全文获取类型
收费全文 | 6638篇 |
免费 | 496篇 |
国内免费 | 539篇 |
专业分类
林业 | 1266篇 |
农学 | 1285篇 |
基础科学 | 220篇 |
364篇 | |
综合类 | 2461篇 |
农作物 | 537篇 |
水产渔业 | 310篇 |
畜牧兽医 | 762篇 |
园艺 | 211篇 |
植物保护 | 257篇 |
出版年
2024年 | 22篇 |
2023年 | 87篇 |
2022年 | 143篇 |
2021年 | 166篇 |
2020年 | 187篇 |
2019年 | 212篇 |
2018年 | 126篇 |
2017年 | 196篇 |
2016年 | 212篇 |
2015年 | 201篇 |
2014年 | 292篇 |
2013年 | 315篇 |
2012年 | 404篇 |
2011年 | 453篇 |
2010年 | 407篇 |
2009年 | 390篇 |
2008年 | 392篇 |
2007年 | 423篇 |
2006年 | 385篇 |
2005年 | 310篇 |
2004年 | 289篇 |
2003年 | 236篇 |
2002年 | 187篇 |
2001年 | 165篇 |
2000年 | 153篇 |
1999年 | 131篇 |
1998年 | 115篇 |
1997年 | 130篇 |
1996年 | 116篇 |
1995年 | 106篇 |
1994年 | 102篇 |
1993年 | 100篇 |
1992年 | 95篇 |
1991年 | 95篇 |
1990年 | 87篇 |
1989年 | 49篇 |
1988年 | 45篇 |
1987年 | 32篇 |
1986年 | 29篇 |
1985年 | 14篇 |
1984年 | 14篇 |
1983年 | 3篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1955年 | 2篇 |
排序方式: 共有7673条查询结果,搜索用时 31 毫秒
971.
Polyethylene glycol influences feeding behaviour of dairy goats browsing on bushland with different herbage cover 总被引:1,自引:1,他引:0
The effects of supplemental polyethylene glycol (PEG) on feeding behaviour, diet composition, intake and milk production were studied in late lactating Sarda goats browsing on a lentisk-based bushland with different herbage cover. The goats were allotted to two groups that browsed 7 h daily on 2 plots characterised by different cover proportions of woody and herbaceous species. These proportions were 90% woody vs. 10% herbaceous species in one plot (W — woody, close bushland with low herbage cover) and 70% woody vs. 30% herbaceous species in the other plot (H — herbaceous, open bushland with high herbage cover). Each group was divided into two sub-groups: one fed with 50 g day− 1 of PEG 4000 MW (PEG+) and the other receiving no PEG supplementation (PEG−). The feeding behaviour was different between groups; goats in herbaceous plot (H-goats), had a longer grazing time, expressed as percentage of total observation time than the counterparts (64% vs. 58%, P < 0.05). The intake was not affected by the treatments. The percentage of the species eaten by the goats was different in the two groups: H-goats ate more herbaceous and less woody species than goats in woody plot (W-goats). In each group the PEG tended to affect the botanical composition of the diet. PEG dosed goats ate more tanniferous species than control counterparts (P = 0.12). The average daily milk production was 473, 591, 541 and 589 ml, respectively in W-PEG−, W-PEG+, H-PEG− and H-PEG+ sub-groups with significant effect of herbage cover × PEG dose interaction (P < 0.05). The milk urea was affected by herbage cover (P < 0.001) and PEG supplementation (P < 0.001). In conclusion, the effect of PEG is strictly dependent on pasture composition. In open bushland, featured by moderate woody cover, the effect of PEG is more intense on diet selection, due to its marked post-ingestive effects. In close bushland, characterised by high proportion of tanniferous species prevails the efficacy of PEG as a performance booster. 相似文献
972.
In this paper we discuss the use of genetic engineering in livestock production. We examine the main two different aspects of genetic engineering: cloning and transgenesis. After commenting what has been expected from both techniques in livestock production in the last 25 years, the practical difficulties for implementing cloning and transgenesis are examined. Apart from technical difficulties, problems derived from the detection of genetically superior animals and evaluation of the clones and the transgenic animals make these techniques less interesting than they appear to be. Most of the observed variability of the economically interesting traits is not genetic, genetic evaluation needs a large number of animals and cloning success will represent a serious loss of genetic variability and the loss of the flexibility needed for markets in constant evolution. There is a risk in transgenic animals of production of new intermediate biochemical products that may be toxic, allergenic or carcinogenic. The benefits produced by transgenic animals hitherto hardly justify this risk. The expectations that genetic engineering produced 25 years ago should be re-examined, considering the risks and the high investment required. 相似文献
973.
