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   6086篇
  免费   3459篇
林业   149篇
农学   428篇
  1256篇
综合类   2篇
农作物   82篇
水产渔业   2479篇
畜牧兽医   3906篇
植物保护   1243篇
  2022年   2篇
  2021年   123篇
  2020年   462篇
  2019年   1014篇
  2018年   889篇
  2017年   929篇
  2016年   921篇
  2015年   804篇
  2014年   790篇
  2013年   951篇
  2012年   439篇
  2011年   451篇
  2010年   583篇
  2009年   237篇
  2008年   213篇
  2007年   58篇
  2006年   88篇
  2005年   80篇
  2004年   84篇
  2003年   83篇
  2002年   97篇
  2001年   66篇
  2000年   98篇
  1999年   7篇
  1997年   11篇
  1996年   4篇
  1995年   4篇
  1994年   5篇
  1993年   6篇
  1990年   4篇
  1989年   2篇
  1988年   1篇
  1986年   2篇
  1985年   1篇
  1984年   1篇
  1983年   2篇
  1982年   3篇
  1981年   1篇
  1980年   1篇
  1979年   1篇
  1978年   6篇
  1977年   5篇
  1976年   1篇
  1975年   3篇
  1974年   2篇
  1973年   1篇
  1970年   4篇
  1968年   1篇
  1966年   1篇
  1963年   1篇
排序方式: 共有9545条查询结果,搜索用时 31 毫秒
941.
The vascular endothelial growth factor (VEGF) is essential for follicular development by promoting follicular angiogenesis, as well as for the proliferation and survival of granulosa cells. The biological effects of VEGF are regulated by two membrane receptors, VEGFR1 and VEGFR2, and two soluble receptors, sVEGFR1 and sVEGFR2, which play an antagonistic role. Thus, the objective of this study was to identify the mRNA expression pattern of total VEGF, VEGFR1, VEGFR2, sVEGFR1 and sVEGFR2 in bovine preselected follicles (PRF) and post‐selected follicles (POF). The mRNA expression of these five genes in both granulosa cells (GC) and theca cells (TC) was compared between follicles classified as PRF and POF based on their diameter and on their ratio of estradiol/progesterone (E2/P4). Results showed a lower expression of mRNA of sVEGFR1 and sVEGFR2 in POF than in PRF (p < .05). Regarding the mRNA expression of total VEGF, VEGFR1 and VEGFR2, there was no difference between POF and PRF follicles (p > .05). Our results showed that the mRNA expression of VEGFR2 and sVEGFR1 was more abundant than the expression of VEGFR1 and sVEGFR2, while GC was the main source of mRNA for total VEGF. On the other hand, TC was the follicular compartment where the receptors were most expressed. Our results suggest that non‐dominant follicles maintain a greater concentration of the mRNA expression of both membrane and soluble VEGF receptors. On the other hand, follicular dominance is related to a reduction in the mRNA expression of sVEGFR1 and sVEGFR2, which may favour VEGF binding with VEGFR2 and, hence, improve the follicular health and development.  相似文献   
942.
Bovine trypanotolerance is a heritable trait associated to the ability of the individuals to control parasitaemia and anaemia. The INHBA (BTA4) and TICAM1 (BTA7) genes are strong candidates for trypanotolerance‐related traits. The coding sequence of both genes (3951 bp in total) were analysed in a panel including 79 Asian, African and European cattle (Bos taurus and B. indicus) to identify naturally occurring polymorphisms on both genes. In general, the genetic diversity was low. Nineteen of the 33 mutations identified were found just one time. Seventeen different haplotypes were defined for the TICAM1 gene, and 9 and 12 were defined for the exon 1 and the exon 2 of the INHBA gene, respectively. There was no clear separation between cattle groups. The most frequent haplotypes identified in West African taurine samples were also identified in other cattle groups including Asian zebu and European cattle. Phylogenetic trees and principal component analysis confirmed that divergence among the cattle groups analysed was poor, particularly for the INHBA sequences. The European cattle subset had the lowest values of haplotype diversity for both the exon1 (monomorphic) and the exon2 (0.077 ± 0.066) of the INHBA gene. Neutrality tests, in general, did not suggest that the analysed genes were under positive selection. The assessed scenario would be consistent with the identification of recent mutations in evolutionary terms.  相似文献   
943.
