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   2166篇
  免费   87篇
  国内免费   4篇
林业   158篇
农学   118篇
基础科学   61篇
  485篇
综合类   147篇
农作物   282篇
水产渔业   190篇
畜牧兽医   478篇
园艺   77篇
植物保护   261篇
  2024年   49篇
  2023年   45篇
  2022年   67篇
  2021年   85篇
  2020年   110篇
  2019年   143篇
  2018年   142篇
  2017年   129篇
  2016年   139篇
  2015年   71篇
  2014年   84篇
  2013年   212篇
  2012年   111篇
  2011年   131篇
  2010年   104篇
  2009年   67篇
  2008年   120篇
  2007年   57篇
  2006年   45篇
  2005年   38篇
  2004年   33篇
  2003年   32篇
  2002年   24篇
  2001年   17篇
  2000年   27篇
  1999年   16篇
  1998年   15篇
  1997年   12篇
  1996年   16篇
  1995年   13篇
  1994年   7篇
  1993年   4篇
  1992年   8篇
  1991年   6篇
  1990年   8篇
  1989年   6篇
  1988年   3篇
  1987年   7篇
  1986年   4篇
  1985年   5篇
  1984年   9篇
  1983年   4篇
  1982年   2篇
  1981年   4篇
  1979年   5篇
  1975年   3篇
  1973年   3篇
  1972年   2篇
  1971年   3篇
  1967年   3篇
排序方式: 共有2257条查询结果,搜索用时 781 毫秒
51.
A risk analysis was undertaken in an attempt to improve improvised rump straps on donkeys carrying tourists at the World Heritage Site at Petra, Jordan. Tail-base lesions were identified in 63 of the 86 donkeys. Observations and questionnaires were used to collect data relating to the straps, donkey health and human attitudes. The worse lesions were associated with padded rather than unpadded straps, if tightly fitted. Padding could be a cause of, or a response to lesions, but results suggest that it did not effectively aid healing. Significantly worse lesions occurred with unclean than with clean straps and, contrary to many recommendations, cotton straps were associated with worse lesions than were synthetic straps. Since this was an exploratory study, findings should be considered to generate (not to test) hypotheses and any resulting interventions will require monitoring. Further possible risks are discussed, referring to medical and veterinary literature and applied expertise in working equines.  相似文献   
52.
Many fish species undergo natural starvation periods. Adaptation to starvation is possible through the activation of behavioral, biochemical and physiological mechanisms. Knowledge of the effect of dietary nutrients on the intermediary metabolism during starvation and refeeding can be useful to improve fish health and optimize aquaculture production. To analyze the effect of dietary nutrients on liver metabolism of Siberian sturgeon (Acipenser baerii) submitted to starvation and refeeding, four isoenergetic diets differing in nutrient composition were designed: LP-St (38 % protein, 12 % lipid, 36 % carbohydrate), HP-St (44 % protein, 10 % lipid, 30 % carbohydrate), LP-L (38 % protein, 18 % lipid, 25 % carbohydrate) and HP-L (44 % protein, 16 % lipid, 22 % carbohydrate). Four groups of fish were fed 3 weeks to satiety with the corresponding diet, starved for 2 weeks and then refeed 5 weeks to satiety on the same diet. Starvation mobilized the hepatic lipid store to a greater extent than glycogen. Starvation increased superoxide dismutase activity irrespective of the diet, while low protein diets (LP-St and LP-L) increased catalase activity. The oxidative damage decreased after 5 weeks of refeeding. Refeeding the starved fish on the HP-St diet promoted the greatest growth performance. In addition to reporting for the first time the effect of diet composition on growth, liver composition and antioxidant activities in Siberian sturgeon submitted to starvation and refeeding, our findings suggest that refeeding on HP-St diet stimulated the use of dietary carbohydrates and allowed a protein sparing effect in Siberian sturgeon.  相似文献   
53.
