全文获取类型
收费全文 | 139581篇 |
免费 | 8020篇 |
国内免费 | 277篇 |
专业分类
林业 | 5805篇 |
农学 | 4958篇 |
基础科学 | 924篇 |
18187篇 | |
综合类 | 19647篇 |
农作物 | 5369篇 |
水产渔业 | 7383篇 |
畜牧兽医 | 74337篇 |
园艺 | 1897篇 |
植物保护 | 9371篇 |
出版年
2020年 | 1248篇 |
2019年 | 1497篇 |
2018年 | 2317篇 |
2017年 | 2595篇 |
2016年 | 2419篇 |
2015年 | 2092篇 |
2014年 | 2563篇 |
2013年 | 5927篇 |
2012年 | 4548篇 |
2011年 | 5380篇 |
2010年 | 3679篇 |
2009年 | 3713篇 |
2008年 | 5356篇 |
2007年 | 5167篇 |
2006年 | 4818篇 |
2005年 | 4420篇 |
2004年 | 4255篇 |
2003年 | 4371篇 |
2002年 | 4091篇 |
2001年 | 5023篇 |
2000年 | 4768篇 |
1999年 | 3852篇 |
1998年 | 1499篇 |
1997年 | 1479篇 |
1996年 | 1367篇 |
1995年 | 1625篇 |
1994年 | 1356篇 |
1993年 | 1378篇 |
1992年 | 2785篇 |
1991年 | 2924篇 |
1990年 | 2897篇 |
1989年 | 2923篇 |
1988年 | 2628篇 |
1987年 | 2615篇 |
1986年 | 2560篇 |
1985年 | 2422篇 |
1984年 | 1992篇 |
1983年 | 1728篇 |
1982年 | 1171篇 |
1979年 | 1672篇 |
1978年 | 1299篇 |
1977年 | 1152篇 |
1976年 | 1114篇 |
1975年 | 1184篇 |
1974年 | 1386篇 |
1973年 | 1462篇 |
1972年 | 1398篇 |
1971年 | 1221篇 |
1970年 | 1257篇 |
1969年 | 1159篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Pituitaries were collected from late follicular phase (n = 5), mid-luteal phase (n = 5), and anestrous ewes (n = 4) to assess changes in intrapituitary LH heterogeneity at selected reproductive states. After homogenization, an aliquot of each pituitary extract was desalted by flow dialysis against water and chromtofocused on a pH 10.5 to 4.0 gradient. Concentrations of LH in pituitary extracts and chromatofocusing fractions were determined by RIA. The LH in pituitary extracts resolved into 13 isoforms during chromatofocusing, which were coded with letters beginning with the most basic isoform. Follicular and mid-luteal phase ewes exhibited similar distributions of intrapituitary LH among its isoforms. Relative to follicular and luteal phase ewes, anestrous ewes had lower percentages of isoforms D and E as well as higher percentages of isoforms G, H, J and K. Isoform F, the predominant molecular form of LH, constituted a similar percentage in all treatment groups (P > .05). Thus, the distribution of intrapituitary LH among its isoforms did not change significantly between the mid-luteal and follicular phases of the estrous cycle, but higher percentages of the weakly basic and acidic forms of LH were present during anestrus. These observations suggest that intrapituitary LH heterogeneity changes minimally throughout the estrous cycle of ewes during the breeding season. 相似文献
992.
To evaluate the changes in plasma and platelet serotonin (5-HT) as markers of the serotoninergic system in equines, 5-HT content was measured by high performance liquid chromatography (HPLC) in deproteinized plasma obtained from peripheral blood samples of 12 clinically healthy horses, before and after feeding. 5-HT was measured in platelet rich plasma (PRP) and in platelet poor plasma (PPP). 5-HT in platelets (p5-HT) was obtained by subtracting free 5-HT in PPP (f5-HT) from 5-HT in PRP. After food ingestion, significant increases in p5-HT and f5-HT (p < 0.001), and no changes in the f5-HT/p5-HT ratio were recorded. Increase in the total circulating 5-HT might account both for initiating peristaltic activity and for increasing the f5-HT levels. Augmented 5-HT uptake by platelets could reflect the hypothetical increased activity of the serotoninergic neurons. Besides showing the feasibility to obtain f5-HT and p5-HT through HPLC determination of 5-HT in PRP and PPP equine plasma, these findings are consistent with the postulation that 5-HT is released from enterochromaffin cells following a mechanical and chemical stimulation. 相似文献
993.
