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   2635篇
  免费   173篇
  国内免费   217篇
林业   461篇
农学   90篇
基础科学   107篇
  1004篇
综合类   746篇
农作物   48篇
水产渔业   21篇
畜牧兽医   449篇
园艺   17篇
植物保护   82篇
  2024年   19篇
  2023年   53篇
  2022年   93篇
  2021年   110篇
  2020年   107篇
  2019年   139篇
  2018年   96篇
  2017年   129篇
  2016年   137篇
  2015年   134篇
  2014年   160篇
  2013年   182篇
  2012年   214篇
  2011年   183篇
  2010年   133篇
  2009年   176篇
  2008年   114篇
  2007年   151篇
  2006年   104篇
  2005年   98篇
  2004年   68篇
  2003年   54篇
  2002年   40篇
  2001年   34篇
  2000年   43篇
  1999年   30篇
  1998年   23篇
  1997年   25篇
  1996年   24篇
  1995年   28篇
  1994年   20篇
  1993年   19篇
  1992年   20篇
  1991年   15篇
  1990年   16篇
  1989年   14篇
  1988年   7篇
  1987年   9篇
  1986年   3篇
  1985年   1篇
排序方式: 共有3025条查询结果,搜索用时 15 毫秒
1.
2.
The aim of this study was to analyse the association between single-nucleotide polymorphisms within INHA and ACVR2B and litter size in Dazu black goats. In total, twenty-two SNPs were genotyped in 190 individuals by SNaPshot and resequencing. The results showed that three SNPs (SNP_1, SNP_12 and SNP_13 in this study) were detected to have significant additive genetic effect on the recorded goat litter size (p < .05). The SNP_1 (NC_030809.1), a non-synonymous substitution of G for T at chr2-g. 28314990 in the exon 2 of INHA gene (NM_001285606.1), resulted in homozygote 2 (HOM2) contributed 0.25 and heterozygote (HET) contributed 0.12 larger litter than homozygote 1 (HOM1). Meanwhile, SNP_12 (Chr22-g. 11721225 A > T) and SNP_13 (Chr22-g. 11721227 A > C) (NC_030829.1) simultaneously mutated at the first and third position of a triplet AAA (lysine, K) in the exon 4 of ACVR2B gene (XM_018066623.1) had estimated genetic effects of HOM1 (0.00) and HOM2 (0.03) larger than HET (−0.12). In conclusion, one SNPs (chr2-g. 28314990 T > G) within the exon 2 of INHA and two SNPs (Chr22-g. 11721225 A > T and Chr22-g. 11721227 A > C) i n the exon 4 of ACVR2B gene were highly recommended as candidate markers of litter size in Dazu black goats. A large-scale association study to assess the impact of these variants on litter size is still necessary.  相似文献   
3.
Surface litter protects rangeland soils against wind and water erosion and provides food and nesting materials for wildlife and insects. However, the ability of grassland systems to provide these services depends on the little studied topic of seasonal surface litter decomposition. Seasonal and annual surface litter decomposition rates were determined between 2014 and 2015 in central and western South Dakota at three mixed-grass prairie locations. Residue bags containing surface litter were placed in the field in late fall (1 November) of 2014 and removed after the winter (1 April), spring (1 July), and summer + fall seasons (1 November) of 2015. The litter was analyzed for total C, total N, acid detergent fiber (ADF), and acid detergent lignin (ADL). Average winter temperatures ranged from −5oC to −15oC, while summer temperatures ranged from 10oC to 35oC. Litter decomposition was lowest during the winter (0.57−0.86 g [kg × day]−1) and greatest during the summer + fall (2.12−2.69 g [kg × day]−1). Over the entire season, 40.8−62% of the surface litter decomposed. Winter litter decomposition was positively correlated with air temperature (r = 0.62, P < 0.01) and snow depth (r = 0.61, P < 0.01), and negatively correlated with C/N ratio (r = −0.65, P < 0.01), ADF (r = −0.35, P < 0.05), and ADL (r = −0.25, P < 0.05) concentrations. These findings indicate that winter decomposition cannot be ignored and that winter surface litter decomposition increases with snow depth.  相似文献   
4.
