收费全文 | 223161篇 |
免费 | 14151篇 |
国内免费 | 908篇 |
林业 | 23073篇 |
农学 | 13944篇 |
基础科学 | 2993篇 |
41322篇 | |
综合类 | 14759篇 |
农作物 | 17205篇 |
水产渔业 | 15972篇 |
畜牧兽医 | 79849篇 |
园艺 | 6189篇 |
植物保护 | 22914篇 |
2021年 | 2499篇 |
2020年 | 2886篇 |
2019年 | 3688篇 |
2018年 | 3925篇 |
2017年 | 4290篇 |
2016年 | 4720篇 |
2015年 | 4271篇 |
2014年 | 5628篇 |
2013年 | 16164篇 |
2012年 | 5483篇 |
2011年 | 7185篇 |
2010年 | 7108篇 |
2009年 | 7746篇 |
2008年 | 6657篇 |
2007年 | 5521篇 |
2006年 | 6258篇 |
2005年 | 5454篇 |
2004年 | 5340篇 |
2003年 | 5107篇 |
2002年 | 4414篇 |
2001年 | 4826篇 |
2000年 | 4474篇 |
1999年 | 4427篇 |
1998年 | 4006篇 |
1997年 | 4071篇 |
1996年 | 3810篇 |
1995年 | 4334篇 |
1994年 | 3714篇 |
1993年 | 3398篇 |
1992年 | 3497篇 |
1991年 | 3708篇 |
1990年 | 3350篇 |
1989年 | 3343篇 |
1988年 | 2937篇 |
1987年 | 2957篇 |
1986年 | 2864篇 |
1985年 | 3186篇 |
1984年 | 3090篇 |
1983年 | 2993篇 |
1982年 | 2594篇 |
1981年 | 2587篇 |
1980年 | 2560篇 |
1979年 | 2756篇 |
1978年 | 2546篇 |
1977年 | 2393篇 |
1976年 | 2267篇 |
1975年 | 2055篇 |
1974年 | 2129篇 |
1973年 | 2041篇 |
1972年 | 1774篇 |
2. Calcium regulation of skeletal muscle sarcoplasmic reticulum was compared between broiler and Leghorn chickens. Calcium regulation matured from the 2nd to the 11 th week and, at any age, broiler chickens showed significandy lower calcium transport rates and transport efficiencies. The mechanism of calcium transport in broiler chickens was more energy‐consuming than that of the Leghorn chickens.
3. Sarcoplasmic calcium regulation is pivotal for muscle metabolism. As in porcine malignant hyperthermia, weaker calcium regulation might lead to hyperactivation of skeletal muscle, followed by elevated lactic acid concentrations and cardiovascular failure. 相似文献
2. The EMC was 600 mm wide, 450 mm deep and 450 mm high at the rear; it had a softwood perch and at one side a 250 mm wide nest box (containing litter or artificial turf) with a dust bath direcdy above. It housed 4 birds and provided 675 cm2/bird in the main cage with an additional 281 cm2 /bird in the nest box. The nest box and dust bath had automatically controlled doors which were closed at night. There were 18 EMC; in the first trial these were compared with 6 control cages with perch but without next box or dust bath.
3. Hens spent 32 to 37% of day time on the perch, 5 to 7% in the dust bath and 5 to 6% in the nest. At night 92 to 98% roosted on the perch.
4. Initially only 55 to 70% of eggs were laid in the nest box partly because some eggs were laid before dawn. Once the door was retimed to open 3h before lights‐on the proportion rose to 91 to 96%. Very few eggs were laid in the dust bath. Pre‐laying behaviour lasted longer in treatments with nest boxes (55 to 76min) than in control cages (48min); disturbance was slight in all treatments, but lowest in control cages.
5. Dust baths were well used, with on average 61% of hens dust bathing during a 3‐h afternoon observation period compared with only 17% in control cages. Two birds could use the dust bath simultaneously.
6. It was concluded that although a number of minor design features still required attention the EMC has potential to reduce the disadvantages of conventional cages for welfare while retaining their advantages and has possible commercial application. 相似文献
2. The rate of lay observed with a Step‐Up treatment which gave increases in photoperiod from 8L:16D at 18 weeks to 15L:9D at 27 weeks of age was not significantly different from that of treatments which held the birds on 11L:13D during peak egg production but gave increments up to 15L:9D later in the laying year.
3. A control group maintained on 11L:13D from 20 to 72 weeks laid 295 eggs per bird housed and a further group held on 8L:16D from 0 to 72 weeks laid 284 eggs per bird. These yields were lower than the Step‐Up treatment (299 eggs) but show the potential of modern hybrid stocks to lay prolifically even without light stimulation.
4. It is concluded that the stocks tested in this experiment showed no advantage when given lighting programmes in the first laying year which were designed to minimise the adverse effects of photorefractoriness. 相似文献
2. Hepatocytes that stained with trypan blue were considered to be dead or dying before fixation and represented damaged cells.
3. The proportion of trypan blue‐stained hepatocytes in the livers of ascitic birds was slightly less than half that observed in the hypoxia‐induced birds but significandy more than the proportion of stained cells observed in control birds.
4. Liver damage in the ascitic birds was also assessed biochemically by an altered enzyme profile.
5. The study demonstrated that increased trypan blue uptake in the livers of ascitic birds reared at sea‐level may be the consequence of hypoxia stress caused by reduced oxygen utilisation. 相似文献
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