974.
Phenotypic and functional analysis of bovine peripheral blood dendritic cells before parturition by a novel purification method
下载免费PDF全文
![点击此处可从《Animal Science Journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Tao Zhuang Megumi Urakawa Hidetoshi Sato Yuko Sato Teruaki Taguchi Tsuyoshi Umino Shiro Katto Koutaro Tanaka Kozue Yoshimura Naokazu Takada Hiroko Kobayashi Megumi Ito Michael T. Rose Yoshio Kiku Yuya Nagasawa Haruki Kitazawa Kouichi Watanabe Tomonori Nochi Tomohito Hayashi Hisashi Aso 《Animal Science Journal》2018,89(7):1011-1019
Dendritic cells (DCs) are specialized antigen presenting cells specializing in antigen uptake and processing, and play an important role in the innate and adaptive immune response. A subset of bovine peripheral blood DCs was identified as CD172a+/CD11c+/MHC (major histocompatibility complex) class II+ cells. Although DCs are identified at 0.1%–0.7% of peripheral blood mononuclear cells (PBMC), the phenotype and function of DCs remain poorly understood with regard to maintaining tolerance during the pregnancy. All cattle used in this study were 1 month before parturition. We have established a novel method for the purification of DCs from PBMC using magnetic‐activated cell sorting, and purified the CD172a+/CD11c+ DCs, with high expression of MHC class II and CD40, at 84.8% purity. There were individual differences in the expressions of CD205 and co‐stimulatory molecules CD80 and CD86 on DCs. There were positive correlations between expression of cytokine and co‐stimulatory molecules in DCs, and the DCs maintained their immune tolerance, evidenced by their low expressions of the co‐stimulatory molecules and cytokine production. These results suggest that before parturition a half of DCs may be immature and tend to maintain tolerance based on the low cytokine production, and the other DCs with high co‐stimulatory molecules may already have the ability of modulating the T‐cell linage. 相似文献
975.
A. Gurgul T. Szmatoła K. Ropka‐Molik I. Jasielczuk K. Pawlina E. Semik M. Bugno‐Poniewierska 《Zeitschrift für Tierzüchtung und Züchtungsbiologie》2016,133(4):264-276
The study is aimed at identifying selection footprints within the genome of Limousin cattle. With the use of Extended Haplotype Homozygosity test, supplemented with correction for variation in recombination rates across the genome, we created map of selection footprints and detected 173 significant (p < 0.01) core haplotypes being potentially under positive selection. Within these regions, a number of candidate genes associated inter alia with skeletal muscle growth (GDF15, BMP7, BMP4 and TGFB3) or postmortem proteolysis and meat maturation (CAPN1 and CAPN5) were annotated. Noticeable clusters of selection footprints were detected on chromosomes 1, 4, 8 and 14, which are known to carry several quantitative trait loci for growth traits and meat quality. The study provides information about the genes and metabolic pathways potentially modified under the influence of directional selection, aimed at improving beef production characteristics in Limousin cattle. 相似文献
976.
We evaluated support for four alternate hypotheses explaining the distribution of breeding Brown-headed Cowbirds (Molothrus ater) in forests at varying distances from the forest edge in three Midwestern USA landscapes with varying amounts of forest fragmentation
(core forest area ranged from 5 to 70%). We focused on breeding cowbirds’ use of forest because of the risk of nest parasitism
to forest-dwelling hosts and to identify factors affecting breeding cowbird habitat selection. We compared distances of cowbird
locations in the forest from the forest edge (“edge distances”) to distances of random forest locations in the entire landscape
or within individual cowbird home ranges. We analyzed 1322 locations of 84 cowbirds across three landscapes. We found support
for the landscape context hypothesis that breeding cowbird preference for forest edge varied with landscape context. Ninety
percent of cowbird locations were within 150–350 m of forest edge, despite the overall availability of forest at greater distances
from edge (as far as 500–1450 m) both within cowbird home ranges and the entire forested landscape. Cowbird preference for
edge varied by landscape context largely due to differences in the availability of forest edge. In a highly fragmented forest
cowbirds utilized the entire forest and likely viewed it as “all edge.” In less fragmented forests, cowbirds preferred edge.
