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   164728篇
  免费   8781篇
  国内免费   105篇
林业   6577篇
农学   4746篇
基础科学   1130篇
  18603篇
综合类   31687篇
农作物   6398篇
水产渔业   7522篇
畜牧兽医   85240篇
园艺   1892篇
植物保护   9819篇
  2018年   1950篇
  2017年   2168篇
  2016年   2086篇
  2015年   1776篇
  2014年   2145篇
  2013年   6107篇
  2012年   4088篇
  2011年   5054篇
  2010年   3295篇
  2009年   3330篇
  2008年   5036篇
  2007年   4639篇
  2006年   4526篇
  2005年   4163篇
  2004年   4174篇
  2003年   4139篇
  2002年   3991篇
  2001年   5119篇
  2000年   5143篇
  1999年   3893篇
  1998年   1639篇
  1997年   1666篇
  1995年   1946篇
  1994年   1802篇
  1993年   1714篇
  1992年   3606篇
  1991年   3883篇
  1990年   3711篇
  1989年   3788篇
  1988年   3500篇
  1987年   3668篇
  1986年   3809篇
  1985年   3662篇
  1984年   2927篇
  1983年   2657篇
  1982年   1799篇
  1981年   1674篇
  1979年   2702篇
  1978年   2242篇
  1977年   1847篇
  1976年   1811篇
  1975年   1854篇
  1974年   2403篇
  1973年   2509篇
  1972年   2481篇
  1971年   2334篇
  1970年   2178篇
  1969年   2000篇
  1968年   1641篇
  1967年   1746篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
92.
93.
Four commercially available semiquantitative milk progesterone tests (Ovucheck-Praxistest: Cambridge Veterinary Science/Smith Kline), Progestassay-Milchprogesteron (Pitman-Moore/Janssen), Reprostrip-Progesteron-Schnelltest (Noctech/Albrecht), Enzygnost-Milchprogesteron (IQ, 'Bio' UK/Hoechst Veterin?r) were examined for their accuracy by using them for the determination of progesterone levels of 64 milk samples, i.e. 1556 single assays. Several test series were performed, using codified samples and changing sequences. Three or four test persons respectively, performed the tests independently and classified the samples semiquantitatively. These test results were then compared to the results acquired by measuring the progesterone levels of the same samples by means of an approved quantitative, labor-bound progesterone test (Hormonost: Biolab). These control tests were performed at a specialized routine labor, by different personnel and at a different location. Lastly, in 48 out of the 64 sampled animals the reproductive status could be evaluated clinically and was taken into account as well. Samples yielding high progesterone levels, i.e. greater than or equal to 9 ng/ml were classified correctly in 84.4 to 96.5% of the cases, whereas samples with low levels (less than or equal to 2.5 ng/ml) were classified correctly in 68.8 to 90.0% of the cases only. Samples ranging between this spectrum (greater than 2.5 less than 9 ng/ml) were classified correctly only in 42.1 to 52.6% of the cases. However, this range appears to be of the most interest for the veterinary practitioner since cows in proestrus or early interestrus tend to have mild progesterone levels within these values. On the other hand, clinical findings are often insufficient for a proper diagnosis just in these animals.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
94.
A review of investigations of the functional anatomy of the equine foot is presented. Emphasis is placed on the relationships of structures involved in the major diseases of the foot.  相似文献   
95.
Gold, bronze, and black (normal pigmentation) body colors in Tilapia mossambica are controlled by a single autosomal gene with incomplete dominant gene action: GG fish are black gg fish are gold; Gg fish are bronze. Because gold body color is produced by the recessive genotype, it is easy to produce and to maintain a truebreeding population of gold T. mossambica for commercial purposes. If all other colors are culled, the population will breed true, because gold × gold will always produce 100% gold offspring. The G gene will be a valuable genetic marker for many genetic studies, such as the production of gynogenetic T. mossambica .  相似文献   
96.
In this study, the effects of nitrogen (N) rate (60, 120, 180 and 240 kg N ha?1 applied in three equal dressings at seeding and after the first and second cuttings) and stubble height (7, 14 and 21 cm) on the dry matter (DM) yield, crude protein (CP) content, and CP yield of a sorghum–sudangrass hybrid [Sorghum bicolor (L.) Moench × Sorghum sudanense (Piper) Stapf., cv. Pioneer 988] in the three‐cut system was investigated. The N rate had no significant effect in the first and third cuttings, but in the second cutting DM yields increased significantly with increase in N rate. The highest yield of 9.1 t ha?1 was obtained with 80 kg N ha?1 for the average of 2 years at the second cutting, but no significant difference was found among the 40, 60 and 80 kg N ha?1 rates. CP content and yield were not significantly affected by N rate at the first and third cuttings, but CP content and yield were significantly affected by application of N at the second cutting. Stubble height had a significant effect on CP content at the third cutting. However, it had no significant effect on CP content at the first and second cuttings. Stubble height had a significant effect on the CP yield at the first cutting, but no significant effect on CP yield at the second and third cuttings.  相似文献   
97.
A porcine in vitro fertilization (IVF) system and seminal quality parameters of frozen–thawed boar semen were used to assess the effectiveness of two different thawing rates of frozen boar semen, and to address the question of whether differences between fertility of ejaculates could be predicted in a limited field trial. In the first experiment, two thawing procedures were analysed (37°C, 30 s; 50°C, 12 s) and no differences in sperm quality were found. However, when the procedure was 50°C, 12 s the IVF results showed a higher number of sperm per penetrated oocyte and a near 10 points higher rate of pronuclear formation. In the second experiment, the fertility results obtained in the limited field trial show to be efficient enough for application in a commercial use, especially for three of the employed boars (fertility ≥80%). In this limited study, the conventional seminal parameters are not accurate enough to discriminate good and bad boars in relation to fertility. On the contrary, parameters of in vitro penetrability are more precise to predict subsequent fertilities. As conclusion, the IVF fertilization system seems to be a good tool to evaluate the quality of frozen–thawed boar semen previous to its commercial way, to verify the bank semen storage quality and a good way to assay new sperm freezing procedures, as it is the more precise evaluating method in estimating the potential fertilizing ability.  相似文献   
98.
99.
A genomic library of Babesia bovis DNA from the Mexican strain M was constructed in plasmid pUN121 and cloned in Escherichia coli. Several recombinants which hybridized strongly to radioactively labeled B. bovis genomic DNA in an in situ screening were selected and further analyzed for those which specifically hybridized to B. bovis DNA. It was found that pMU-B1 had the highest sensitivity, detecting 25 pg of purified B. bovis DNA, and 300 parasites in 10 microliters of whole infected blood, or 0.00025% parasitemia. pMU-B1 contained a 6.0 kb B. bovis DNA insert which did not cross-hybridize to Babesia bigemina, Trypanosoma evansi, Plasmodium falciparum, Anaplasma marginale, Boophilus microplus and cow DNA. In the Southern blot analysis of genomic DNA, pMU-B1 could differentiate between two B. bovis geographic isolates, Mexican strain M and Thai isolate TS4. Thus, the pMU-B1 probe will be useful in the diagnosis of Babesia infection in cattle and ticks, and in the differentiation of B. bovis strains.  相似文献   
100.
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