篇
  2012年   4871篇
  2011年   5969篇
  2010年   3881篇
  2009年   4039篇
  2008年   5818篇
  2007年   5455篇
  2006年   5096篇
  2005年   4736篇
  2004年   4487篇
  2003年   4610篇
  2002年   4109篇
  2001年   4913篇
  2000年   4977篇
  1999年   3934篇
  1998年   1727篇
  1997年   1674篇
  1996年   1505篇
  1995年   1780篇
  1994年   1586篇
  1993年   1437篇
  1992年   3009篇
  1991年   3239篇
  1990年   3043篇
  1989年   3124篇
  1988年   2822篇
  1987年   2768篇
  1986年   2885篇
  1985年   2734篇
  1984年   2203篇
  1983年   1969篇
  1979年   2137篇
  1978年   1730篇
  1977年   1414篇
  1976年   1465篇
  1975年   1556篇
  1974年   1832篇
  1973年   1854篇
  1972年   1752篇
  1971年   1552篇
  1970年   1523篇
  1969年   1573篇
  1967年   1391篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
Osteolytic defects were detected radiographically in the distal sesamoid bone of a 16-month-old Bralers heifer, in the middle phalanx of a 14-month-old American Gray Brahman bull, and in the distal phalanx of a 3-year-old American Gray Brahman bull. The articular cartilage was damaged in each animal because of osteolysis or pathologic fracture. After each animal was anesthetized and positioned in lateral recumbency, the lesions were curetted and packed with cancellous bone harvested from the same animal's tuber coxae. Basic postoperative management involved stall rest and immobilization of the graft site with a fiberglass cast (42 to 79 days), after which a support bandage was used for approximately 2 weeks. Recurrence of lameness has not been observed in these animals for 60 months, 58 months, and 21 months, respectively. These cases exemplify the benefit of using an autogenous cancellous bone graft for treatment of severe osteolysis of a digit in cattle.  相似文献   
182.
The medical records from 44 dogs with traumatic luxation of the cubital joint were reviewed. Closed reduction was performed in 35 dogs, and open reduction in 9 dogs. Excellent or good results were achieved in 31 of 35 dogs (88.6%) treated by closed reduction and in 5 of 9 dogs (55.6%) treated by open reduction. Collateral ligament repair, performed in 5 dogs, did not appear to affect the prognosis. Factors such as body weight and type and duration of external coaptation also did not appear to influence the outcome. Increased dysfunction associated with open reduction most likely was caused by the chronic nature of the dislocation, iatrogenic damage to the articular cartilage during reduction, or both. A good prognosis for return to normal function with minimal gait abnormality can be expected in most cases of traumatic luxation of the cubital joint treated by early closed reduction.  相似文献   
183.
Abstract. Recent developments in in situγ ray spectrometry offer a new approach to measuring the activity of radionuclides such as 137Cs and 40K in soils, and thus estimating erosion or deposition rates and field moist bulk density (ρm). Such estimates would be rapid and involve minimal site disturbance, especially important where archaeological remains are present. This paper presents the results of a pilot investigation of an eroded field in Scotland in which a portable hyper pure germanium (HPGe) detector was used to measure γ ray spectra in situ. The gamma (γ) photon flux observed at the soil surface is a function of the 137Cs inventory, its depth distribution characteristics and ρm. A coefficient, QCs, derived from the forward scattering of 137Cs γ ray photons within the soil profile relative to the 137Cs full energy peak (662 keV), was used to correct the in situ calibration for changes in the 137Cs vertical distribution in the ploughed field, a function of tillage, soil accumulation and ρm. Based on only 8 measurements, the agreement between in situγ ray spectrometry and soil sample measurements of 137Cs inventories improved from a non significant r2=0.05 to a significant r2=0.62 (P<0.05). Erosion and deposition rates calculated from the corrected in situ137Cs measurements had a similarly good agreement with those calculated from soil cores. Mean soil bulk density was also calculated using a separate coefficient, QK, derived from the forward scattering γ photons from 40K within the soil relative to the 40K full energy peak (1460 keV). Again there was good agreement with soil core measurements (r2=0.64; P<0.05). The precision of the in situ137Cs measurement was limited by the precision with which QCs can be estimated, a function of the low 137Cs deposition levels associated with the weapons testing fallout and relatively low detector efficiency (35%). In contrast, the precision of the in situ ρm determination was only limited by the spatial variability associated with soil sampling.  相似文献   
184.
