收费全文 | 208784篇 |
免费 | 13587篇 |
国内免费 | 698篇 |
林业 | 22829篇 |
农学 | 13854篇 |
基础科学 | 2944篇 |
40180篇 | |
综合类 | 11044篇 |
农作物 | 17000篇 |
水产渔业 | 15720篇 |
畜牧兽医 | 70881篇 |
园艺 | 6140篇 |
植物保护 | 22477篇 |
2021年 | 2409篇 |
2020年 | 2812篇 |
2019年 | 3615篇 |
2018年 | 3689篇 |
2017年 | 4021篇 |
2016年 | 4593篇 |
2015年 | 4192篇 |
2014年 | 5540篇 |
2013年 | 16079篇 |
2012年 | 4963篇 |
2011年 | 6505篇 |
2010年 | 6792篇 |
2009年 | 7480篇 |
2008年 | 5982篇 |
2007年 | 4946篇 |
2006年 | 5771篇 |
2005年 | 5033篇 |
2004年 | 4893篇 |
2003年 | 4677篇 |
2002年 | 4044篇 |
2001年 | 4476篇 |
2000年 | 4148篇 |
1999年 | 4241篇 |
1998年 | 3964篇 |
1997年 | 4045篇 |
1996年 | 3768篇 |
1995年 | 4291篇 |
1994年 | 3675篇 |
1993年 | 3369篇 |
1992年 | 3284篇 |
1991年 | 3435篇 |
1990年 | 3084篇 |
1989年 | 3033篇 |
1988年 | 2664篇 |
1987年 | 2751篇 |
1986年 | 2572篇 |
1985年 | 2941篇 |
1984年 | 2930篇 |
1983年 | 2836篇 |
1982年 | 2526篇 |
1981年 | 2474篇 |
1980年 | 2479篇 |
1979年 | 2431篇 |
1978年 | 2312篇 |
1977年 | 2204篇 |
1976年 | 2052篇 |
1975年 | 1804篇 |
1974年 | 1824篇 |
1973年 | 1734篇 |
1971年 | 1488篇 |
2. There were some differences between the two ages; these could largely be reconciled by considering the likely differences in the proportion of feather protein.
3. The results were compared with similar values for the chicken and goose; overall there is a striking similarity, both in absolute concentrations and relative proportions of amino acids. 相似文献
2. Addition of 10 g ascorbic acid/kg to the control diet (without excess tyrosine) produced no beneficial effects on performance. Excess dietary tyrosine caused depressions in all measures of performance.
3. Adding 0.1, 1, 10 or 20 g ascorbic acid/kg to the diet containing excess tyrosine tended to improve performance.
4. Subcutaneous injection of 50 mg ascorbic acid/bird d to chicks receiving excess tyrosine brought about a significant improvement in body‐weight gain.
5. The elevation of plasma free tyrosine caused by excess dietary tyrosine decreased as dietary ascorbic acid increased.
6. Ascorbic acid can alleviate, though not completely counteract, the adverse effect of excess dietary tyrosine. 相似文献
2. In the early stage of growth the feather shaft is held in place by cell indentations, whereas later, bridges are seen which probably hold the feather more firmly. 相似文献
2. Male broilers grew linearly up to 76 d whereas the females grew more slowly after 35 d. At 364 d the difference in live body weight was small.
3. Up to 56 d, males had a slightly higher eviscerated yield. There was no significant difference between the sexes beyond this age.
4. The proportional yield of breast muscle was variable with age, whereas that of thigh and drumstick increased with age in the male, particularly at 175 and 364 d.
5. After 35 d the proportion of skin was larger in females, whereas the gizzard as a proportion of live body weight in both sexes decreased rapidly between 21 and 76 d. Blood loss during killing was always higher in male broilers after 21 d. 相似文献
First European Symposium on Poultry Welfare. Edited by L. Yding S0RENSEN. Published by the Danish Branch of World's Poultry Science Association. 1981. 238pp. D.Kr.90. ISBN 87–88162–00–1. 相似文献
2. Phenotypic correlations between total plasma triglyceride concentration and fat content were + 0.50, + 0.37 and + 0.39 for the three groups of male broilers and +0.33 for the group of females. Triglyceride content of plasma very low density lipoprotein (VLDL) plus low density lipoprotein (LDL) showed similar correlations with fat content.
3. Selection of birds from a flock of 550 male broilers on the basis of high or low plasma VLDL plus LDL triglyceride concentrations produced groups with significant differences in body fat content but little difference in body weight.
4. The results suggest that measurements of plasma triglyceride concentration could be used as an indirect means of selecting for decreased body fat content in broiler breeding programmes. 相似文献
2. The Leghorn egg was characterised by a thinner and weaker shell compared with the Sinai and the crossbreds, at all the experimental temperatures.
3. In contrast to other reports, high ambient temperatures for a long period had only mild effects on egg‐shell quality.
4. The results suggest that gradual acclimation to high ambient temperatures might improve the efficiency of the physiological mechanisms involved in the hen's response to heat. Consequently, the reproductive process adapts to the hot environmental conditions.
5. The results indicate that the Sinai breed might be used for future selection of a breed, highly resistant to extreme environmental conditions and with an improved shell quality. 相似文献
2. Age at first egg, body‐weight gain and egg production were affected by energy allowance. Birds on the lower energy allowances came into lay later than birds on the higher energy allowances and at a lower body weight.
3. Body‐weight gain decreased with decreasing energy allowance. The decrease in egg output in response to decreasing energy allowance resulted from more birds ceasing to lay and fewer birds laying on more than 3 d per week. Similar changes in the distribution of rates of lay were observed on each treatment as the flock aged.
4. The relationship between body‐weight gain and egg number on each treatment was negative from 21 to 36 weeks, but became less consistent with age.
5. Protein intake had little effect on body weight. At the lowest energy allowance, egg number and egg weight decreased with increasing protein allowance. This effect was not observed on the higher energy allowances. 相似文献
2. Injecting the growing embryo with amino acids equivalent to the amount of protein used resulted in higher weight of chicks at all stages up to 56 d. 相似文献