收费全文 | 340040篇 |
免费 | 17795篇 |
国内免费 | 721篇 |
林业 | 25039篇 |
农学 | 14307篇 |
基础科学 | 3031篇 |
48794篇 | |
综合类 | 48973篇 |
农作物 | 17788篇 |
水产渔业 | 18802篇 |
畜牧兽医 | 148480篇 |
园艺 | 6634篇 |
植物保护 | 26708篇 |
2019年 | 3827篇 |
2018年 | 4633篇 |
2017年 | 5132篇 |
2016年 | 5219篇 |
2015年 | 4584篇 |
2014年 | 5961篇 |
2013年 | 16647篇 |
2012年 | 8768篇 |
2011年 | 11384篇 |
2010年 | 8494篇 |
2009年 | 8848篇 |
2008年 | 10660篇 |
2007年 | 9887篇 |
2006年 | 10080篇 |
2005年 | 9346篇 |
2004年 | 9124篇 |
2003年 | 8905篇 |
2002年 | 8225篇 |
2001年 | 9294篇 |
2000年 | 8927篇 |
1999年 | 7680篇 |
1998年 | 4477篇 |
1997年 | 4516篇 |
1996年 | 4235篇 |
1995年 | 4963篇 |
1994年 | 4391篇 |
1993年 | 4068篇 |
1992年 | 6289篇 |
1991年 | 6540篇 |
1990年 | 6344篇 |
1989年 | 6308篇 |
1988年 | 5727篇 |
1987年 | 5778篇 |
1986年 | 5881篇 |
1985年 | 5941篇 |
1984年 | 5206篇 |
1983年 | 4746篇 |
1982年 | 3652篇 |
1981年 | 3450篇 |
1980年 | 3394篇 |
1979年 | 4764篇 |
1978年 | 4077篇 |
1977年 | 3641篇 |
1976年 | 3398篇 |
1975年 | 3446篇 |
1974年 | 4022篇 |
1973年 | 4040篇 |
1972年 | 3722篇 |
1971年 | 3512篇 |
1970年 | 3362篇 |
The prevalence of subaortic stenosis-like heart murmurs in Golden Retriever dogs appears to be very much greater than previously suspected.
Most cases of aortic stenosis represent mild lesions, are detected as incidental findings, and result in no loss of longevity or quality of life. Moderate or severe stenosis may result in exertional weakness, syncope, or sudden death. Signs of congestive heart failure are rare unless mitral valve insufficiency is present.
Generally, electrocardiography and radiography are unrevealing in this disorder. Doppler echocardiography may be the best diagnostic tool to detect even mild cases of aortic stenosis.
At present, therapy is limited due to the technical expertise and expense required. The prognosis depends on the severity of the lesion.
相似文献2. The organism persisted for at least one year in an empty trial house at the laboratory in which naturally‐infected broiler breeder birds had previously been housed. A similar survival period was seen in a building which had housed an infected layer breeder flock, although infection was not detected in a subsequent pullet flock.
3. Salmonella enteritidis was also frequently found surviving outside poultry houses in small pockets of litter and fan dust which had been left after cleansing and disinfection of the site. On some poultry units S. enteritidis was also found in wild bird droppings.
4. Salmonella contamination appeared to persist preferentially in association with dust particles swept from the floor and in food troughs and S. enteritidis survived at least 26 months in artificially contaminated poultry food. 相似文献
2. Chicks from the two genotypes were reared in wire‐floored cages and divided at random into 3 groups. Birds were fed on high protein (HP, 12.99 MJ ME, 238 g crude protein/kg and 12.94 MJ ME, 216 g crude protein/kg from 0 to 3 and 3 to 7 weeks, respectively), medium protein (MP, 12.99 MJ ME, 219 g crude protein/kg and 12.87 MJ ME, 201 g crude protein/kg from 0 to 3 and 3 to 7 weeks), and low protein (LP, 12.94 MJ ME, 205 g crude protein/kg and 12.75 MJ ME, 184 g protein/kg from 0 to 3 and 3 to 7 weeks) diets.
3. The LP diets resulted in a significantly lower daily body weight gain of males from 0 to 3 weeks. Dietary protein content had no effect on body weight gain from 3 to 7 weeks, body weight at 7 weeks, and the food intake of birds. Carcase composition of birds from both genotypes was unaffected by dietary protein.
4. Naked neck birds had significandy higher body weights at 7 weeks. Yields of carcase and breast of Na/na males were significantly higher than those of na/na males. There were no significant differences between females from the two genotypes as regards carcase yield.
5. It was concluded that the dietary protein requirements of naked neck birds were similar to those for normally feathered birds. 相似文献
2. We carried out post‐mortem examinations on 111 embryos that died within this period to assess the causes of this mortality.
3. Malpositioning and severe oedema were the predominant symptoms of dead‐in‐shell embryos with 55% being malpositioned and 41% showing severe oedema. Of these, 22 embryos (24%) showed both symptoms. Malpositioning generally results from incorrect setting of the eggs or inadequate turning and oedema was significantly correlated with the amount of water lost from the eggs which in turn was correlated with egg size.
4. Myopathy, gross lesions of internal organs, haemorrhage, bacterial infections and congenital deformities were found in less than 10% of chicks examined for these symptoms. 相似文献
2. Birds kept at the cool temperatures throughout life ate most food and gave the best production during lay. Minimum food intake and poorest performance were obtained with birds kept at the hot temperatures throughout life.
3. Performance in the hot environment during lay was improved by rearing birds in the cool environment, the response being related to an increased food intake. Food intake in the cool environment during lay was reduced, with only minor effects on performance, in birds which had been reared in the hot environment.
4. The results of the present study show that production responses during lay are affected by the temperatures experienced by hens during both growth and lay. 相似文献