994.
Weiland E Wieczorek-Krohmer M Kohl D Conzelmann KK Weiland F 《Veterinary microbiology》1999,66(3):171-186
The arterivirus porcine reproductive and respiratory syndrome virus (PRRSV) contains six structural proteins the roles of which are not completely understood. In a preceding study, immunization with the dutch isolate I10 of PRRSV had led to the development of MAbs against four structural proteins [Wieczorek-Krohmer, M., 1994. Herstellung und Charakterisierung von monoklonalen Antik?rpern gegen das Virus des Porzinen Reproduktiven und Respiratorischen Syndroms (PRRSV). Inaugural-Dissertation, Ludwig-Maximilians-Universit?t, München] here finally identified by reaction with individual plasmid-expressed PRRSV proteins as products of ORFs 3 (GP3), 4 (GP4), 5 (GP5) and 7 (N). Surprisingly, the MAbs against GP5 revealed the presence of two antigenically distinct virus populations in the isolate I10, the population PRRSV-'PPV', isolated from plaques and the PRRSV-'EPV', gained by end point dilution. MAbs against GP3, GP4 and N reacted with both I10 populations as well as with natural PRRSV isolates. However, the anti-GP5 MAbs exclusively recognized PRRSV-'PPV'. In this study immunization of mice with both separated I10 populations confirmed that solely PRRSV-'PPV' possesses the property to induce an immune response ultimately leading to the establishment of MAbs against GP5. Whereas the 15 anti-GP5 MAbs (derived from four independent fusions) reacted exclusively with PRRSV-'PPV' of the isolate I10, anti-GP4 MAbs detected their target antigen on various isolates of European origin and were able to neutralize them. As indicated by competition assays and selection of neutralization-resistant virus mutants, all GP5 MAbs are directed against a single antigenic site on the ORF 5 protein. Both groups of neutralizing antibodies bound to the surface of purified virions demonstrating that the recognized epitopes represent surface structures of the virion envelope. However, anti-GP5 MAbs mediated the binding of more gold granules than anti-GP4 MAbs. Comparison of the neutralizing effect of anti-GP4 and anti-GP5 MAbs revealed the anti-GP5 MAbs as the more efficient antibodies. For the complete neutralization of about 100 ID50 of PRRSV-'PPV' anti-GP5 culture supernatant was effective up to a dilution of 1:1280 whereas the most effective anti-GP4 antibodies exhibited a comparable effect only up to 1:64. These results indicate that PRRSV GP5 in principle is a major target for neutralizing antibodies, as is found for other arteriviruses, but that in nature 'ORF 5 escape mutants' may develop as easily as in vitro. 相似文献
995.
996.
The purpose of these studies was to determine the time course for changes in feed intake, blood metabolites, and lipogenic activity in adipose tissue in response to the initiation of porcine somatotropin (pST) treatment and following withdrawal from treatment in barrows. An initial study was conducted to determine the impact of chronic pST treatment (4 wk of daily injection; 0 vs 4 mg/d) on adipose tissue lipid metabolism in barrows (initial weight 67 kg). Feed efficiency was improved 27%, backfat thickness was decreased 43%, and glucose and lactate oxidation and incorporation into lipid in adipose tissue was reduced 70 to 86% in pST-treated pigs. Palmitate esterification was decreased 44%, whereas palmitate oxidation was unaffected. In vitro metabolism of lactate, glucose, and palmitate in liver slices was not affected by pST treatment. The time-course for changes in intake and adipose tissue metabolism in response to 7 d of pST (0 vs 4 mg/d) treatment and 7 d of withdrawal was examined in subsequent studies in barrows (initial weight 75 kg). Feed intake during pST treatment was significantly (P < .05) less than in control pigs within 24 h of the initiation of treatment and remained low through 3 d after withdrawal. Adipose tissue biopsies were obtained on d 0, 1, 2, 4, and 7 of the treatment phase and on d 2, 4, and 7 after withdrawal from 7 d of treatment. Maximal inhibition of lipogenesis by pST treatment in adipose tissue in vitro was observed on d 4 (-68%) and d 7 (-69%). Similarly, fatty acid synthase activity declined during the treatment period, with the greatest change noted on d 7 (-26%). After withdrawal from treatment, lipogenesis gradually increased, returning to control values 7 d after withdrawal. Levels of IGF-I began to increase from d 1 to d 7 of treatment, continually decreased during withdrawal, and were normalized by the end of the withdrawal period. Plasma urea nitrogen concentrations decreased during treatment, increased during the withdrawal phase, and were normalized 4 d after the last pST treatment. Overall results indicate that most of the metabolic changes in response to pST occur within 1 wk of treatment and return to pretreatment values after 7 d of withdrawal from treatment. 相似文献
997.