We consider how variation in cowbird edge preference might relate to patterns in host abundance, host diversity, and host
quality because cowbird movements indicate they are capable of using forest farther from edges. 相似文献
977.
Variability of the seeds and cones and key taxonomic characters selection ofLarix in Northeast China
The paper presents the results of biometrical study of the seeds and cones of larch taxa from Northeast of China. Fourteen
characters are considered and their analyses showed that the seeds and cones of the larch taxa were relative greatly variable.
Cluster analysis and principal component analysis for all 6 larch taxa were made. The analysis results proved that the dominant
larch population in Toudao Farm fell within the range of variation ofLarix olgensis. The key taxonomic characters are the length of seed wing, length of cone, length of seed, the length/width ratio of seed
and number of scales.
This research was supported by Natural Science Foundation of Heilongjiang Province.(C9515)
Responsible Editor: Chai Ruihai 相似文献
978.
Undesirable characteristic of rapeseed oil is a relatively high level of linolenic acid (18:3), which is easily oxidized leading
to rancidity and a shortened shelf life of the oil. Previous attempts to reduce linolenic acid levels in rapeseed oil through
breeding have been impaired by complex genetics and strong environmental sensitivity of this trait. Therefore, our objective
was to develop molecular markers for low linolenic acid that could facilitate the breeding of low linolenic rapeseed. Bulked
segregant analysis was employed to identify two RAPD markers associated with 18:3 in a doubled haploid population segregating
for linolenic and erucic acid levels. Based on analysis of individual DH lines, the markers RM350 and RM574, representing
two independent loci, accounted for a total of 39% of the genetic variability in this population. This marker RM350 alone
accounted for 25% genetic variation for this trait with no evidence of recombination. Significant interlocus interaction found
between the markers RM350 and RM574 suggested that epistasis was involved in the genetic control of 18:3 level in this population.
Another marker designated as RM322, which was independent of the other two, was found significantly associated with the erucic
acid level and oil content. RAPD markers identified in this study should be a useful tool for the early detection of low linolenic,
or low or high erucic acid genotypes in rapeseed breeding programs based on doubled haploids.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
979.
In Accelerated Recurrent Selection (ARS) schemes, selection is based on the predicted performance of progeny families, estimated as the average of the parental families. These schemes can halve cycle time compared to simple recurrent selection methods. They also have a lower requirement for evaluation of families in yield trials, although they do require more seed production. ARS therefore provides options in cycle time, effective population size, response to selection and cost which have not been available before. Example schemes are compared by computer simulation with truncation selection and with optimal family selection, where contributions of families to the next generation are optimised to give the maximum response to selection at a specified effective population size. In many circumstances, ARS schemes compare favourably. Difficulties in combining estimates of selection intensity and of effective population size when comparing the merits of different breeding schemes are discussed. It is suggested that unless one is interested in response to selection over periods greater than 50 years, the weight given to effective population size in ranking different schemes should be small. 相似文献
980.
B.R. Ntare 《Euphytica》1999,107(2):141-147
Selection of superior crosses of groundnut (Arachis hypogaea L.) in early generations would increase the probability of identifying
superior lines. The objective of this study was to determine the potential of selecting for physiological traits identified
in a yield model [crop growth rate (C), reproductive duration (DR) and partitioning (p)] in segregating populations. Forty
populations and nine parental lines were evaluated in replicated trials in 1992 (F2, 1993 (F3) and 1994 (F4) at three locations
in Niger. Physiological traits were estimated from final yield and biomass as well as data on flowering and maturity. Regressions
from two different parent-offspring generations (F2: F3 and F3: F4) were calculated. The results were compared to determine
if early generation performance accurately predicts the performance of cross bulks in later generations. Differences were
observed among populations and parents for all traits. Effects of locations were significant for C, p and DR in F2 and F3
but nonsignificant for yield and C in F4. Regression coefficients from F3: F2 were 0.10 ± 0.08 for C, 0.45 ± 0.17 for p, 0.10
± 0.03 for DR and 0.16 ± 0.03 for pod yield. Based on F3: F4 regression, the coefficients were 0.12 ± 0.23 for C, 0.46 ± 0.17
for p and 0.57 ± 0.17 for yield. Parent-offspring correlations were in most cases similar to the regression values. It was
concluded that selection for yield and model components in early generation bulks may inneffective.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献