全文获取类型
收费全文 | 301185篇 |
免费 | 17396篇 |
国内免费 | 5953篇 |
专业分类
林业 | 29519篇 |
农学 | 21960篇 |
基础科学 | 7533篇 |
51164篇 | |
综合类 | 36053篇 |
农作物 | 19783篇 |
水产渔业 | 18651篇 |
畜牧兽医 | 105183篇 |
园艺 | 7641篇 |
植物保护 | 27047篇 |
出版年
2022年 | 2554篇 |
2021年 | 4426篇 |
2020年 | 4566篇 |
2019年 | 5406篇 |
2018年 | 5230篇 |
2017年 | 6101篇 |
2016年 | 6472篇 |
2015年 | 6564篇 |
2014年 | 7731篇 |
2013年 | 18706篇 |
2012年 | 9453篇 |
2011年 | 11724篇 |
2010年 | 10507篇 |
2009年 | 10835篇 |
2008年 | 10487篇 |
2007年 | 9518篇 |
2006年 | 10015篇 |
2005年 | 9053篇 |
2004年 | 7520篇 |
2003年 | 7090篇 |
2002年 | 6242篇 |
2001年 | 7240篇 |
2000年 | 7121篇 |
1999年 | 6944篇 |
1998年 | 5303篇 |
1997年 | 5195篇 |
1996年 | 4851篇 |
1995年 | 5333篇 |
1994年 | 4718篇 |
1993年 | 4326篇 |
1992年 | 5101篇 |
1991年 | 5087篇 |
1990年 | 4719篇 |
1989年 | 4611篇 |
1988年 | 3898篇 |
1987年 | 3884篇 |
1986年 | 3692篇 |
1985年 | 3988篇 |
1984年 | 3641篇 |
1983年 | 3401篇 |
1982年 | 2873篇 |
1981年 | 2771篇 |
1980年 | 2731篇 |
1979年 | 3079篇 |
1978年 | 2819篇 |
1977年 | 2560篇 |
1976年 | 2424篇 |
1975年 | 2336篇 |
1974年 | 2437篇 |
1973年 | 2362篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
To determine whether turkey herpesvirus (HVT) impairs the aspecific and specific defense against an avian pneumovirus (APV) infection, specific-pathogen-free turkeys were inoculated at 7 days of age with HVT and 1, 5, or 7 wk later with APV. Clinical signs, APV replication, and development of antibodies against APV were evaluated. No differences were found between the birds that received both HVT and APV and those that received only APV. 相似文献
102.
103.
104.
T. Sivakumar Virendranath & G. C. Srivastava 《Journal of Agronomy and Crop Science》2001,186(1):43-46
Grain yield and yield components (grains per ear, grain weight, 1000-grain weight, ear weight, ear seed ratio and dry matter partitioning between ear and seed) were examined in a wheat genotype (PBW-343) with well-filled grains and a Triticale genotype (DT-46) with poorly filled grains (showing grain shrivelling) grown in pots. Six days after anthesis (DAA), benzyladenine (BA) @2 µg ear−1 and abscisic acid (ABA) @4 µg ear−1 were injected at the base of the mother shoot ear in both species. It was observed that, in both wheat and Triticale , BA increased the grain weight, grain number and partitioning of dry matter between ear and seed, whereas ABA decreased the grain weight, grain number and dry matter partitioning between ear and seed. However, these decreases were slower in Triticale than in wheat. BA treatment increased the grain dry matter accumulation, which in turn resulted in better filling of grains and increased the grain weight in both wheat and Triticale . The average grain weight of Triticale was lower than that of wheat. Thus, it appears that variation in grain weight between wheat and Triticale might be due to different availabilities of growth-promoting phytohormones such as cytokinins and assimilates. 相似文献
105.
106.
107.
108.
G H Loneragan D A Dargatz P S Morley M A Smith 《Journal of the American Veterinary Medical Association》2001,219(8):1122-1127
OBJECTIVE: To evaluate trends in feedlot cattle mortality ratios over time, by primary body system affected, and by type of animal. DESIGN: Retrospective cohort study. ANIMALS: Approximately 21.8 million cattle entering 121 feedlots in the United States during 1994 through 1999. PROCEDURES: Yearly and monthly mortality ratios were calculated. Numbers of deaths were modeled by use of Poisson regression methods for repeated measures. Relative risks of death over time and by animal type were estimated. RESULTS: Averaged over time, the mortality ratio was 12.6 deaths/1,000 cattle entering the feedlots. The mortality ratio increased from 10.3 deaths/1,000 cattle in 1994 to 14.2 deaths/1,000 cattle in 1999, but this difference was not statistically significant (P = 0.09). Cattle entering the feedlots during 1999 had a significantly increased risk (relative risk, 1.46) of dying of respiratory tract disorders, compared with cattle that entered during 1994, and respiratory tract disorders accounted for 57.1% of all deaths. Dairy cattle had a significantly increased risk of death of any cause, compared with beef steers. Beef heifers had a significantly increased risk of dying of respiratory tract disorders, compared with beef steers. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggested that although overall yearly mortality ratio did not significantly increase during the study, the risk of death attributable to respiratory tract disorders was increased during most years, compared with risk of death during 1994. The increased rates of fatal respiratory tract disorders may also reflect increased rates of non-fatal respiratory tract disorders, which would be expected to have adverse production effects in surviving animals. 相似文献
109.
A. Massip 《Reproduction in domestic animals》2001,36(2):49-55
This review contains two parts. The first part is devoted to the significant steps in cryopreservation of mammalian embryos with emphasis on cattle and sheep that serve as models of reference. These steps are: (1) shortening of cooling and warming processes; (2) addition and dilution of cryoprotectant in one step; (3) introduction of plastic straw as a freezing and dilution container; (4) the choice of ethylene glycol as the quite universal cryoprotectant because of its low toxicity and high permeability; (5) vitrification, a cryopreservation method which enable passage from the liquid to the solid state by extreme elevation of viscosity due to high concentration of cryoprotectants and very rapid cooling. There are several vitrification solutions which contain dimethyl sulphoxide, glycerol, ethylene glycol, or a mixture of them, as basic cryoprotectants. The second part considers some factors affecting the efficiency of cryopreservation concerning (i) the origin of embryos and (ii) the stage of development and species. The origin of embryos (in vivo versus in vitro): in vitro embryos show a chilling and freezing sensitivity associated with their lipid content which can be modified by the culture conditions. Both conventional freezing and vitrification have been used and it seems that vitrification is more adapted to in vitro embryos when some modifications of initial protocols are carried out, particularly the rate of cooling. Thus considerable progress has been achieved by using the open pulled straw method of Vajta which enables the use of a minimum volume of freezing medium (0.5 μl) and a very high cooling rate that permits rapid traversal of the damaging temperature zone, corresponding to chilling sensitivity. The stage of development and species: not only are there differences between species at the same stage of development but in the same species all stages of development do not survive equally under the same freezing protocol. In cattle for example, oocytes and early stages of development in vivo or in vitro do not survive whereas compacted morulae and blastocysts survive very well. In the pig hatched blastocysts survive better than the other stages. Horse embryos have special characteristics that pose problems for successful freezing. In conclusion, a lot of work remains to be done to define fundamental characteristics of embryos of certain species (pig, horse) and of embryos of some stages or of oocytes. 相似文献
110.
The isolation of (+)-totarol as active compound against Mycobacterium tuberculosis is reported from Chamaecyparis nootkatensis outerbark. 相似文献