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   10866篇
  免费   701篇
  国内免费   863篇
林业   559篇
农学   639篇
基础科学   226篇
  4947篇
综合类   3876篇
农作物   488篇
水产渔业   264篇
畜牧兽医   605篇
园艺   255篇
植物保护   571篇
  2024年   89篇
  2023年   327篇
  2022年   430篇
  2021年   504篇
  2020年   487篇
  2019年   477篇
  2018年   422篇
  2017年   658篇
  2016年   719篇
  2015年   535篇
  2014年   594篇
  2013年   886篇
  2012年   1015篇
  2011年   783篇
  2010年   560篇
  2009年   528篇
  2008年   479篇
  2007年   550篇
  2006年   484篇
  2005年   395篇
  2004年   275篇
  2003年   220篇
  2002年   157篇
  2001年   134篇
  2000年   123篇
  1999年   83篇
  1998年   83篇
  1997年   76篇
  1996年   66篇
  1995年   59篇
  1994年   27篇
  1993年   38篇
  1992年   38篇
  1991年   38篇
  1990年   20篇
  1989年   19篇
  1988年   21篇
  1987年   16篇
  1986年   7篇
  1985年   3篇
  1984年   1篇
  1982年   2篇
  1962年   2篇
排序方式: 共有10000条查询结果,搜索用时 421 毫秒
991.
针对山地水土流失、土壤瘠薄及有机梨对原生态的高标准生产要求,研究出有机肥和碎秸秆混合压埋改良土壤培肥保水技术。研究结果表明,改造后的土壤腐殖质化程度高,营养成分含量高,铵态氮(N)含量105.08mg/kg,速效磷(PzOs)含量41.88mg/kg,速效钾(KzO)含量426.17mg/kg,可溶性有机质含量为2.21%。修反坡梯田、修水盆和有机肥秸秆压埋改造后的土壤,显著提高了土壤含水量,提高7.1%。果园树盘覆草,显著减少了地面蒸发量,覆草比不覆草日减少地面蒸发量3.61mm。  相似文献   
992.
ABSTRACT

The study was carried out (a) to identify the changes in the soil organic carbon (SOC) content during the different phases of slash-and-burn cultivation—i.e., before slash-and-burn (Phase 1), after slash-and-burn (Phase 2), and after harvest (Phase 3); and (b) to determine the status of soil organic carbon content in the primary undisturbed forest (Site 1) and in the secondary forests, where slash-and-burn cultivation was taken up ~25 yr (Site 2), 15 yr (Site 3), and 5 yr back (Site 4). The undisturbed forest holds the largest amount of SOC % (5.25) followed by 25 yr (3.07), 5 yr (2.86), and 15 yr (2.27) fallow. The mean percentages of SOC in the 0- to 15-cm layer fell from 3.07 in Phase 1 to 2.53 and 2.37, respectively, in Phases 2 and 3; in the subsurface 15- to 30-cm layer, they fell from 1.95 to 1.62 and 1.63, respectively. Although, the SOC in Phase 3 still seems sufficient to support another round of cultivation, further studies are needed to examine crop yields in successive cultivation cycles, suitability of other rice varieties, and weed and pest types and rates of invasion. Tribal population dynamics is another major concern, which needs assessment for monitoring future land requirements.  相似文献   
993.
Abstract

AIM: To find evidence for localisation in the uterus, and fetal infection, of Leptospira spp. in farmed deer in the lower North Island of New Zealand during and shortly after the breeding season.

METHODS: Between February and July 2008, 116 blood samples, 120 kidneys, 120 uteri and 27 fetuses were collected from 120 mixed-age hinds from lines from nine farms, at a deer slaughter premises. Serum samples were tested for antibodies against Leptospira borgpetersenii serovar Hardjo-bovis and Leptospira interrogans serovar Pomona, using the microscopic agglutination test (MAT). For both serovars, a titre of >1:48 was considered positive. Samples from kidneys, uteri and fetal tissue were subjected to bacterial culture, using Ellinghausen-McCullough-Johnson-Harris (EMJH) medium, and real-time PCR, using DNA gyrase subunit B gene primers.

