全文获取类型
收费全文 | 1280篇 |
免费 | 86篇 |
国内免费 | 87篇 |
专业分类
林业 | 195篇 |
农学 | 46篇 |
基础科学 | 22篇 |
221篇 | |
综合类 | 417篇 |
农作物 | 92篇 |
水产渔业 | 209篇 |
畜牧兽医 | 170篇 |
园艺 | 41篇 |
植物保护 | 40篇 |
出版年
2024年 | 4篇 |
2023年 | 18篇 |
2022年 | 25篇 |
2021年 | 37篇 |
2020年 | 44篇 |
2019年 | 49篇 |
2018年 | 29篇 |
2017年 | 43篇 |
2016年 | 75篇 |
2015年 | 54篇 |
2014年 | 76篇 |
2013年 | 117篇 |
2012年 | 111篇 |
2011年 | 89篇 |
2010年 | 78篇 |
2009年 | 60篇 |
2008年 | 75篇 |
2007年 | 57篇 |
2006年 | 58篇 |
2005年 | 63篇 |
2004年 | 48篇 |
2003年 | 41篇 |
2002年 | 30篇 |
2001年 | 31篇 |
2000年 | 21篇 |
1999年 | 17篇 |
1998年 | 15篇 |
1997年 | 17篇 |
1996年 | 12篇 |
1995年 | 10篇 |
1994年 | 8篇 |
1993年 | 5篇 |
1992年 | 9篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1987年 | 2篇 |
1983年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有1453条查询结果,搜索用时 31 毫秒
981.
982.
Basky Z 《Pest management science》2003,59(10):1152-1158
Russian wheat aphid, Diuraphis noxia (Kurdjumov) is a severe pest of cereals in South Africa and in the USA. In order to reduce D noxia damage, intensive resistance breeding programs have been undertaken, resulting in D noxia-resistant cultivars that are now widely used in South Africa and in the USA. However, there appear to be differences in the ability of different populations of D noxia to damage these resistant cereal cultivars. To determine whether different biotypes of D noxia are present, damage to eight wheat cultivars was compared when they were exposed to either Hungarian or South African aphid strains. It appeared that the Hungarian CVS MV Magdaléna, MV Magvas, and MV 17 were susceptible to D noxia from both Hungary and South Africa. The susceptible South African CV Betta was also severely damaged regardless of the country of origin of the aphids. None of the cultivars resistant in South Africa (Caledon, SST 333, SST 972 and Halt) were, however, resistant to Hungarian populations of D noxia. These cultivars, which carry resistance genes originating from the breeding lines PI 262660, PI 137739 and PI 372129, were severely damaged by the Hungarian D noxia. Apart from the highly resistant CV Halt, the resistant cultivars used in this study were developed in South Africa, with the biotype present there. Damage to all cultivars tested was significantly more severe in response to Hungarian than to South African D noxia, indicating that a more damaging aphid biotype occurs in Hungary. However, D noxia has not yet become a pest of wheat in Hungary, possibly due to a difference in cultural practices. 相似文献
983.
Weed competition in crops constitutes the main limiting factor for groundnut crop yields in the Congolese humid zone. In order to control the problems related to weed interference in the crop, the most appropriate weeding period for good production has been investigated. The critical period of weed interference has been determined from a groundnut crop experiment weeded at different times during two crop cycles. The data obtained allowed the authors to propose a suitable weeding regime that would reduce the costs and frequencies of weeding and enable local farmers to plan their work more effectively. 相似文献
984.
R.L.Heathcote 《干旱区资源与环境》1993,(Z1)
<正> 1. Introduction The revived concern for environmental hazards implicit in the contemporary debatesupon Global Change questions, the Ozone Hole and the International Decade of NaturalDisaster Reduction, raises once more the question of disaster relief. Much of the ensuingconcerm for disaster relief however has focused upon technical questions of supplying aid in 相似文献
985.
南水北调大型泵结构设计研究 总被引:19,自引:10,他引:9
值此南水北调工程正式开工,回顾了我国大型泵的发展历史;介绍了国外典型大型泵的结构,具有转速高、齿轮传动,配套电机体积小,泵站建设总投资少等优点;阐述了吸取国外先进经验改进我国大型泵结构设计的必要性及可行性;提出要加大科研力度,使大型泵设计实现第二次革命,推动排灌事业的发展. 相似文献
986.
苏垦8312系利用双丰1号四倍体与MHE_2/76F_2所为亲本,经过系统选择、多倍体组合配制、二次组合筛选选育而威,经我国西部地区(新疆、宁夏等5省区)区域试验、生产示范鉴定结呆表明,苏垦8312在块根产量、含糖率和产糖量方面,平均比对照品种分别提高12.04%、0.75度和17.56%。是具有根型好、丰产、抗(耐)病能力铰强的标准型品种。 相似文献
987.
