全文获取类型
收费全文 | 2560篇 |
免费 | 154篇 |
国内免费 | 6篇 |
专业分类
林业 | 72篇 |
农学 | 61篇 |
基础科学 | 14篇 |
259篇 | |
综合类 | 458篇 |
农作物 | 73篇 |
水产渔业 | 88篇 |
畜牧兽医 | 1533篇 |
园艺 | 38篇 |
植物保护 | 124篇 |
出版年
2021年 | 20篇 |
2020年 | 35篇 |
2019年 | 33篇 |
2018年 | 46篇 |
2017年 | 42篇 |
2016年 | 49篇 |
2015年 | 25篇 |
2014年 | 62篇 |
2013年 | 114篇 |
2012年 | 97篇 |
2011年 | 90篇 |
2010年 | 70篇 |
2009年 | 77篇 |
2008年 | 105篇 |
2007年 | 82篇 |
2006年 | 89篇 |
2005年 | 83篇 |
2004年 | 87篇 |
2003年 | 94篇 |
2002年 | 71篇 |
2001年 | 81篇 |
2000年 | 68篇 |
1999年 | 62篇 |
1998年 | 38篇 |
1997年 | 28篇 |
1996年 | 29篇 |
1995年 | 27篇 |
1994年 | 18篇 |
1993年 | 27篇 |
1992年 | 64篇 |
1991年 | 58篇 |
1990年 | 54篇 |
1989年 | 31篇 |
1988年 | 54篇 |
1987年 | 42篇 |
1986年 | 54篇 |
1985年 | 53篇 |
1984年 | 33篇 |
1983年 | 29篇 |
1979年 | 34篇 |
1977年 | 19篇 |
1976年 | 24篇 |
1975年 | 22篇 |
1974年 | 17篇 |
1973年 | 31篇 |
1972年 | 22篇 |
1971年 | 20篇 |
1970年 | 17篇 |
1969年 | 24篇 |
1967年 | 17篇 |
排序方式: 共有2720条查询结果,搜索用时 15 毫秒
131.
T. A. Williams M. T. Abberton D. R. Evans W. Thornley I. Rhodes 《Journal of Agronomy and Crop Science》2000,185(2):121-128
Two medium leaf size varieties of white clover (Trifolium repens L.) were grown in mixture with a range of grass species and varieties in a field experiment conducted over 4 years. The clovers were grown singly or as a blend in swards with a tetraploid perennial ryegrass (Lolium perenne L.), a diploid Italian ryegrass (Lolium multiflorum L.) or tetraploid hybrids between these two species (Lolium x boucheanum Kunth). These grasses were also grown in monoculture. Swards were managed in a regime involving two periods of sheep grazing, two periods of cattle grazing and a silage cut in the same year for 4 years. The objectives of this experiment were 3‐fold: (1) to examine the yield and persistency of white clover in highly productive systems with modern perennial, hybrid and Italian ryegrasses; (2) to compare the performance of white clover varieties grown singly in mixture with grasses with their performance together as a blend, and (3) to examine the persistency of new Italian ryegrass varieties, exemplified by AberComo. The clover content of the sward remained high throughout the experiment, with a mean of 31 % overall. Differences in clover yield between plots were largely attributable to the effects of companion grasses. Total yields were very similar in plots containing different clover varieties and in plots where these varieties had been grown singly or as a blend. Grass yields in mixture were greater than those in monoculture but showed similar relative rankings. The diploid Italian ryegrass variety AberComo showed an unexpectedly high yield in the 4th year of the experiment. The implications of these results for strategies for productive, sustainable agriculture using modern white clover and ryegrass varieties are discussed. 相似文献
132.
T A Yazwinski R L Kilgore B L Presson M L Williams R K Fulton L Pote T E Greenway 《American journal of veterinary research》1985,46(1):163-164
Twenty calves at each of 2 Arkansas locations were inoculated with infective Fasciola hepatica metacercariae. After 56 days, the calves at each site were randomly assigned by weight to 2 treatment groups of 10 calves/group; vehicle control or clorsulon at the rate of 7 mg/kg of body weight. All treatments were given orally as a suspension. Calves were killed 6 weeks after treatment and F hepatica counts were performed for all animals. At the 2 sites, mean levels of efficacy were 96% and 91%. Adverse reactions to clorsulon or the vehicle were not observed in the calves. 相似文献
133.