A 6-year study was conducted in 4 dairy herds in Iowa in which Johne's disease was diagnosed previously. Fecal specimens were collected at 6-month intervals from animals 2 years of age and over for mycobacteriologic examination. Serum samples were obtained at 3-month intervals and tested by enzyme-linked immunosorbent assay (ELISA). The antigen used in the ELISA was a potassium chloride extract of a field strain of Mycobacterium paratuberculosis. The ELISA reactions were observed in 87% of the cows from which M. paratuberculosis was isolated. Dairy producers that participated in the Johne's control program reported reduced economic losses. Increased income was attributed to improved milk production, increased value of vaccinated animals sold as replacements to other dairy herds in which Johne's disease had been diagnosed, and the increased market value of slaughter animals removed from the herd.  相似文献   
185.
An ultrasonic linear array scanner with a transrectal probe was used to observe ovarian and uterine changes associated with the reproductive cycle in llamas. 'Waves' of follicular development and regression occurred in unstimulated females, during which the dominant follicle reached a maximum size of 9 to 13 mm; both ovaries were equally active. Ovulation was induced by mating in 80 per cent of cases, and when mating was accompanied by the administration of human chorionic gonadotrophin the ovulation rate increased to 90 per cent and the time to ovulation decreased from two to three days to one to two days. Some spontaneous ovulations occurred. Corpora lutea reached a maximum size of 12 mm (non-pregnant) or 14 mm (pregnant) after seven or 16 days, respectively. The lifespan of the corpus luteum was approximately 11 days in non-pregnant llamas and the regression time was advanced by the administration of prostaglandin or embryo recovery. Pregnancy could be diagnosed as early as 19 days after mating.  相似文献   
186.
Squamous cell carcinoma is an uncommon tumor of the cornea in dogs. A 12-year-old Shih Tzu with a history of chronic keratoconjunctivitis sicca was diagnosed as having squamous cell carcinoma of the corneoscleral limbus. The lesion was treated by surgical resection and cryotherapy. One year after surgery, multiple tumors, apparently unrelated to the first tumor, appeared on the same cornea. Keratoconjunctivitis sicca may have predisposed the dog to corneal squamous cell carcinoma development.  相似文献   
187.
The persistence and mobility of chlorsulfuron and metsulfuron were studied in Italy at four field locations. Chlorsulfuron was applied pre-emer-gence to wheat at 15 and 30 g a.i. ha?1 in the autumn of 1986 and metsulfuron early post-emergence at 8 g a.i. ha?1 in the spring of 1987. Soil samples were taken periodically at each location for 1 year. Trials were conducted until 1989 at three locations. After the wheat harvest in 1989, chlorsulfuron and metsulfuron residues in the soil were measured by bioassay in plots that had been treated annually for 3 years and also in plots that had received single treatments. Chlorsulfuron at both application rates dissipated to levels below 0.3 g a.i. ha?1 over 12 months at three locations while, at the fourth, which received only 250 mm rain during the study, significant levels of residues were still present after 15 months. The degradation of chlorsulfuron applied at 30 g a.i. ha?1 followed first-order kinetics, with half-lives of 149, 70, 59 and 51 days at the four locations. Chlorsulfuron mobility was high at three locations while, in the silty clay soil of the fourth, the herbicide was confined to the upper 30 cm, despite the high rainfall. Metsulfuron applied in spring was detected only in the first soil sample collection. It was present in all sampled layers at one location but, in the others, was confined to the surface layer. There was no evidence of herbicide accumulation in the soil with repeated applications. Persistance et mobilité du chlorsulfuron et du met-sulfuron dans différents sols et sous différentes conditions climatiques La persistance et la mobilité du chorsulfuron et du metsulfuron au champ ont étéétudiées en Italie sur quatre sites. Le chlorsulfuron était appliqué en pré-levée sur le blé aux doses de matière active (m.a.) 15 et 30 g ha?1à l'automne 1986 et le metsulfuron en post-levée précoce à la dose 8 g m.a. ha?1 au printemps 1987. Des échantillons de sol étaient prélevés périodiquement sur chaque site pendant 1 an. Les essais ont été conduits jusqu'en 1989 sur trois des sites. En 1989, après la récolte du blé. les résidus de chlorsulfuron et de metsulfuron dans le sol ont été mesurés par des essais biologiques dans des parcelles qui avaient été traitées chaque année pendant 3 ans, ainsi que dans des parcelles qui n'avaient reçu qu'un seul traitement. Le chlorsulfuron aux deux doses disparaissait jusqu'à des niveaux inférieurs à 0.3 g m.a. ha?1 en 12 mois sur trois des sites, alors que sur le quatrième, qui n'a reçu que 250 mm de