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   141381篇
  免费   7365篇
  国内免费   83篇
林业   5667篇
农学   4345篇
基础科学   915篇
  16116篇
综合类   26605篇
农作物   5408篇
水产渔业   6726篇
畜牧兽医   73124篇
园艺   1629篇
植物保护   8294篇
  2018年   1763篇
  2017年   1943篇
  2016年   1842篇
  2015年   1537篇
  2014年   1827篇
  2013年   5219篇
  2012年   3525篇
  2011年   4391篇
  2010年   2979篇
  2009年   2878篇
  2008年   4424篇
  2007年   4106篇
  2006年   3881篇
  2005年   3656篇
  2004年   3757篇
  2003年   3703篇
  2002年   3494篇
  2001年   4679篇
  2000年   4699篇
  1999年   3530篇
  1998年   1503篇
  1997年   1450篇
  1995年   1688篇
  1994年   1552篇
  1993年   1472篇
  1992年   3156篇
  1991年   3354篇
  1990年   3280篇
  1989年   3274篇
  1988年   3021篇
  1987年   3075篇
  1986年   3182篇
  1985年   3028篇
  1984年   2476篇
  1983年   2248篇
  1982年   1457篇
  1981年   1392篇
  1980年   1299篇
  1979年   2211篇
  1978年   1789篇
  1977年   1482篇
  1976年   1428篇
  1975年   1518篇
  1974年   1923篇
  1973年   1976篇
  1972年   1964篇
  1971年   1830篇
  1970年   1709篇
  1969年   1593篇
  1967年   1367篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
103.
104.
105.
106.
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   
107.
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   
108.
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   
109.
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   
110.
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   
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