RESULTS: Thirty-four of 116 (29.3%) serum samples were positive for serovar Hardjo-bovis, and 13 (11.2%) for serovar Pomona. Seven of 120 kidneys were positive for serovar Hardjo-bovis by culture, and five of these, but no others, were positive by real-time PCR. Of 120 uteri, none was culture- or PCR-positive. None of 27 fetal samples was culture-positive but one was positive by real-time PCR. The dam of the PCR-positive fetus was culture-negative from the kidney, but had an MAT titre of 1:192 for Hardjo-bovis.

CONCLUSIONS: Attempts to isolate Leptospira spp. from the genital tracts and early fetuses of farmed deer were unsuccessful. However, molecular evidence suggested fetal infection in one case. This finding justifies further study of the role of leptospires in the genital tract and fetus and its association with reproductive loss in farmed deer.  相似文献   
994.
Abstract

A trial was performed to evaluate effects of mussel meal in laying hen diets. The study included 96 hens (Lohmann Selected Leghorn). Four diets with 0, 3, 6 or 9% inclusion level of mussel meal, replacing the same quantities of fish meal, were included. At 26 weeks of age, five eggs from each treatment were collected and analysed for internal egg quality. The different contents of mussel meal had no significant effect on production performance or egg quality parameters except from laying percentage and egg yolk pigmentation. Mussel meal concentration up to 6% tended to improve laying percentage compared to the 0% group. Yolk pigmentation increased significantly with increasing levels of mussel meal. There were no differences between the different diets in fatty acid pattern regarding docosahexaenoic acid (DHA), docosapentaenoic acid (DPA) and linolenic acid in the egg yolk. Eicosapentaenoic acid (EPA) decreased significantly when fishmeal was replaced by mussel meal.  相似文献   
995.
Abstract

