收费全文 | 202625篇 |
免费 | 13335篇 |
国内免费 | 644篇 |
林业 | 22757篇 |
农学 | 13820篇 |
基础科学 | 2927篇 |
39671篇 | |
综合类 | 9589篇 |
农作物 | 17058篇 |
水产渔业 | 15485篇 |
畜牧兽医 | 66825篇 |
园艺 | 6167篇 |
植物保护 | 22305篇 |
2021年 | 2435篇 |
2020年 | 2804篇 |
2019年 | 3605篇 |
2018年 | 3614篇 |
2017年 | 3975篇 |
2016年 | 4588篇 |
2015年 | 4175篇 |
2014年 | 5544篇 |
2013年 | 16081篇 |
2012年 | 4764篇 |
2011年 | 6287篇 |
2010年 | 6719篇 |
2009年 | 7425篇 |
2008年 | 5781篇 |
2007年 | 4719篇 |
2006年 | 5595篇 |
2005年 | 4792篇 |
2004年 | 4699篇 |
2003年 | 4485篇 |
2002年 | 3870篇 |
2001年 | 3911篇 |
2000年 | 3563篇 |
1999年 | 3799篇 |
1998年 | 3918篇 |
1997年 | 3986篇 |
1996年 | 3731篇 |
1995年 | 4254篇 |
1994年 | 3631篇 |
1993年 | 3333篇 |
1992年 | 3061篇 |
1991年 | 3184篇 |
1990年 | 2878篇 |
1989年 | 2843篇 |
1988年 | 2471篇 |
1987年 | 2537篇 |
1986年 | 2399篇 |
1985年 | 2834篇 |
1984年 | 2835篇 |
1983年 | 2770篇 |
1982年 | 2489篇 |
1981年 | 2455篇 |
1980年 | 2457篇 |
1979年 | 2375篇 |
1978年 | 2274篇 |
1977年 | 2199篇 |
1976年 | 2022篇 |
1975年 | 1768篇 |
1974年 | 1787篇 |
1973年 | 1718篇 |
1971年 | 1463篇 |
2. In 3 treatments the diet was supplemented with 0, 100 and 150 mg probiotic/kg food.
3. In the 100 mg probiotic group, egg production improved by 5%, and shell thickness improved slightly, with fewer thin‐shelled eggs than in the control (8.6% compared to 18.6%).
4. The initial serum cholesterol concentration of 170.2 mg/dl in control birds remained similar throughout the 10‐week experimental period, whereas in the 150 mg group the initial value of 176.5 mg/dl decreased to 114.3 mg by week 10.
5. Yolk cholesterol concentration was 14.69 mg in the control group and 11.28 and 11.37 mg/g in the 100 and 150 mg probiotic groups respectively. Overall mean total egg cholesterol was thus reduced by probiotic supplementation. 相似文献
2. Daytime perching did not differ significantly between the treatments. At night, over the whole year, 81% of birds in the 480 mm cages and 86% in the 520 mm cages roosted on the perch. This figure reached about 95% in the 560 and 600 mm cages, significantly more at most ages than in the 480 mm cages.
3. Feather damage was slightly less, but claw problems slightly more, in the 2 wider treatments than in the 2 narrower treatments. Birds in the wider cages were calmer when approached or handled by humans than those in the narrower cages. This may have been associated with variation in space allowance between the treatments.
4. There was a trend for lower production in the 480 mm cages than in the other treatments which may have been associated with the reduced feeding space in this treatment. There were few other treatment differences in production traits.
5. The balance of the evidence from this study is that when perches are provided in laying cages for medium weight hybrids, 140 mm of perch space per hen is adequate. For the amelioration of a number of the welfare problems of conventional cages, provision of perches should be combined with other modifications. 相似文献
2. Birds in all treatments gained similar amounts of weight from 12–14 weeks of age. Thereafter, birds in C maintained gains, the body weight gains of birds in EC and EH declined moderately and birds in H lost weight.
3. Birds in C produced more metabolic heat. Those in H showed increased heat production as they became older.
4. Protein retention was greater in turkeys in C, EC and EH. It was lower in H, becoming negative between 18–20 weeks of age. Birds in H suffered body fat losses which increased with age.