The aggressiveness of four Phytophthora infestans isolates collected from wild and cultivated potato species (sect. Petota ) and the level of resistance of nine Petota species were assessed in the highland tropics of Ecuador. For this, isolates of P. infestans were inoculated on whole plants of Petota species in the field and net house and six epidemiological components – infection frequency (IF), incubation period (IP), latent period (LP), lesion size (LS), lesion growth rate (LGR), and relative area under the lesion expansion curve (RAULEC) – were measured during a single infection cycle. Additionally, host specificity was determined by testing for a significant host by pathogen interaction using the same components. The results showed significant differences among isolates of the EC-1 clonal lineage for IP, IF, and RAULEC. Significant differences among isolates were not found for the other components measured. There were significant differences in resistance among the accessions of Petota hosts tested. RAULEC, LGR, LP, and LS were in general more adequate in differentiating among the more resistant and more susceptible accessions but the importance of each component varied with host species. There was slight and inconsistent evidence for the existence of host specificity in some isolates of Petota hosts. IP was the only component for which a significant host by isolate interaction was observed and in most cases the isolates had the greatest aggressiveness on their hosts of origin. 相似文献
988.
Landscape connectivity can have profound consequences for distribution and persistence of populations and metapopulations.
Evaluating functional connectivity of a landscape for a species requires a measure of dispersal rates through landscape elements
at a spatial scale sufficient to encompass movement capabilities of individuals over the entire landscape. We evaluated functional
connectivity for a rock-dwelling mammal, the mountain vizcacha (Lagidium viscacia), in northern Patagonia. Because of the strict association of mountain vizcachas with rocks, we hypothesized that connectivity
for this species would be influenced by geology. We used molecular genetic estimates of gene flow to test spatially explicit
models of connectivity created with GIS cost-distance analysis of landscape resistance to movement. We analyzed the spatial
arrangement of cliffs with join counts and local k-function analyses. We did not capture and genotype individuals, but sampled
at the population level through non-invasive collection of feces of mountain vizcachas. The model of landscape connectivity
for mountain vizcachas based on geology was corroborated by the pattern of genetic structure, supporting the hypothesis that
functional connectivity for mountain vizcachas is influenced by geology, particularly by the distribution of appropriate volcanic
rocks. Analysis of spatial arrangement of cliffs indicated that occupied cliffs are clustered and confirmed that rivers act
as barriers to dispersal for mountain vizcachas. Our methods could be used, within certain constraints, to study functional
landscape connectivity in other organisms, and may be particularly useful for cryptic or endangered species, or those that
are difficult or expensive to capture. 相似文献
989.
Brigitte Moser Jochen A. G. Jaeger Ulrike Tappeiner Erich Tasser Beatrice Eiselt 《Landscape Ecology》2007,22(3):447-459
Patch-based landscape metrics can be biased by the boundaries and the extent of a reporting unit if the boundaries fragment
patches. We call this the “boundary problem”. The effective mesh size m
eff is a convenient method to quantify landscape fragmentation, that is based on the probability that two points chosen randomly
in a region will be connected, e.g., not be separated by roads, railroads, or urban development. The cutting-out (CUT) procedure,
used in the original computation of m
eff, suffers from the boundary problem because the boundaries of the reporting units are considered to be additional barriers.
Therefore, m
eff will be underestimated, particularly if reporting units are embedded within the broader landscape. In this paper, we present
a solution to overcome this limitation by a new method called “cross-boundary connections” (CBC) procedure. It attributes
the connections between two points that are located in different reporting units to both reporting units. We systematically
compare the CBC procedure to the CUT procedure and show that the boundary problem is intrinsic to the CUT procedure, while
the CBC procedure is independent of the size and administrative boundaries of reporting units. In addition, we elucidate the
superior performance of the new procedure in the case study of South Tyrol where m
eff is being used for sustainability reporting on the level of municipalities. The new CBC procedure eliminates the bias due
to the boundaries and the size of reporting units in measuring landscape fragmentation through m
eff. 相似文献
990.
The purpose of this study was to examine the effect of day length on seminal characteristics, testicular size, sexual behavior and testosterone concentration (T4) in dominant and subordinate Pelibuey rams. Six groups of three animals were evaluated every 2 weeks over a 12 month period. Dominant (D), medium (M) and subordinate (S) animals in each group were identified through a food competition test every 15 days. All rams ejaculated and produced semen throughout the year. A linear hierarchy was clearly established and maintained during the year. S rams had lower levels (P < 0.05) of semen volume, sperm concentration and testicular volume than D rams, regardless of season. Scrotal circumference was not influenced (P > 0.05) by their social position. D rams were more affected by the photoperiod, decreasing (P < 0.05) reaction times, increasing (P < 0.05) T4 and producing more (P < 0.05) sperm per ejaculation than S rams during short days (14.65 ± 1.22 vs 26.92 ± 1.65 s; 8.68 ± 0.44 vs 7.37 ± 0.40 ng/mL and 3.37 ± 0.17 vs 2.04 ± 0.16 sperm/109, respectively). Semen volume and sperm concentration were significantly (P < 0.05) greater during short days in all rams, regardless of their social status, with the exception of sperm concentration in D rams where no variation was found. M rams displayed variable values in the range between the D and S rams. It was concluded that the magnitude of the seasonal effects was not sufficient to prevent the rams being used for breeding throughout the year and that seasonal variation within the variables affected the social ranks differently. 相似文献