The study evaluates the first three parities of 144 Swedish Landrace×Yorkshire sows from three certified organic and three conventional commercial piglet-producing herds. Sows and litters were weaned at seven and five weeks post partum (pp) in organic and conventional herds, respectively. Herdsmen judged sow behaviour and maternal ability at farrowing, two weeks pp and at weaning. Herdsmen collected registrations of litter size, litter weight and sow fat status at farrowing and at weaning. Piglet mortality and oestrous signs were registered continuously during the nursing period. Sows in organic herds were scored to be more cautious among piglets, and to have better nursing behaviour and maternal ability in early lactation than sows in conventional herds. Piglet mortality was higher in organic than in conventional herds. Oestrus during lactation was only observed among sows in organic herds, and sows showing oestrus during lactation were fatter at weaning.  相似文献   
996.
Herbage selenium (Se) concentration is generally low in Norway. It is unknown whether feeding practices on Norwegian organic farms fulfil the dietary needs of Se and vitamin E for sheep and dairy cattle. Therefore we analysed Se in soil and herbage, and Se and vitamin E in animal blood in the indoor feeding season at 14 organic dairy and 14 organic sheep farms. The herbage Se concentration was low. Approximately 50 and 35% of all samples in the first and second cut, respectively, had Se concentrations below the detection limit of 0.01 mg/kg dry matter (DM). The median (10th, 90th percentile) Se concentrations were <0.01 (<0.01, 0.03) and 0.02 (<0.01, 0.06) mg/kg DM in the first and second cuts, respectively. Whole blood Se concentrations were 0.10 (0.04, 0.15) μg/g in dairy cattle and 0.14 (0.03, 0.26) μg/g in sheep. Vitamin E concentrations were 4.2 (2.7, 8.4) mg/l in dairy cattle and 1.3 (0.9, 2.4) mg/l in sheep. None of the soil or plant variables explained the variation in herbage Se concentration, although Se in soil and plant tended to be correlated. Herbage Se concentration was inadequate to meet the dietary Se requirements. Vitamin E requirement was only met in dairy herds. We recommend Se and vitamin E supplementation to ruminants on organic farms.  相似文献   
997.
本试验旨在研究基于不同地区取样点土壤成分条件下,木质素降解菌的生长特性,并为筛选出降解农作物秸秆中木质素的优势菌奠定基础.通过对扬州、南京、临沂、潍坊和连云港地区5个取样点的土壤成分测定,并利用选择培养基和马铃薯葡萄糖琼脂(PDA)-RB亮蓝与PDA-Bavendamm平板显色反应鉴定不同土壤中木质素降解菌生长量及其木质素降解酶特性,依据各取样点菌株的生长特性最终选择优势菌株.结果表明,扬州取样点土壤中木质素降解菌密度[(5.67±0.58) 105g-1]极显著高于另外4个取样点(P<0.01),所获得的木质素降解菌的产酶特性亦优于其他取样点.可知,5个取样点土壤中木质素降解菌密度存在明显差异,并与土壤中总有机碳含量呈强正相关(r=0.84,P<0.01).  相似文献   
998.
本文研究了6株不同饲用乳酸茵固态发酵棉籽粕产有机酸情况。结果表明:德氏乳杆菌产酸量最高,可达18.56g/kg。为了增加产酸量,利用响应面分析法对德氏乳杆菌固态发酵棉籽粕培养基进行了优化,建立了二次回归方程:Y1=25.07—1.0325*X1—0.47625*X2—1.28875*X5—1.8;5125*X1*X1+0.67*X1十X2+O.17*X1*X5—1.59575.X2*X2-0.6775.X2*X3—1.06;575*X5*X3,并确定了最佳培养基配方为:棉粕916.44g/kg,麸皮83.56g/kg,糖蜜27.91g/kg,水480.24g/kg。在此条件下,使德氏乳杆菌固态发酵棉籽粕产有机酸含量达到25.64g/kg,比优化前提高了39.65%。  相似文献   
999.
在有机物污染物监测当中,快速溶剂萃取(Ase)属于一种样品前的处理技术,指出了由于快速溶剂萃取所具有的快捷、高效、半固体待测物等方面的特性。对有机物污染前的处理现状进行了分析,探讨了水环境监测下该技术的应用效能,对有机污染物监测技术在未来水环境当中的发展前景进行了详细阐述。  相似文献   
1000.
《Southern Forests》2013,75(2):117-122
Vermicompost leachate (VCL) is a liquid generated by earthworms and microorganisms during the conversion of organic matter into vermicompost. Vermicompost leachate is rich in nutrients and believed to contain both known and unknown plant growth substances, which are beneficial to plants. Four-year-old seeds (artificially aged) of Eucalyptus dunnii, E. nitens and E. smithii were subjected to VCL at different concentrations. In comparison to the control, the percentage of emerged seedlings of all three species improved significantly (p < 0.05) by more than two to three times when treated with VCL. Treated seedlings showed better growth performance than untreated ones at both first and second harvest, 75 and 150 d after sowing, respectively. Eucalyptus dunnii achieved the highest seedling vigour index (SVI) at the lowest VCL concentrations of 1:5. In contrast, the SVI of E. nitens was maximum at the highest concentrations of VCL (1:20). The best SVI of E. smithii was obtained at VCL concentrations of 1:10. These results suggest that the influence of VCL on SVI of the eucalypt species tested was concentration-dependent. The absolute seedling vigour rate per day was also improved with all the tested concentrations of VCL, with some exceptions for E. dunnii seedlings. The order of response to VCL by the species can be summarised as E. smithii > E. nitens > E. dunnii. The results of this study suggest that VCL can be used to supplement chemical fertilisers. The liquid nature of VCL should be convenient for application both under nursery and field conditions. However, trials under field conditions are required.  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