134.
Association of blood cholesterol with occurrence of fat necrosis in cows and tall fescue summer toxicosis in steers 总被引:2,自引:0,他引:2
J A Stuedemann T S Rumsey J Bond S R Wilkinson L P Bush D J Williams A B Caudle 《American journal of veterinary research》1985,46(9):1990-1995
Factors associated with fat necrosis in cows and tall-fescue summer toxicosis in steers were studied. In the cow study, fescue pastures were fertilized, using 3 rates of N: high N (703 to 483 kg and 0 kg of N/ha/year from broiler litter in 1972 to 1974 and 1975, respectively), moderate N, and low N (224 and 74 kg of N/ha/yr from NH4NO3, 1972 to 1975, respectively). Bermuda grass pastures were fertilized at 2 rates of N: moderate N and low N (280 and 20 kg of N/ha/year from NH4NO3, 1972 to 1975, respectively). Fat necrosis developed only in cows grazing tall fescue, with an occurrence of 60%, 8%, and 3% for high-N, moderate-N, and low-N pastures, respectively. Cows grazing the high-N fescue, and to some extent those grazing the moderate-N fescue, had clinical signs of summer fescue toxicosis. Plasma cholesterol concentrations were lowest in cattle grazing the high-N fescue, averaging 114 mg/dl, followed by 134 and 127 mg/dl in cattle grazing the moderate-N and low-N fescue, respectively. In the steer grazing study, 24 paddocks of 0.49 ha each were seeded with tall-fescue lines G1-307 or G1-306 or with tall-fescue cultivars, KY-31 or Kenhy. All paddocks were fertilized with 170 kg of N/ha/year. Serum cholesterol concentrations were lower in steers grazing on G1-307 than in steers grazing on G1-306 or cultivars. Serum total lipids followed a similar trend, with a positive correlation (r = 0.49) between cholesterol and total lipids.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
135.
Defences of the bovine mammary gland against infection and prospects for their enhancement 总被引:4,自引:0,他引:4
The equine alternative complement pathway has been partially characterized and compared to the equine classical activation pathway. A dose-dependent lysis of RbRBC was observed with peak lytic values noted within 10 minutes at 37°C when rabbit red blood cells (RbRBC) were used as an alternative pathway activator. Sheep red blood cells (SRBC) sensitized with rabbit hemolysin or partially purified equine IgM antibodies were equally sensitive to lysis. Dilution of the commercial hemolysin by reduced lysis from 90% to 38% in the presence of constant cell numbers. Hemolysis of SRBC peaked at 10 minutes and the majority of lysis occurred within 10 minutes. Dilution of equine sera by as little as decreased hemolytic activity for SRBC to 21.5% from greater than 90% with undiluted sera. The alternative pathway protein, equine factor B, was tested using RbRBC and monitored by its differential susceptibility to heat treatment at 50°C. This treatment led to almost complete inactivation after a 15-minute incubation. An apparent heat-dependent decay of certain classical pathway components was also observed after 50°C treatment. This sensitivity was indicated by a reduction in the lytic activity for sensitized SRBC. Treatment for 15 minutes at 56°C with either RbRBC or SRBC was sufficient to abolish hemolytic activity in all equine sera tested. Chelation of cations with 0.04 M EDTA blocked expression of alternative and classical pathway activation; however, chelation of Ca++ ions with 10 mM EGTA containing 1 mM Mg++ ions permitted lysis of the RbRBC but not the SRBC. A dose-related Mg++-ion dependence for RbRBC hemolytic activity was observed as the concentration of Mg++ was increased to 1.0 mM. In addition, our results obtained with pre-colostral foal serum strongly suggest that natural antibody to RbRBC was of little importance in the lysis observed with these cells. These results also show that the equine alternative pathway activation may require Ca++ ions. If Ca++ ions are required, the equine alternative pathway is quite different from any other mammalian complement system so far described. Our results suggest that the alternative pathway of activation is of major importance in the equine complement system. Confirmation of this hypothesis requires both purification of the components involved as well as further characterization. 相似文献
136.