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   
944.
Paraoxonase‐1 (PON1) is an enzyme found in serum and follicular fluid that protects cell membrane and circulating lipids against oxidative damage. The aims of this study were to measure the direct effects of recombinant PON1 (rPON1) on bovine oocyte maturation at the molecular level (gene expression) and to measure the carry‐over effects of PON1 on pre‐implantation embryo development in vitro. COCs were submitted to IVM with the addition of 0.0, 0.02, 0.04 and 0.08 mg ml?1 of rPON1, corresponding to an average PON1 arylesterase enzyme activity of 2.2 ± 0.4, 15.5 ± 1.5, 30.2 ± 3.0 and 57.9 ± 5.0 U ml?1, respectively. The results indicated that addition of rPON1 during IVM improved embryo development in a dose‐dependent manner as D7 embryo development was 22.2%, 29.4%, 32.2% and 37.0% for the treatment groups, respectively (p = 0.02). In conclusion, addition of PON1 enzyme during IVM exerted dose‐related positive effects on embryo development rates to blastocysts.  相似文献   
945.
This study investigated the effects of long‐term extenders on post‐thaw sperm quality characteristics following different holding times (HT) of boar semen at 17 and 10°C. Sperm‐rich fractions, collected from five boars, were diluted in Androhep® Plus (AHP), Androstar® Plus (ASP), Safecell® Plus and TRIXcell® Plus (TCP) extenders. The extended semen samples were held for 2 hr at 17°C (HT 1) and additionally for 24 hr at 10°C (HT 2), after they were evaluated and frozen. CASA sperm motility and motion patterns, mitochondrial membrane potential (MMP), plasma membrane integrity (PMI) and normal apical ridge (NAR) acrosome integrity were assessed in the pre‐freeze and frozen‐thawed semen. The Vybrant Apoptosis Assay Kit was used to analyse the proportions of viable and plasma membrane apoptotic‐like changes in spermatozoa. Results indicated that boar variability, extender and HT significantly affected the sperm quality characteristics, particularly after freezing‐thawing. Differences in the pre‐freeze semen were more marked in the sperm motion patterns between the HTs. Pre‐freeze semen in HT 2 showed significantly higher VCL and VAP, whereas no marked effects were observed in the sperm membrane integrity and viability (YO‐PRO‐1?/PI?) among the extenders. Post‐thaw sperm TMOT and PMOT were significantly higher in the AHP and ASP extenders of HT 2 group, whereas VSL, VCL and VAP were markedly lower in the TCP extender. Furthermore, spermatozoa from the AHP‐ and ASP‐extended semen of HT 2 group were characterized by higher MMP, PMI and NAR acrosome integrity following freezing‐thawing. In most of the extenders, the incidence of frozen‐thawed spermatozoa with apoptotic‐like changes was greater in HT 1. The findings of this study indicate that holding of boar semen at 10°C for 24 hr in long‐term preservation extenders modulates post‐thaw sperm quality characteristics in an extender‐dependent manner. These results will further contribute to the improvement in the cryopreservation technology of boar semen.  相似文献   
946.
This study investigated the immediate (6 h or less) effects of fibrinogen and albumin contained in transfused equine origin fresh frozen plasma on those proteins when measured in sick neonatal foals. Fibrinogen and albumin concentrations were measured in the administered plasma and in 31 sick foals at admission to a referral neonatal intensive care unit. Additional samples were obtained from the foals at 2 and 6 h following transfusion. No changes in albumin concentration were recognised. The main determinant of fibrinogen concentration following transfusion was the concentration of fibrinogen in the foal at admission. Importantly, intravenous transfusion of equine fresh frozen plasma did not result in immediate (6 h or less) increases or decreases in the fibrinogen concentration in the recipient foals. Fibrinogen from the donor contained within transfused plasma will not directly affect fibrinogen concentrations measured at later times.  相似文献   
947.