盐胁迫对扁桃砧木叶片SOD、POD和CAT活性的影响   总被引:4,自引:0,他引:4  
以温室盆栽的3种扁桃砧木为材料,研究了不同浓度的NaCl、Na2SO4和不同胁迫时间对其叶片活性氧代谢变化的影响.结果表明:NaCl和Na2SO4胁迫下,石头扁桃,桃扁桃和毛桃叶片中POD、CAT活性均随盐浓度的增加而升高,而随着盐胁迫时间的延长其活性有所下降,但都高于对照.SOD活性均随盐浓度的增加先升高后下降,而且后期下降幅度很大.与NaCl胁迫相比,Na2SO4胁迫下这3种砧木叶片中POD、CAT和SOD活性低.从2种盐胁迫下POD、CAT和SOD活性变化规律初步判断这3种砧木的抗盐能力强弱为桃扁桃>毛桃>石头扁桃.  相似文献   
54.
In 1931 Carpantier reported bovine TB (BTB) in Iranian cattle. Some eighty years on with a national test-and-slaughter programme in place for over four decades, the efforts to vanquish Mycobacterium bovis (M. bovis) infection in cattle have been in vain as the vast majority of the 30 Iranian provinces still have reports of BTB in their cattle herds every year. This paper reviews the present epidemiology of BTB in Iran and in the region and evaluates the success of government policy in controlling this disease.  相似文献   
55.
The temporal dynamics of appetite (weight of food consumed per day) were analysed for fish fed to satiation after a 1 or 2-week period of feed deprivation. Three species were compared: two omnivores (minnow, Phoxinus phoxinus and gibel carp, Carassius auratus gibelio) and a carnivore (three-spined stickleback, Gasterosteus aculeatus). All species showed compensatory changes in appetite and growth during the re-feeding period but in the stickleback, there was a lag of a week before the compensatory response was detected. The temporal dynamics of appetite differed between the three species but not within a species. Appetite of the minnow declined towards control levels from the onset of re-feeding. In gibel carp, appetite increased to a peak and then declined to control levels. In the stickleback, appetite was initially below control levels, increased to a maximum, then declined towards control levels. The differences between the species might have been artefacts of the experimental protocols used but could also reflect underlying differences in the control of appetite in these species of fish.  相似文献   
56.
The aim of this study was to investigate the effects of onion on serum uric acid levels and hepatic Xanthine Dehydrogenase/Xanthine Oxidase activities in normal and hyperuricemic rats. Hyperuricemia was induced by intraperitoneal injection of 250 mg kg(-1) potassium oxonate in rats. Oral administration of onion at 3.5 and 7.0 mg kg(-1) day(-1) for 7 days was able to reduce serum uric acid levels in hyperuricemic rats with no significant effects on the level of this compound in the normal animals. In addition, onion when tested in vivo on rat liver homogeneities elicited significant inhibitory actions on the Xanthine Dehydrogenase (XDH) and Xanthine Oxidase (XO) activities. This effect resulted less potent than that of allopurinol. However, the hypouricemic effect observed in the experimental animal did not seem to parallel the change in XDH and XO activities, implying that the onion might be acting via other mechanisms apart from simple inhibition of enzyme activities. Such hypouricemic action and enzyme inhibitory activity of onion makes it a possible alternative for allopurinol, or at least in combination therapy to minimize the side-effects of allopurinol, in particular in long-term application.  相似文献   
57.
Background: Previous studies have shown that some cytokines have protective effects on cartilage in joint diseases. In the current study, effects of IL-4 against morphological changes and tissue degradation induced by IL-1α on bovine nasal cartilage (BNC) explants were investigated. Methods: Fresh BNC samples were prepared from a slaughterhouse under sterile conditions. BNC explants culture was treated with both IL-lα (10 ng/ml) and IL-4 (50 ng/ml) at the same time for 28 days. The morphological characteristics of explants were assessed by using histology techniques and invert microscopy. Matrix metalloproteinase-1 (MMP-1) production was assessed within different days by using Western blotting. Results: IL-lα induced prominent cartilage morphology degradation. The pro and active form of MMP-1 band substantially increased at day 21 of culture. In the presence of both IL-lα and IL-4, chondrocytes preserved their ordinary normal phenotype with intact extracellular matrix. In addition, a significant reduction in pro-MMP-1and inhibition of active MMP-1 was seen. Conclusion: In conclusion, IL-4 could be regarded as a potential candidate in cartilage protecting against the degradation changes of IL-lα. It seems that the preservation effect of IL-4 is associated with significant reduction of MMP-1. Key Words: Chondrocyte, Interleukin-1α, Interleukin-4, Matrix metalloproteinase-1, Bovine nasal cartilage  相似文献   
58.
