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   5919篇
  免费   386篇
  国内免费   206篇
林业   326篇
农学   428篇
基础科学   70篇
  1036篇
综合类   816篇
农作物   390篇
水产渔业   398篇
畜牧兽医   2368篇
园艺   140篇
植物保护   539篇
  2023年   39篇
  2022年   62篇
  2021年   121篇
  2020年   108篇
  2019年   126篇
  2018年   155篇
  2017年   174篇
  2016年   173篇
  2015年   144篇
  2014年   183篇
  2013年   272篇
  2012年   307篇
  2011年   349篇
  2010年   268篇
  2009年   250篇
  2008年   324篇
  2007年   310篇
  2006年   255篇
  2005年   246篇
  2004年   221篇
  2003年   280篇
  2002年   340篇
  2001年   324篇
  2000年   192篇
  1999年   151篇
  1998年   41篇
  1997年   43篇
  1996年   35篇
  1995年   46篇
  1994年   45篇
  1993年   38篇
  1992年   47篇
  1991年   41篇
  1990年   54篇
  1989年   51篇
  1988年   34篇
  1987年   39篇
  1986年   30篇
  1985年   34篇
  1984年   52篇
  1983年   27篇
  1982年   26篇
  1980年   23篇
  1979年   28篇
  1978年   28篇
  1977年   27篇
  1975年   25篇
  1974年   37篇
  1972年   30篇
  1971年   37篇
排序方式: 共有6511条查询结果,搜索用时 15 毫秒
191.
This paper reports on the genital lesions observed in adult male and female goats from a commercial flock in the Extremadura region of southwestern Spain, following an outbreak of contagious agalactia syndrome caused by Mycoplasma agalactiae and M. putrefaciens. Although both species were isolated from several organs, M. putrefaciens was the only agent isolated from the genital lesions reported here, characterized by desquamative salpingitis and cystic catarrhal metritis in females and by testicular degeneration in males. Mycoplasma putrefaciens was isolated from the testes of only one of the males examined.  相似文献   
192.
193.
甘肃省泾川县三元杂交肉牛生长发育的研究报告   总被引:1,自引:1,他引:0  
本研究测定并对比研究了德秦黄、德利黄、德西黄、利西黄、利秦黄等三元杂交牛及二元杂交牛初生 ,3月龄、6月龄、1 2月龄、1 8月龄的生长发育指标 ,结果表明 :三元杂交牛比二次杂交牛体尺、体重均有较大幅度的增长、初步筛选出适宜泾川养殖习惯的较佳杂交组合。在农户饲养条件下 ,三元杂交初生重 38kg,比二元杂交牛提高 1 2 .8%~ 5 6.6% ;1 8月龄母牛体重 377.8kg,提高 1 5 .2 %~94.1 % ;1日龄~ 1 8月龄日增重 5 2 8.2 g,增长 9.2 %~ 68.7% ;管围提高幅度为 2 4 .1 %~ 32 .3% ;臀围为 1 .7%~ 32 .3% ;体高为 2 .9%~ 1 5 3% ,体斜长为 5 .5 %~ 1 3.9% ,胸围为 1 1 .4%~ 2 0 .5 %。尻长为 5 .1 %~ 1 1 .8% ,腰角宽为 0~ 2 1 .4% ,髋宽为 0~ 3%。适宜的杂交组合为德秦黄、德利黄、德西黄。三元杂交牛 6~ 1 2月龄生长发育受阻 ,主要是饲料营养水平低所致 ,建议从 2月龄开始用优质饲草和配和配合饲料 ,1 8月龄体重达到 40 0 kg以上 ,再经 3个月育肥 ,2 1月龄达到体重 5 0 0 kg的出栏标准。  相似文献   
194.
195.
ABSTRACT Logistic regression models for wheat Fusarium head blight were developed using information collected at 50 location-years, including four states, representing three different U.S. wheat-production regions. Non-parametric correlation analysis and stepwise logistic regression analysis identified combinations of temperature, relative humidity, and rainfall or durations of specified weather conditions, for 7 days prior to anthesis, and 10 days beginning at crop anthesis, as potential predictor variables. Prediction accuracy of developed logistic regression models ranged from 62 to 85%. Models suitable for application as a disease warning system were identified based on model prediction accuracy, sensitivity, specificity, and availability of weather variables at crop anthesis. Four of the identified models correctly classified 84% of the 50 location-years. A fifth model that used only pre-anthesis weather conditions correctly classified 70% of the location-years. The most useful predictor variables were the duration (h) of precipitation 7 days prior to anthesis, duration (h) that temperature was between 15 and 30 degrees C 7 days prior to anthesis, and the duration (h) that temperature was between 15 and 30 degrees C and relative humidity was greater than or equal to 90%. When model performance was evaluated with an independent validation set (n = 9), prediction accuracy was only 6% lower than the accuracy for the original data sets. These results indicate that narrow time periods around crop anthesis can be used to predict Fusarium head blight epidemics.  相似文献   
196.