用经改进的聚丙烯酰胺凝胶电泳铁染法,测定了二花脸猪血浆转铁蛋白多态类型,分析了其基因型频率和基因频率,并探讨了猪血浆转铁蛋白多态性与产仔性能和增重速度的相关性。  相似文献   
5.
讨论了一类与区域分解算法有关的空间分解问题,将二阶问题的已有的空间分解结果推广到四阶问题的相应空间中。还证明了一致重迭性条件与Badea条件的等价性。  相似文献   
6.
羊草草原枯枝落叶中能量特征 分析   总被引:1,自引:0,他引:1  
在草原生态系统中,以枯枝落叶为主体的能量的输入,积累和释放,是能量流动的主要环节。每年因植物的枯死,转移到枯枝落叶的能量为3949.679KJ.m62.a,其季节变化呈指数形式。现存枯枝落叶中贮存的能量为3424.10KJ.m^2,经1年分解后,残留在枯枝落叶的能量为2282.43KJ.m^2,释放出的能量为1141.60KJ.m^2,能量释放的季节动态呈S型曲线。  相似文献   
7.
The objective of the study was to categorize objectively nine breeds of healthy dogs according to pelvic limb standing posture. A total of 135 dogs from different breeds were used and the standing angles of the hip, stifle, and tarsal joints, together with the percentages of the greater trochanter, patella and tuber calcanei heights, with respect to crista iliaca height, were used as discriminant variables for the categorization of pelvic limb posture. All included breeds were allocated to three groups of the standing pelvic limb posture. The best discriminant variables between the three groups were the percentage of patellar height, and the standing angles of the stifle and tarsal joints. German shepherds, Anatolian shepherds, golden retrievers, Rottweilers, Belgian Malinois and Dobermann pinschers were well separated between 89% and 100% success rate for the categorization. The minimal success rate was determined in Berner sennenhunds as the ratio of 75%. It was also determined that Dobermann pinschers had the straightest pelvic limbs, while German shepherds had the most angulated pelvic limbs. Further studies are required to document the impact of postural differences in active and passive structure diseases of the locomotor system of the pelvic limb among dog breeds.  相似文献   
8.
9.
A late term broodmare presented with low‐grade intestinal colic. Clinical findings were consistent with a small intestinal lesion requiring surgical intervention. The risks of general anaesthesia to the fetus, combined with clinical findings in an otherwise quiet natured horse influenced the decision to choose an alternative standing left flank approach first. A small intestinal resection and anastomosis was performed successfully and the mare delivered a healthy foal without complications a month later. Standing colic surgery might be a preferred alternative to general anaesthesia in certain circumstances as discussed in this case report.  相似文献   
10.
Benefits of organic farming on soil fauna have been widely observed and this has led to consider organic farming as a potential approach to reduce the environmental impact of conventional agriculture. However, there is still little evidence from field conditions about direct benefits of organic agriculture on soil ecosystem functioning. Hence, the aims of this study were to compare the effect of organic farming versus conventional farming on litter decomposition and to study how this process is affected by soil meso- and macrofauna abundances. Systems studied were: (1) organic farming with conventional tillage (ORG), (2) conventional farming with conventional tillage (CT), (3) conventional farming under no-tillage (NT), and (4) natural grassland as control system (GR). Decomposition was determined under field conditions by measuring weight loss in litterbags. Soil meso- and macrofauna contribution on decomposition was evaluated both by different mesh sizes and by assessing their abundances in the soil. Litter decomposition was always significantly higher after 9 and 12 months in ORG than in CT and NT (from 2 to 5 times in average), regardless decomposer community composition and litter type. Besides, mesofauna, macrofauna and earthworm abundances were significantly higher in ORG than in NT and CT (from 1.6 to 3.8, 1.7 to 2.3 and 16 to 25 times in average, respectively for each group). These results are especially relevant firstly because the positive effect of ORG in a key soil process has been proved under field conditions, being the first direct evidence that organic farming enhances the decomposition process. And secondly because the extensive organic system analyzed here did not include several practices which have been recognized as particularly positive for soil biota (e.g. manure use, low tillage intensity and high crop diversity). So, this research suggests that even when those practices are not applied, the non-use of agrochemicals is enough to produce positive changes in soil fauna and so in decomposition dynamics. Therefore, the adoption of organic system in an extensive way can also be suggested to farmers in order to improve ecosystem functioning and consequently to achieve better soil conditions for crop production.  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