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   30266篇
  免费   1297篇
  国内免费   8篇
林业   1068篇
农学   892篇
基础科学   174篇
  4561篇
综合类   3978篇
农作物   1158篇
水产渔业   1146篇
畜牧兽医   16480篇
园艺   335篇
植物保护   1779篇
  2021年   222篇
  2020年   266篇
  2019年   332篇
  2018年   600篇
  2017年   664篇
  2016年   566篇
  2015年   364篇
  2014年   552篇
  2013年   1662篇
  2012年   863篇
  2011年   1114篇
  2010年   808篇
  2009年   761篇
  2008年   1077篇
  2007年   1053篇
  2006年   890篇
  2005年   713篇
  2004年   721篇
  2003年   661篇
  2002年   584篇
  2001年   745篇
  2000年   743篇
  1999年   544篇
  1998年   244篇
  1997年   227篇
  1995年   269篇
  1992年   400篇
  1991年   415篇
  1990年   432篇
  1989年   440篇
  1988年   344篇
  1987年   353篇
  1986年   383篇
  1985年   342篇
  1984年   331篇
  1983年   303篇
  1979年   481篇
  1978年   328篇
  1977年   370篇
  1976年   468篇
  1975年   485篇
  1974年   523篇
  1973年   480篇
  1972年   438篇
  1971年   354篇
  1970年   372篇
  1969年   434篇
  1968年   441篇
  1967年   446篇
  1966年   438篇
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
951.
OBJECTIVE: To determine whether findings of urinalyses could be used to reliably distinguish gonadectomized cats with urine-marking behavior from those with no problem urination. DESIGN: Case control study. ANIMALS: 58 gonadectomized cats (47 males and 11 females) with urine-marking behavior (ie, marking of vertical surfaces) and 39 (26 males and 13 females) without problem urination or urinary tract-associated conditions. PROCEDURE: Urine was collected by cystocentesis from all cats. Findings of urinalyses of cats with urine-marking behavior were analyzed statistically for sex-related differences and differences between cats that marked vertical surfaces only and those that marked both vertical and horizontal surfaces; findings of urinalyses of control cats were compared between sexes. Subsequently, results of urinalyses of cats with urine-marking behavior were compared with those of control cats. RESULTS: With regard to variables measured via urinalysis, there were no differences between male and female cats within either group. Among cats with urine-marking behavior, there were no differences between those that only marked vertically and those that marked vertically and horizontally. Analyses of data from all cats with urine-marking behavior and control cats revealed no differences that could be associated with urine marking. CONCLUSIONS AND CLINICAL RELEVANCE: These data suggest that urine-marking behavior by gonadectomized cats is an aspect of normal behavior. Clinicians are advised to focus on behavioral history of house-soiling cats to differentiate between urine-marking behavior and inappropriate urination; for the latter, urinalysis is appropriate to rule out lower urinary tract disorders.  相似文献   
952.
953.
954.
955.
OBJECTIVE: To compare the sensitivity and positive predictive value of four methods for detection of oestrus in dairy cows with resynchronised oestrous cycles. PROCEDURE: Oestrous cycles in cows in three herds were synchronised for a first round of artificial insemination (AI) and then resynchronised for a second round of AI. Sensitivity and positive predictive value of four aids (pedometers, radiotelemetric transmitters [HeatWatch; HW], tail-paint and heatmount detectors) that were used to detect the resynchronised oestrus were compared. Milk progesterone concentration and pregnancy testing at 12 weeks were used as the reference standard for cows being in oestrus. RESULTS: The mean sensitivity and positive predictive value for detecting the resynchronised oestrus, for each aid that was used, was > 80%. Tail-paint was significantly more sensitive at detecting oestrus compared to heatmount detectors (P = 0.002), but not significantly more sensitive than pedometers (P = 0.07) or HW (P = 0.55) for detecting oestrus (91.3, 85.7; 81.4 and 88.4%, respectively). Positive predictive value of HW for detecting oestrus was greater than tail-paint (P = 0.014) and heatmount detectors (P = 0.024) but not pedometers (P = 0.25; 100, 91.7, 92.9 and 87.5%, respectively). Positive predictive value of heatmount detectors was greater than pedometers in herd C (93.4% vs 73.3%; P = 0.035) but not in herds A (95.0% vs 90.0%; P = 0.56) or B (90.8% vs 100%; P = 0.10). No other significant differences in sensitivity or positive predictive value of detection of oestrus were found between aids. CONCLUSION: Tail-paint, heatmount detectors, pedometers and HW provide a high sensitivity (> 80%) and positive predictive value (> 85%) of detecting oestrus in dairy cows with resynchronized oestrous cycles.  相似文献   
956.
957.
1. A readily identifiable strain of Escherichia coli K12 was used as a 'marker' organism to determine the sources, routes and patterns of microbial cross-contamination during mechanical defeathering of broiler chicken carcases. 2. Inoculation of scald water with the marker organism led to a relatively even pattern of carcase contamination during subsequent defeathering. Microbial cross-contamination was greater by this route of inoculation than by either surface inoculation of a 'seeder' carcase or oral inoculation of a live bird one day before slaughter. 3. Dispersal of the marker organism was strongly influenced by the mechanical action of the defeathering machines. Forward transmission of the marker occurred by aerosol or large airborne droplets and particulates such as feathers. Moving carcases through the defeathering machines when these were non-operational clearly reduced backward transmission of the marker. 4. Although microbial dispersal was unaffected by increasing the spacing between individual carcases or installing a water curtain at the entry and exit of the defeathering machines, shielding of carcases with aluminium baffles reduced counts of the marker organism from contaminated carcases by > 90%. 5. The results imply that microbial cross-contamination of broiler chicken carcases during defeathering occurs mainly via the airborne route, which could be contained by physical means.  相似文献   
958.
1. Although the poultry industry uses state-of-the-art equipment and up-to-date services, in Brazil it generally makes decisions involving all its production variables based on purely subjective criteria. This paper reports the use of artificial neural networks to estimate performance in production birds belonging to a South Brazilian poultry farm. 2. Recorded data from 22 broiler production breeder flocks were obtained, from April, 1998 to December, 1999, which corresponded to 689 data lines of weekly recordings. 3. These data were processed by artificial neural networks using the software NeuroShell 2 version 4.0 (Ward Systems Group). The artificial neural network models generated were compared and selected based on their largest determination coefficient (R2), lowest Mean Squared Error (MSE), as well as on a uniform scatter in the residual plots. The authors conclude that it is possible to explain the performance variables of production birds, with the use of artificial neural networks. 4. The method allows the decisions made by the technical staff to be based on objective, scientific criteria, allows simulations of the consequences related to these decisions, and reports the contribution of each variable to the variables under study.  相似文献   
959.
960.
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