précipitations pendant l'étude, des résidus étaient présents en quantité significative après 15 mois. La dégradation du chlorsulfuron appliquéà la dose 30 g m.a. ha?1 suivait une cinétique du premier ordre, avec des demi-vies 149, 70, 59 et 51 j sur les quatre sites. La mobilité du chlorsulfuron était élevée sur trois des sites alors que sur le quatrième oú le sol était une argile limoneuse, l'herbicide était confiné aux 30 cm superficiels en dépit de précipitations élevées. Le metsulfuron appliqué au printemps n'était détecté que dans le premier prélèvement. Sur un des sites, il était présent dans toutes les couches, mais était confinéà la couche superficielle sur les autres sites. Après des applications répétées, aucune accumulation d'herbicide n'était observée dans le sol. Persistenz und Mobilität von Chlorsulfuron und Metsulfuron in verschiedenen Böden und bei unterschiedlicher Witterung Die Persistenz und Mobilität von Chlorsulfuron und Metsulfuron wurden in Italien an 4 Standorten unter Freilandbedingungen untersucht. Chlorsulfuron wurde im Herbst 1986 bei Weizen im Vorauflauf mit 15 und 30 g AS ha?1 und Metsulfuron im Frühjahr 1987 im frühen Nachauflauf mit 8 g AS ha?1 angewandt. Von jeder Versuchsfläche wurden über einen Zeitraum von 1 Jahr periodisch Bodenproben entnommen. Die Versuche wurden an 3 Standorten bis 1989 durchgeführt. Nach der Weizenernte 1989 wurden mit Biotests die Rückstände von Chlorsulfuron und Metsulfuron in den Böden mit jährlicher (über 3 Jahre) sowie mit einmaliger Behandlung bestimmt. Die Chlorsulfuron-Rückstände nahmen an 3 Standorten bei beiden Aufwandmengen nach 12 Monaten auf 0,3 g AS ha?1 ab. Am 4. Standort, wo während der Untersuchung nur 250 mm Niederschlag gefallen waren, wurden nach 15 Monaten noch signifikante Rückstände gefunden. Der Abbau von Chlorsulfuron folgte an allen Standorten bei einer Aufwandmenge von 30 g AS ha?1 einer Kinetik erster Ordnung mit Halbwertszeiten von 149, 70, 59 und 51 Tagen. Die Mobilität von Chlorsulfuron war an 3 Standorten hoch, während es in dem schluffigen Tonboden des 4. Standortes trotz hohen Niederschlags in den oberen 30 cm verblieb. Das im Frühjahr ausgebrachte Metsulfuron wurde nur in der 1. Bodenprobenserie gefunden. An einem Standort lagen in allen beprobten Bodenschichten Rückstände vor, doch sonst waren sie auf die oberste Bodenschicht beschränkt. Bei wiederholter Anwendung konnte keine Herbizidakkumulation im Boden festgestellt werden.  相似文献   
188.
189.
The records of 61 horses undergoing tooth repulsion for treatment of alveolar periostitis were reviewed. Seventeen of 36 horses (47%) in which maxillary teeth were removed had serious postoperative complications, such as infection of a second tooth, bone sequestration, chronic sinusitis, draining tracts, retained dental packing, feed impaction of the alveolus or sinus, suture-line dehiscence, or skin-flap sloughs. Eight horses required at least one additional surgical procedure. Eight of 25 horses (32%) in which mandibular teeth were removed had serious postoperative complications, and four horses required an additional surgical procedure. Hospitalization lasted 2 to 61 days (median, 22 days) for maxillary teeth and 3 to 35 days (median, 8 days) for mandibular teeth. Long-term follow-up (at least 5 months) was possible in 47 horses. Twenty-four of 30 horses (80%) with maxillary tooth repulsion healed without further problems; six horses had persistent nasal discharge. Fourteen of 17 horses (82%) with mandibular tooth repulsion healed with no further problems or with only minor complications; three horses had a chronic draining tract.  相似文献   
190.
Twenty-nine pruritic, atopic dogs were entered into a double-blind, placebo-controlled, crossover study to evaluate the efficacy of an investigational antiallergenic compound, AHR-13268. Fourteen dogs were evaluated by a veterinary dermatologist (at intervals) and the owner (daily). Fifteen dogs were evaluated only by the owner. The mean (+/- SE) owner scores for pruritus, erythema, and lesions with placebo treatment (higher score = worse signs) were 3.24 (+/- 0.12), 2.73 (+/- 0.12), and 2.61 (+/- 0.09), respectively. With drug treatment, the corresponding scores were 2.89 (+/- 0.12), 2.50 (+/- 0.12), and 2.25 (+/- 0.09). Scores for pruritus and lesions (but not erythema) were significantly better with drug treatment than with placebo treatment. Investigator scores showed similar trends, but the differences were not great enough to be statistically significant. Overall, 11/29 (38%) owners reported their dogs had moderate or better improvement from drug capsules, and 4/29 dogs (14%) improved on placebo capsules. A variety of adverse effects were reported following both drug (9/29 dogs) and placebo (8/29 dogs) capsule administration, but were mild and well tolerated. Results of this study indicate that AHR-13268 has potential for empiric treatment of allergic inhalant dermatitis in some 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