5. Birds in EH and EC gained similar amounts of body weight. Birds in EH exhibited lower heat production and greater energy retention than those in EC.
6. Body temperatures were higher in H than in the other environments. 相似文献
2. A panel of 180 MAbs was generated, from which 14 were chosen for their specificity.
3. These MAbs were characterised immunohistologically on frozen sections of embryonic, newly‐hatched and adult, male and female, left gonads.
4. Three of these MAbs are described: AGC5, AGC7, and AGC13 which recognise respectively germinal epithelial cells, supporting cells and germ cells.
5. These MAbs may be useful in following the developmental pathways in the chicken gonad and analysing the interactive role(s) taking place between gonadal stromal cells and primordial germ cells. 相似文献
2. The Light Sussex and Gold‐Pencilled Hamburgh bantams used in this study were known to be Co/ Co db+ / db+ and co+ / co+ Db/ Db respectively.
3. In both F2 generations the least eumelanin restricted segregants were as the parent mated with the Buff Rock, thus demonstrating the Buff Rock bantam tested to be Co/ Co Db/ Db, or possess similar alleles at these loci.
4. The Buff Rock bantam male was found to be heterozygous for a eumelanin inhibitor gene which appears to be dominant white (I). 相似文献
2. Culture media deficient in either sodium or chloride ions reduced the volume of SEF secreted by up to a quarter. Ionic composition of the fluid was little altered, and sodium was transported against a concentration gradient.
3. Amiloride, an inhibitor of Na+/H+ exchange, reduced SEF production by half, whereas ionic composition and osmolality of the fluid was unchanged.
4. Likewise, acetazolamide, an inhibitor of carbonic anhydrase, reduced SEF production by the blastoderm, and left the ionic composition and osmolality of the fluid unaltered.
5. Neither furosemide or 4,4´‐diisothiocyanatostilbene‐2,2´‐disulfonic acid at the concentrations used changed the volume of SEF formed or its ionic composition.
6. It is concluded that the secretion of SEF is dependent upon the active transport of sodium across the blastoderm; an amiloride‐sensitive Na+/H+ exchanger and carbon dioxide hydration catalysed by carbonic anhydrase are also involved.
7. Furthermore, it is proposed that fluid transport across the blastoderm is the result of local osmotic gradients, not from a sub‐embryonic fluid hyperosmotic to albumen, as has been suggested previously. 相似文献
2. A combination of 3 growing climates, 3 laying climates (temperate, hot dry, hot humid), and 2 dietary energy concentrations (10.03 and 11.70 MJ ME/kg with protein concentrations proportional to energy) were tested for 46 weeks using 432 point‐of‐lay pullets.
3. Both growing and laying climates significantly affected most traits measured during the laying period. The patterns of egg production showed good persistency in all environments and differences observed reduced with age.
4. The difference between the high and low energy intake reduced at high ambient temperatures. Feeding low energy diets did not affect mean age at first egg, rate of lay or the egg output in the hot dry environment, nor egg weight in either hot dry or hot humid climates. Rather, feeding low energy diets resulted in improved body weight change in all 3 laying environments.
5. The growing climate and diet interacted significandy on body weight change, while highly significant interactions between laying climate and diet occurred on rate of lay, food and energy intake and egg weight during the laying period. 相似文献
2. The hen‐housed laying performance of hens offered the DPM‐containing diets was significantly better for number of eggs laid and total egg mass produced but the performance of the control groups was lower than would normally be expected of those stocks. The reason for the lower rate of lay and higher mortality of hens fed on the control diet was not identified. The amount of food consumed increased as the energy content of the diet decreased and the efficiency of food conversion (EFC) of the lightweight strain was reduced; there was no significant effect on the EFC of the medium‐weight hens.
3. The inclusion of DPM in the diets had no effect on albumen quality or on the incidence of hair‐cracked, cracked or broken eggs. However, with both strains of hens, shell weight and shell thickness were poorer when 100 or 200 g DPM/kg diets were fed. 相似文献