This study aimed to evaluate three regimes for oestrus and ovulation synchronization in Farafra ewes in the subtropics. During autumn, 43 ewes were assigned to (i) controlled internal drug releasing (CIDR)‐eCG group, treated with CIDR for 12 days and eCG at insert withdrawal, n = 13; (ii) PGF2α‐PGF2α group, treated with two PGF2α injections at 11 days interval, n = 14; and (iii) GnRH‐PGF2α‐GnRH group, treated with GnRH, followed 5 days later with PGF2α and 24 h later with a second GnRH, n = 16. Oestrus‐mating detection was carried out at 4 h intervals starting on day 0 [the day of CIDR withdrawal (CIDR‐eCG group), the day of second PGF2α treatment (PGF2α‐PGF2α group) and the day of PGF2α treatment (GnRH‐PGF2α‐GnRH group)]. Ovarian dynamics was monitored by ultrasound every 12 h beginning on day 0 and continued for 4 days. Blood samples were obtained daily for progesterone (P4) and oestradiol 17β (E2) estimation starting on day 0 and continued for 4 days. The obtained results showed that, oestrus expression, ovulation and conception were greater (p < 0.05) in CIDR‐eCG and PGF2α‐PGF2α groups than in GnRH‐PGF2α‐GnRH group. All ewes of PGF2α‐PGF2α group presented, on day of second PGF2α injection with mature CL (P4 > 2.0 ng/ml), compared to 42.9% in GnRH‐PGF2α‐GnRH group (p = 0.01). The peak of oestrus occurred 32–52, 48–60 and 28–96 h after the end of treatment in CIDR‐eCG, PGF2α‐PGF2α and GnRH‐PGF2α‐GnRH groups, respectively. Ovulation started 48 h after treatment in all groups and extended for 24, 36 and 48 h for CIDR‐eCG, PGF2α‐PGF2α and GnRH‐PGF2α‐GnRH groups, respectively. Results demonstrated that oestrus and ovulation synchronization could be efficiently achieved in Farafra ewes using either CIDR‐eCG or PGF2α‐PGF2α regimes; however, the GnRH‐PGF2α‐GnRH treatment induced a more spread oestrus and ovulation that may make the protocol inadequate for timed artificial insemination.  相似文献   
59.
Tropical Animal Health and Production - A cross-sectional study was carried out to determine the prevalence and risk factors of bovine mastitis caused by Streptococcus agalactiae from farms in and...  相似文献   
60.
Abstract In an attempt to identify appropriate feeding rates for multispecies of fish raised in fertilized earthen ponds, the present work was conducted over a 19‐week experimental period to establish the growth performance, production and body composition of Nile tilapia, common carp and silver carp fed 0.0%, 0.5%, 1.0%, 3.0%, 5.0% biomass and to apparent satiation (treatments). Twelve ponds were stocked with a similar number and weight of each fish species. Two ponds were assigned to each of the treatments, and a 25% protein pelleted fish feed was used to feed fish at the specified rate of feeding. At the end of the experiment, growth, weight gain, survival, yield and body composition of fish groups were affected by the treatments. The economic effectiveness also varied among treatments. The most conspicuous attribute of the feeding rates was its lack of influence on growth (g day?1), weight gain (g per fish), yield (kg ha?1) or body composition of silver carp. The results of whole‐body proximate analysis indicated that various feeding rates had either an irregular pattern or no effects on the protein and ash gain per 100 g of fish body weight (bw) gain. The most notable exceptions were significant (P < 0.05) increases in body fat and gross energy gains in Nile tilapia, common carp and silver carp accompanied by decreases in percentages of moisture (but not in silver carp) as feeding rate increased. Among the six different feeding levels, feeding to apparent satiation (feed amount was equivalent to 2.67% of fish bw day?1) appeared to be optimal, as it significantly (P < 0.05) supported the highest fish production, income and net profit compared with all other treatments except for the 3% feeding level, for which the differences in those measurements were comparable.  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