Predicting male fertility on non‐invasive sperm traits is of big importance to human and animal reproduction strategies. Combining the wide range of parameters monitored by computer‐assisted sperm analysis (CASA) with some molecular traits (e.g. mtDNA content) may help to identify markers of the male fertility. The aim of this study was to characterize variation in the mtDNA copy number in equine sperm and to investigate whether mtDNA content is correlated with quality traits of stallion spermatozoa and the age of the male. Ejaculates collected from 53 fertile stallions were divided into four age groups (3–5, 6–10, 11–14 and >15 years) and were subjected to a complex investigation including conventional analysis, CASA, flow cytometry and mtDNA content (real‐time PCR). The mean (±SD) number of mtDNA copies equalled 14 ± 9 and varied from 3 to 64. Considering the great number of sperm parameters monitored in this study, only few of them were correlated with the mtDNA content: ejaculate volume (a positive correlation), the amplitude of lateral head displacement (ALH; a negative correlation) and the high mitochondrial activity index (a negative correlation). The stallion age was not correlated with the mtDNA copy number. This study provides the first set of data on mtDNA content in equine sperm and confirms phenomena previously described for humans and dog on associations between sperm mtDNA content and selected motility parameters monitored by the CASA. Basing our study on spermatozoa from fertile stallions could however limit the extent of detected associations.  相似文献   
948.
The negative impact of heat stress on health and productivity of dairy cows is well known. Heat stress can be quantified with the temperature–humidity index (THI) and is defined as a THI ≥ 72. Additionally, animal welfare is affected in cows living under heat stress conditions. Finding a way to quantify heat stress in dairy cows has been of increasing interest over the past decades. Therefore, the objective of this study was to evaluate concentrations of faecal glucocorticoid metabolites [i.e. 11,17‐dioxoandrostanes (11,17‐DOA)] as an indirect stress parameter in dairy cows without heat stress (DOA 0), with heat stress on a single day (acute heat stress, DOA 1) or with more than a single day of heat stress (chronic heat stress, DOA 2). Cows were housed in five farms under moderate European climates. Two statistical approaches (approach 1 and approach 2) were assessed. Using approach 1, concentrations of faecal 11,17‐DOA were compared among DOA 0, DOA 1 and DOA 2 samples regardless of their origin (i.e. cow, unpaired comparison with a one‐way anova ). Using approach 2, a cow was considered as its own control; that is 11,17‐DOA was treated as a cow‐specific factor and only paired samples were included in the analysis for this approach (paired comparison with t‐tests). In approach 1 (p = 0.006) and approach 2 (p = 0.038), 11,17‐DOA values of cows under acute heat stress were higher compared to those of cows without heat stress. Our results also indicate that acute heat stress has to be considered as a confounder in studies measuring faecal glucocorticoid metabolites in cows to evaluate other stressful situations.  相似文献   
949.
Because of the importance of reproduction in stock breeding systems, it is necessary to find selection criteria that increase reproductive efficiency. The aim of this study was to estimate genetic parameters for the probability of conception on first service (PROB) in Murrah heifers, and its association with other traits of economic interest [age at first calving (AFC), service period, calving interval and milk yield at 270 days], with the purpose of evaluating their use as selection criteria. Reproductive information and first lactation records of 1200 Murrah heifers were used to perform two‐trait analyses between PROB and the other characteristics. Bayesian inference was used to estimate the variance components, considering PROB as threshold and the other as linear factors. The results demonstrate that this trait has heritability of 0.15, indicating the possibility of a genetic gain by using it for selection. With respect to the genetic correlation estimates, the only high‐magnitude association was with AFC (?0.899), which is the current criterion indicating sexual precocity of females. In the light of the parameters estimated, the first‐service pregnancy rate is an alternative for indication of sexual precocity, although presenting a smaller genetic gain than the current standard AFC. Nevertheless, additional research should be conducted regarding this trait to assess the economic importance of its use in dairy buffalo production systems.  相似文献   
950.
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