R.R. Lesniewski J.D. Kirsch L.E. Mrozinski D.H. Hellwig J. Kotwica G.L. Williams 《Domestic animal endocrinology》1985,2(3):141-150
Experiments were conducted to evaluate the effects of time and temperature on the potential of bovine whole blood (WB) or plasma (PL) to metabolize the ovarian steroids progesterone, estradiol-17β and testosterone. During a radioimmunoassay study (Experiment 1), we observed a temperature and time-dependent reduction (P<0.001) of plasma progesterone concentrations in samples incubated as WB at 5, 15, 25, or 35C for up to 48 hr. Most notable was the observation that 27% of progesterone present in controls was lost when WB was incubated at 5C for 48 hr and a 17% reduction was observed when PL samples were incubated at 35C for 48 hr. Immunoreactive estradiol-17β concentrations (Experiment 2) in PL and WB incubates were not affected by time or temperature. However, immunoreactive testosterone concentrations increased more than 3-fold by 48 hr in WB incubates held at 35C. To examine the latter observation further, 3H-progesteone was incubated with WB at 35C, followed by extraction and thin-layer chromatography (Experiment 3). Results generally supported RIA findings and revealed the presence of significant 17α-hydroxylase, 17–20 lyase and aromatase activity. Heretofore this has not been considered to occur outside major steroid metabolizing organs. 相似文献
137.
E S Williams J C DeMartini S P Snyder 《American journal of veterinary research》1985,46(11):2317-2321
The efficacy of the lymphocyte blastogenesis and complement-fixation tests and fecal culture for detection of Mycobacterium paratuberculosis infection was assessed in bighorn sheep (Ovis canadensis), elk (Cervus elaphus nelsoni), mule deer (Odocoileus hemionus), white-tailed deer (O virginianus), bighorn X mouflon (O musimon) hybrid sheep, and domestic sheep. Spontaneously infected bighorns were tested at the time of capture; experimentally infected animals were tested monthly for 12 months or periodically for 36 months. Lymphocyte blastogenesis tests were conducted with peripheral blood mononuclear cells and protein antigens of M avium, M bovis, and M paratuberculosis. Best diagnostic results were obtained when M avium purified-protein derivative was used as antigen and 20% bovine fetal serum was incorporated in the culture medium; a positive test was defined as a stimulation index greater than or equal to 3.5. Test sensitivity and specificity, respectively, were 82% and 94% in hybrid sheep and were 72% and 100% in domestic sheep. Sensitivity and specificity, respectively, were 39% and 94% in elk and 53% and 92% in deer. When infection was determined in spontaneously infected bighorns by culture of M paratuberculosis and/or the presence of acid-fast bacilli in characteristic microscopic lesions, sensitivity was 75% and specificity was 87%. Fecal cultures and the complement-fixation tests seldom correctly identified infected animals. 相似文献
138.
G M Williams 《Journal of the Association of Official Analytical Chemists》1979,62(4):857-863
Chemical carcinogens are mechanistically classified as genotoxic which interact directly with DNA, and epigenetic which cause chronic tissue injury, hormonal imbalance, and promotional effects. This review evaluates in vitro tests for their contribution to a battery for identifying genotoxic chemical carcinogens. In addition to bacterial mutagenic assays, nonspecific DNA damage/repair tests are recommended for screening chemicals, in particular the hepatocyte primary culture/DNA repair test. 相似文献
139.
The success of many intercropping systems in North America is attributable to the generation of a short term return from an agricultural crop during the early, unprofitable years of a longer term crop that is fruit, nuts or wood. This highly-efficient use of land and related profitability are important not only in the development of intercropping systems per se, but also in other applications which have tremendous potential independent of profitability. For example, intercropping can be effectively used during the establishment phases of hardwood plantations where it is essential to have a high level of weed control, the costs of which are often recovered with the agricultural production. Intercropping can also be used to grow trees rapidly and with a form that can be easily integrated into recreational or park situations, or urban fringe areas. In an urban context, intercropping can be used to grow trees in agricultural areas that are likely to be developed where traditional forestry options are not appropriate and the value of younger, thrifty trees may enhance property values far in excess of the cash value of the wood. 相似文献
140.