A biotype of Aster squamatus (Sprengel) Hieronymus with suspected resistance to the ALS-inhibiting herbicide imazapyr was detected in a chicken farm in the province of Seville, Spain, which had been treated once a year with imazapyr for 10 years. Resistance to imazapyr in this biotype was studied using dose-response experiments, absorption and translocation assays, metabolism studies and ALS activity assays. The rate of imazapyr required to inhibit A squamatus growth by 50% (ED50) was 15 times higher for the R (resistant) than for the S (susceptible) biotype. Cross-resistance existed for the ALS-inhibitors imazamox, imazethapyr, amidosulfuron, nicosulfuron, rimsulfuron, triasulfuron and tribenuron, but not for bensulfuron. Control of A squamatus using alternative herbicides was poor with clopyralid, intermediate with quinclorac, amitrole and MCPA, and excellent with 2,4-D, glufosinate and glyphosate. Absorption of [14C]imazapyr increased over time for both the R and S biotypes, and translocation from the treated leaf to shoots and roots was similar in both biotypes, with most of the radioactivity remaining in the treated leaf. No metabolites of imazapyr were detected in either biotype. Sensitivity of the ALS enzyme (target site) to imazapyr was lower for the R biotype (I50(R) = 4.28 x I50(S)). The mechanism of imazapyr resistance in this R biotype appears to be an altered ALS conferring decreased sensitivity to imazapyr at the whole-plant level.  相似文献   
197.
We conducted two experiments comparing the use of extruded-expelled soybean meal (EESoy) to solvent-extracted soybean meal (SBM) in swine diets. In Exp. 1, the objective was to determine the optimal processing temperature of EESoy for nursery pig growth performance. Pigs (n = 330, 13.2 +/- 2.3 kg of BW) were fed a control diet containing SBM with added fat or one of five diets containing EESoy extruded at 143.3, 148.9, 154.4, 160.0, or 165.6 degrees C. All diets were formulated on an equal apparent digestible lysine:ME ratio. From d 0 to 20, no differences were observed (P > 0.32) in ADG or ADFI (average of 544 and 924 g/d, respectively). However, gain:feed ratio (G/F) improved (quadratic, P < 0.01, range of 0.56 to 0.60) with increasing processing temperature, with the greatest improvement at 148.9 degrees C. In Exp. 2, the objective was to determine the feeding value of EESoy relative to SBM with or without added fat for growing-finishing pigs in a commercial production facility. A total of 1,200 gilts (initially 24.5 +/- 5.1 kg of BW) was used, with 25 pigs per pen and eight replications per treatment. Dietary treatments were arranged in a 2 x 3 factorial, with two sources of soybean meal (SBM or EESoy) and three levels of added fat. Pigs were phase-fed four diets over the experimental period and added fat (choice white grease) levels were 0, 3.4, and 7% initially, with the added fat levels decreasing in the next three dietary phases. Energy levels were based such that the higher energy in EESoy (with or without added fat) was calculated to be equal to that provided by SBM with added fat. From 24.5 to 61.2 kg, pigs fed EESoy had greater (P < 0.07) G/F than those fed SBM. Increasing added fat in either EESoy- or SBM-based diets increased G/F (linear, P < 0.0003). From 61.2 to 122.5 kg, ADG and G/F were unaffected in pigs fed EESoy and/or increasing added fat (P > 0.10). For the overall growing-finishing period, ADG was unaffected (P > 0.61) by increasing energy density of the diet; however, ADFI decreased (P < 0.05) and G/F increased (P < 0.02, range of 0.37 to 0.40) as energy density increased with either EESoy or added fat. Carcass leanness was not affected by dietary treatment. These results indicate that EESoy should be extruded at 148.9 to 154.4 degrees C, and that increasing dietary energy density by using EESoy and/or added fat improves feed efficiency in finishing pigs reared in a commercial environment.  相似文献   
198.
OBJECTIVE: To evaluate the outcome of thermokeratoplasty for treatment of ulcerative keratitis and bullous keratopathy secondary to corneal endothelial disease in dogs. DESIGN: Retrospective study. ANIMALS: 13 dogs. PROCEDURES: Medical records from 1994 to 2001 for dogs evaluated because of ulcerative keratitis and bullous keratopathy and treated with thermokeratoplasty were reviewed. RESULTS: There were 7 spayed females, 5 castrated males, and 1 sexually intact male, ranging from 6 to 16 years of age. Ten dogs had endothelial degeneration, and 3 dogs had breed-related endothelial dystrophy. All dogs had bullous keratopathy, characterized by microbullae formation that was detected via biomicroscopy. Recurrent or nonhealing corneal ulcers were detected unilaterally in 5 dogs and bilaterally in 8 dogs. Mean +/- SD duration from thermokeratoplasty until ulcerations were healed for all dogs was 2.2 +/- 1.1 weeks. All dogs that underwent thermokeratoplasty for nonhealing corneal ulceration secondary to endothelial disease and corneal edema had epithelial wound healing and resolution of corneal ulceration. Mean duration of treatment (ie, topical treatment required until resolution of ulceration) was significantly less after thermokeratoplasty than duration of treatment (with multiple treatments) prior to referral. CONCLUSIONS AND CLINICAL RELEVANCE: It may be necessary to perform thermokeratoplasty of the entire cornea to prevent recurrence of ulcerations in areas that have not been treated with thermokeratoplasty.  相似文献   
199.
Understanding the spread of porcine reproductive and respiratory syndrome virus (PRRSV) in pig populations is essential to the development of effective PRRS prevention and control strategies. Moreover, knowledge of the field dynamics of PRRSV in pigs will provide insights into the clinical relevance of PRRS, and will enable the targeting of interventions. This review of PRRSV includes discussion on the occurrence of outbreaks, the persistence of infection and the fade-out of infection in Dutch breeding herds. The dynamic character of PRRSV infections in endemically infected herds and the relevance of the disease under Dutch field conditions are also highlighted. Furthermore, several strategies aimed at controlling the spread of PRRSV are discussed.  相似文献   
200.
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