998.
G.A. Delis M. Koutsoviti-Papadopoulou V.I. Siarkou G. Kounenis G.C. Batzias 《Research in veterinary science》2010,89(3):418-425
Silicone-made tissue cages were implanted in sheep. Blood serum (SBS) and tissue cage fluid (TCF) samples were collected after amoxicillin intravenous and intramuscular administrations, at the dose of 15 mg/kg. Amoxicillin pharmacodynamics were studied in an artificial culture medium, SBS and TCF with use of a Mannheimia haemolytica and a Pasteurella multocida strain. A concentration-independent antimicrobial activity of amoxicillin was confirmed for levels higher than 0.79–1.75 × MIC. This result favored the use of the percentage of the 24 h dosing interval during which drug levels remain above MIC as the appropriate pharmacokinetic/pharmacodynamic index. The subsequent correlation revealed that intravenous administration could be considered effective against “deep” infections caused by bacteria with MICs < 1 μg/mL or “shallow” infections caused by bacteria with MICs < 0.1 μg/mL. Intramuscular administration could be safely considered effective against both “deep” and “shallow” infections when the MICs of the targeted pathogens are lower than 1 μg/mL. 相似文献
999.
疫病是世界养禽生产效率的主要限制因素。尽管分子生物学方面的基础研究导致了新型疫苗的出现,但集约化生产体系仍受到新的和变异疫病的危害,这常常是多因子感染造成的。本文综述了一些重要疫病复合症的研究进展并对商品蛋鸡群和肉鸡生产效益的影响作一评述。 相似文献
1000.
ismail M 《Veterinary research communications》2007,31(4):467-476
The pharmacokinetics of difloxacin (Dicural) was studied in a crossover study using three groups (n = 4) of male and female Friesian calves after intravenous (i.v.), intramuscular (i.m.) and subcutaneous (s.c.) administrations
of 5 mg/kg body weight. Drug concentration in plasma was determined by high-performance liquid chromatography using fluorescence
detection. The plasma concentration–time data following i.v. administration were best fitted to a two-compartment open model
and those following i.m. and s.c. routes were best fitted using one-compartment open model. The collected data were subjected
to a computerized kinetic analysis. The mean i.v., i.m. and s.c. elimination half-lives (t
1/2β) were 5.56 ± 0.33 h, 6.12 ± 0.42 h and 7.26 ± 0.6 h, respectively. The steady-state volume of distribution (V
dss) was 1.12 ± 0.09 L/kg and total body clearance (ClB) was 2.19 ± 0.1 ml/(min. kg). The absorption half lives (t
1/2ab) were 0.38 ± 0.027 h and 2.1 ± 0.09 h, with systemic bioavailabilities (F) of 96.5% ± 6.4% and 84% ± 5.5% after i.m. and s.c. administration, respectively. After i.m. and s.c. dosing, peak plasma
concentrations (C
max) of 3.38 ± 0.13 μg/ml and 2.18 ± 0.12 μg/ml were attained after (t
max) 1.22 ± 0.20 h and 3.7 ± 0.52 h. The MIC90 of difloxacin for Mannheimia haemolytica was 0.29 ± 0.04 μg/ml. The AUC/MIC90 and C
max/MIC90 ratios for difloxacin following i.m. administration were 120 and 11.65, respectively and following s.c. administration were
97.58 and 7.51, respectively. Difloxacin was 31.7–36.8% bound to calf plasma protein. Since fluoroquinolones display concentration-dependent
activities, the doses of difloxacin used in this study are likely to involve better pharmacodynamic characteristics that are
associated with greater clinical efficacy following i.m. administration than following s.c. administration. 相似文献