全文获取类型
收费全文 | 253638篇 |
免费 | 15214篇 |
国内免费 | 1386篇 |
专业分类
林业 | 23660篇 |
农学 | 14309篇 |
基础科学 | 3194篇 |
43487篇 | |
综合类 | 25401篇 |
农作物 | 17314篇 |
水产渔业 | 16556篇 |
畜牧兽医 | 95901篇 |
园艺 | 6541篇 |
植物保护 | 23875篇 |
出版年
2021年 | 2684篇 |
2020年 | 3022篇 |
2019年 | 3808篇 |
2018年 | 4047篇 |
2017年 | 4507篇 |
2016年 | 4862篇 |
2015年 | 4504篇 |
2014年 | 5809篇 |
2013年 | 16435篇 |
2012年 | 6634篇 |
2011年 | 8624篇 |
2010年 | 7660篇 |
2009年 | 8140篇 |
2008年 | 7756篇 |
2007年 | 6822篇 |
2006年 | 7346篇 |
2005年 | 6664篇 |
2004年 | 6331篇 |
2003年 | 6156篇 |
2002年 | 5482篇 |
2001年 | 5795篇 |
2000年 | 5582篇 |
1999年 | 5183篇 |
1998年 | 4122篇 |
1997年 | 4146篇 |
1996年 | 3899篇 |
1995年 | 4406篇 |
1994年 | 3842篇 |
1993年 | 3550篇 |
1992年 | 4176篇 |
1991年 | 4467篇 |
1990年 | 4150篇 |
1989年 | 4062篇 |
1988年 | 3608篇 |
1987年 | 3695篇 |
1986年 | 3610篇 |
1985年 | 3818篇 |
1984年 | 3642篇 |
1983年 | 3408篇 |
1982年 | 2859篇 |
1981年 | 2762篇 |
1980年 | 2752篇 |
1979年 | 3143篇 |
1978年 | 2853篇 |
1977年 | 2607篇 |
1976年 | 2542篇 |
1975年 | 2326篇 |
1974年 | 2562篇 |
1973年 | 2473篇 |
1972年 | 2201篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
合成了14个含有 N-莰-2-基的磷酰胺和硫代磷酰胺类化合物,用红外、核磁及元素分析确定其结构,并探讨了~(31)Pδ值的规律性变化。初步生物试验表明,磷酰胺类化合物对水稻发芽具有明显的抑制作用,其中化合物Ⅱ1的活性大于对照药剂抗倒胺但低于多效唑。 相似文献
172.
173.
174.
Twelve different mating types among the Hampshire and Landrace breeds were used to determine direct, maternal, heterosis, and recombination effects for performance and carcass traits. Mating types used were two purebred, two F1, two F2, two F3, and four backcross. Carcass data were collected on 238 barrows and 262 gilts over four replications. Traits measured were length (LENG), 10th rib off midline backfat (BF10), longissimus muscle area (LMA), and dressing percentage (DRS%). Average backfat (AVBF) was calculated as the mean of three midline fat depths measured opposite the first rib, last rib, and last lumbar vertebra. The model used to evaluate the carcass traits included main effects of mating type, farrowing season, and sex and included slaughter weight as a covariate. The performance traits of ADG, feed efficiency (FE), daily feed consumption (DFC), lean gain per day (LNGN), and lean efficiency (LNEF) were measured on a pen basis. Comparisons of reciprocal F1 crosses showed that carcasses from pigs sired by Hampshire boars were leaner and had more LMA than those sired by Landrace boars. Heterosis percentages were significant for AVBF (7.2%; P less than .01), BF10 (8.8%; P less than .01), DRS% (1.5%; P less than .01), ADG (11.5%; P less than .01), DFC (10.2%; P less than .01), LNGN (10.6%; P less than .01), and LNEF (6.0%; P less than .05). Epistatic recombination losses in the offspring were significant for LENG (3.6 cm; P less than .05) and approached significance for BF10 (6.1 mm; P less than .10). 相似文献
175.
Pharmacokinetic evaluation of ceftiofur in serum, tissue chamber fluid and bronchial secretions from healthy beef-bred calves.
