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   216302篇
  免费   13691篇
  国内免费   640篇
林业   22918篇
农学   13850篇
基础科学   2933篇
  41116篇
综合类   13008篇
农作物   16936篇
水产渔业   15755篇
畜牧兽医   75161篇
园艺   6181篇
植物保护   22775篇
  2021年   2379篇
  2020年   2803篇
  2019年   3613篇
  2018年   3867篇
  2017年   4235篇
  2016年   4648篇
  2015年   4186篇
  2014年   5526篇
  2013年   16071篇
  2012年   5136篇
  2011年   6919篇
  2010年   7007篇
  2009年   7673篇
  2008年   6315篇
  2007年   5373篇
  2006年   5999篇
  2005年   5192篇
  2004年   5121篇
  2003年   4854篇
  2002年   4206篇
  2001年   4422篇
  2000年   4085篇
  1999年   4126篇
  1998年   3973篇
  1997年   4034篇
  1996年   3761篇
  1995年   4297篇
  1994年   3691篇
  1993年   3387篇
  1992年   3314篇
  1991年   3453篇
  1990年   3168篇
  1989年   3137篇
  1988年   2682篇
  1987年   2764篇
  1986年   2673篇
  1985年   3043篇
  1984年   3020篇
  1983年   2940篇
  1982年   2563篇
  1981年   2515篇
  1980年   2522篇
  1979年   2721篇
  1978年   2493篇
  1977年   2376篇
  1976年   2261篇
  1975年   2062篇
  1974年   2062篇
  1973年   1994篇
  1972年   1668篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
911.
1. Methods for quantitative determination of proteolytic, lipolytic and haemolytic activities and egg yolk factors in semen are described.2. Comparative quantitative determinations of these enzyme activities in the semen of goat, ram, bull, boar, dog and horse have been carried out.3. Special interest attaches to the egg yolk turbidity factor which is found in goat semen and, in small concentrations, also in boar semen.4. As each species appears to have its specific enzyme pattern, regular enzymatic control does not seem to have great significance.  相似文献   
912.
Uptake of I131-labelled triiodothyronine by erythrocytes (EU test) as a test of thyroid function has been studied in cattle. The normal EU value for 141 Swedish Red-and-white cows was 6.99 ± 0.95 (range 4.45–10.23) at an incubation time of one hour. Statistically significantly higher values were obtained for newly-calved cows. The EU values for calves are significantly higher but by 12 to 18 months of age decline to the same level as for older cows.The principle of the test is described and the results obtained compared with those for human beings. The EU test is suitable for screening thyroid function for large groups of animals and for studying associations between thyroid activity and different production qualities.  相似文献   
913.
914.
Summary. Seeds of Polygonam persiaria were collected from seven localities during 1962. In 1963 plants of these seven populations were grown in the field and sampled at regular intervals over the period 21st June to 13th August. At all samplings there were differences between populations in total dry weight, in leaf area index and in total nitrogen content, and populations also differed in the date of flowering. The populations also differed significantly in relative growth rate, leaf area ratio, net assimilation rate and relative nitrogen absorption rate. The variations in net assimilation rate did not appear to be closely associated with variations in leaf area index. Certain growth attributes were correlated with the latitude of origin, and it is tentatively suggested that the populations may represent genetically distinct latitudinal ecotypes.
Études sur des espèces de mauvaises herbes du genre Polygonum L.  相似文献   
915.
916.
917.
1. Two experiments were conducted to evaluate the effects of including ascorbic acid (AA), palm oil or a combination of both in the diets of laying hens during the hot months in a tropical environment.

2. The inclusion of AA improved egg production, food intake and efficiency of utilisation, and decreased the cost of food per kg egg. The addition of 400 mg AA/kg diet gave the most efficient performance.

3. Palm oil inclusion reduced the effect of heat stress and increased egg production, egg weight, food intake and efficiency of utilisation.

4. When 200 mg AA and 50 g palm oil/kg were used, additive responses were obtained with further improvements in egg production, food cost and efficiency.

5. Ascorbic acid and palm oil when fed alone or in combination reduced the incidence of cracked eggs.

6. Thus, 400 mg ascorbic acid/kg of diet, 50 g palm oil or 200 mg AA plus 50 g palm oil/kg diet ameliorated the effects of heat stress in laying hens.  相似文献   

918.
1. The growth of three distinct adipose tissue depots around the abdomen, neck and thigh was measured in a commercial strain of broiler, and related to the changes in activity of lipoprotein lipase within each depot.

2. At 10 d of age there was no significant difference between the weights of the depots. Thereafter the abdominal fat pad exhibited a much greater rate of growth than the other two depots so that by 55 d of age the abdominal fat pad was 4 times the size of the leg depot and 3.4 times the size of the neck depot.

3. Changes in total lipoprotein lipase activity for all three depots showed a similar pattern to the changes in their weight.

4. When [14C]‐VLDL was injected intravenously into birds at 15 and 57 d of age the percentage taken up into the abdominal fat pad was greater than that into the other two depots at both ages. At 57 d of age the abdominal fat pad had a 2.3‐fold greater uptake than at 15 d whereas the other two depots showed no significant differences at the two ages.

5. These results provide further evidence for a key role of lipoprotein lipase in regulating adipose tissue growth in broilers. They also demonstrate that although the abdominal fat pad has been used in a number of studies of adipose tissue metabolism it is not necessarily representative of other depots.  相似文献   

919.
1. One hundred and twenty (60 male and 60 female) 21‐d‐old Ross 1 broiler chicks were reared in cages in rooms kept at 21°C or 31°C and were killed at body weights of 1.0, 1.5, 2.0, 2.5 or 3.0 kg.

2. Birds reared at either of the two temperatures ate similar quantities of food to reach their slaughter weight although at 31°C they took longer to reach it.

3. The meat yields of the birds at each slaughter weight were similar at both rearing temperatures, but at body weights greater than 2.0 kg, the broilers reared at 21 °C had more breast meat than those reared at 31°C.

4. Females ate more food than males to reach each of the slaughter weights. The females deposited more fat and had a greater skin weight than the males and, although they had a similar amount of total meat, they had more breast meat.  相似文献   

920.
Adult populations ofMaladera matrida Argaman (Coleoptera: Scarabaeidae), a beetle highly injurious to agricultural crops, were studied in the coastal plain of Israel from 1985 to 1988. There were two generations annually. Adults emerged in March, April or May and disappeared in October or November. The beetles are on the wing, mate, and feed at twilight and in the dark. Feeding and mating behavior, sex ratio, and preferred food plants were studied.  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