下载免费PDF全文
![点击此处可从《Canadian journal of veterinary research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
S L Halstead R D Walker J C Baker R E Holland G E Stein J G Hauptman 《Canadian journal of veterinary research》1992,56(4):269-274
Ceftiofur is a new broad spectrum cephalosporin marketed for the treatment of acute bovine respiratory disease. In this investigation ceftiofur was administered by intramuscular injection, at 24 h intervals, to healthy beef-bred calves for four days at dosages of 2.2 and 4.4 mg/kg of body weight, with 4 wk intervals between dosing regimens. Serum, tissue chamber fluid (TCF), and bronchial secretion (BS) concentrations of ceftiofur were measured by microbiological assay after the first and fourth dose of each dosing regimen. Peak serum concentrations (Cmax) of 8.8 micrograms/mL and 17.3 micrograms/mL were obtained approximately 2 h (Tmax), the time of mean peak concentration) after single injections of 2.2 mg/kg and 4.4 mg/kg, respectively. The Cmax was increased approximately twofold following multiple doses of 2.2 mg/kg (Cmax = 13.1 micrograms/mL) and 4.4 mg/kg (Cmax = 24.1 micrograms/mL). Ceftiofur accumulated slowly into TCF and peak concentrations were found to be approximately 14% of those observed in serum after the first dose and approximately 24% after multiple dosing. Concentrations of ceftiofur in BS were obtained rapidly with peak concentrations reaching 45% of the serum Cmax after the first dose. After multiple dosing the Cmax for BS was approximately 25% of the serum Cmax. This study found that both the 2.2 mg/kg and 4.4 mg/kg dosing regimens resulted in continuous serum, TCF and BS concentrations of ceftiofur that exceeded the minimal concentration required to inhibit the bacteria most frequently isolated from calves with acute bovine respiratory disease. 相似文献
176.
177.
178.
Abstract. In field and laboratory experiments the conditioner‘Agri-SC’has shown improvements in the structure of loamy sand soils in east Shropshire, UK. It resulted in statistically significant decreases in soil bulk density values and increases in soil porosity and aggregate stability. Further experiments are in progress on both loamy sand and silt loam soils. 相似文献
179.
Ovarian follicular dynamics and embryo yield were studied during 2 different FSH regimens for superovulation of cattle. Twenty heifers were given intramuscular injections of FSH (total of 35 mg NIH) either once daily for 3 days (Group 3×1) or twice daily for 4 days (Group 4×2). At 72 h after the first FSH injection, each animal was injected with 0.75 mg cloprostenol. Inseminations were performed at 12 h and 24 h after the onset of heat. Transrectal ultrasonography was performed on the day of the first FSH injection, the day of cloprostenol injection, the day of insemination and finally on the day of embryo recovery (day 6 or 7 after heat). The numbers of small (2–4 mm), medium (5–9 mm) and large (>10 mm) size follicles were recorded. The total number of corpora lutea, eggs and transferable embryos were recorded on the day of embryo recovery. No differences were found between the 2 groups in either of the parameters studied (p>0.05). It can be concluded that treatment with this FSH preparation once daily for 3 days gives a folliculogenic and superovulatory response similar to a treatment regimen where it is given twice daily for 4 days. 相似文献
180.
R W Morgan J Gelb C S Schreurs D Lütticken J K Rosenberger P J Sondermeijer 《Avian diseases》1992,36(4):858-870
Recombinant strains of herpesvirus of turkeys (HVT) were constructed that contain either the fusion protein gene or the hemagglutinin-neuraminidase gene of Newcastle disease virus (NDV) inserted into a nonessential gene of HVT. Expression of the NDV antigens was regulated from a strong promoter element derived from the Rous sarcoma virus long terminal repeat. Recombinant HVT strains were stable and fully infectious in cell culture and in chickens. Chickens receiving a single intra-abdominal inoculation at 1 day of age with recombinant HVT expressing the NDV fusion protein had an immunological response and were protected (> 90%) against lethal intramuscular challenge at 28 days of age with the neurotropic velogenic NDV strain Texas GB. Recombinant HVT expressing the NDV hemagglutinin-neuraminidase provided partial protection (47%) against the same challenge. Chickens vaccinated with recombinant HVT vaccines had low levels of protection against NDV replication in the trachea when challenged ocularly. Recombinant HVT vaccines and the parent HVT strain provided similar levels of protection to chickens challenged with the very virulent RB1B strain of Marek's disease virus, indicating that insertion of foreign sequences into the HVT genome did not compromise the ability of HVT to protect against Marek's disease. 相似文献