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   142263篇
  免费   2084篇
  国内免费   13篇
林业   5586篇
农学   4654篇
基础科学   1010篇
  17622篇
综合类   19008篇
农作物   5094篇
水产渔业   7235篇
畜牧兽医   73286篇
园艺   1798篇
植物保护   9067篇
  2020年   1246篇
  2019年   1472篇
  2018年   2233篇
  2017年   2496篇
  2016年   2316篇
  2015年   2009篇
  2014年   2470篇
  2013年   5746篇
  2012年   4370篇
  2011年   5178篇
  2010年   3521篇
  2009年   3560篇
  2008年   5164篇
  2007年   4973篇
  2006年   4654篇
  2005年   4271篇
  2004年   4111篇
  2003年   4178篇
  2002年   3811篇
  2001年   4780篇
  2000年   4641篇
  1999年   3747篇
  1998年   1471篇
  1997年   1452篇
  1996年   1341篇
  1995年   1599篇
  1994年   1327篇
  1993年   1359篇
  1992年   2761篇
  1991年   2901篇
  1990年   2866篇
  1989年   2897篇
  1988年   2610篇
  1987年   2592篇
  1986年   2539篇
  1985年   2400篇
  1984年   1957篇
  1983年   1716篇
  1982年   1156篇
  1979年   1649篇
  1978年   1284篇
  1977年   1135篇
  1976年   1100篇
  1975年   1164篇
  1974年   1357篇
  1973年   1448篇
  1972年   1378篇
  1971年   1195篇
  1970年   1247篇
  1969年   1146篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The majority of off-type plants in indica hybrid rice were CMS and its maintainer lines.  相似文献   
992.
    
The extensive pig production in Spain is traditionally characterised by: the use of the Iberian pig, an autochthonous breed perfectly integrated into the environment in which they have developed; a long duration of the productive cycle for about 23–24 months; a high level of animal welfare level, specially in the fattening process with freedom of movement and feeding base on natural sources: acorns and grass, and an equilibrated “dehesa” agro-forestry system where this activity has been developed. Nowadays, the introduction of more intensificated methods due to the increasing demand led to important changes, such as: the shortening of the productive cycle (10–12 months); freeing from the territorial base; changes during the fattening period, fattening with mixed feed and less animal freedom. All these facts may implicate a loss of the animal welfare condition. These circumstances lead us to question it from an ethical point of view.  相似文献   
993.
994.
995.
Pure-bred embryos were transferred within and reciprocally between large (Suffolk) and small (Cheviot) breeds of sheep to establish 4 treatment groups: SinS (Suffolk embryos in Suffolk dams), SinC (Suffolk embryos in Cheviot dams), CinS (Cheviot embryos in Suffolk dams), and CinC (Cheviot embryos in Cheviot dams). The recipient ewes carried single fetuses to term. The maternal plasma concentrations of ovine placental lactogen (oPL), progesterone, IGF-1, FFA, and glucose were measured on d 50, 90, 120, and 140 of pregnancy. Birth weight, body dimensions, and placental characteristics of lambs were recorded at birth. There was a recipient ewe breed × lamb breed × time interaction for the concentration of oPL (P = 0.03), but no such interaction was observed for progesterone (P = 0.42), IGF-1 (P = 0.57), glucose (P = 0.36), or FFA (P = 0.72). There were no differences in oPL (P = 0.28) and progesterone (P = 0.34) concentrations between SinC and SinS ewes. The concentrations of FFA on d 140 (P = 0.008), and those of glucose on d 50 (P = 0.02) and 120 (P = 0.01), were greater in SinC ewes than in SinS ewes. The ewes in CinS had less FFA concentration (P = 0.002) at all time points than CinC ewes. The concentrations of IGF-1 on d 90 were greater (P = 0.004) in CinS ewes than CinC ewes, but did not differ (P = 0.16) on d 50, 120, and 140. The concentrations of glucose on d 50 (P = 0.001), 90 (P = 0.03), and 140 (P = 0.03) were less in CinS ewes compared with CinC ewes. The birth weight of SinC lambs (5.04 ± 0.20 kg) was lighter (P = 0.001) than SinS lambs (5.94 ± 0.19 kg), and body dimensions of SinC lambs were smaller (P = 0.01) than SinS lambs. Neither birth weight nor the body dimensions of CinS lambs differed (P = 0.24) from CinC lambs. Cotyledon number was reduced (P = 0.04) in the CinS (57.5 ± 6.3) compared with the SinS group (74.2 ± 5.9), whereas mean cotyledon weight in CinS (2.42 ± 0.20 g) was greater (P = 0.02) than SinS (1.74 ± 0.21 g). It was concluded that the large genotype lambs were lighter and smaller when born to small genotype dams; however, the birth weight or body dimensions of small genotype lambs did not differ when born to large genotype dams. This study suggests that plasma oPL, progesterone, IGF-1, FFA, and glucose concentrations at different times throughout pregnancy reflect the regulatory effect of the uterine environment on the development of the fetus.  相似文献   
996.
    
Corneal ulceration is a common ophthalmic condition in horses. It is frequently caused by trauma to the corneal surface, followed by secondary infection by commensal or pathogenic organisms including Streptococcus equi subspecies zooepidemicus, Pseudomonas aeruginosa and Staphylococcus spp. Emerging antimicrobial resistance amongst these organisms has raised the need for appropriate antimicrobial therapy selection, to optimise treatment efficacy while minimising further antimicrobial resistance. Medical records of 38 horses presented at the University Veterinary Teaching Hospital Camden for ulcerative keratitis between 2010 and 2020 were reviewed to identify those with positive bacterial cultures and antimicrobial susceptibility profiles (13/38). Common susceptibility patterns were identified and used to guide the empirical treatment of equine bacterial corneal ulcers. Pseudomonas spp. (64.3%), Streptococcus equi subspecies zooepidemicus (14.3%) and Actinobacillus spp. (14.3%) were most commonly identified. Susceptibility to amikacin, gentamicin and ciprofloxacin was observed in 100%, 66.7% and 85.7% Pseudomonas spp. isolates respectively. Resistance to polymyxin B and neomycin occurred in 85.7% and 71.4% of Pseudomonas spp., respectively. All Streptococcus equi subspecies zooepidemicus organisms in this study were susceptible to ceftiofur, cephalexin, penicillin and ampicillin, while they were all resistant to gentamicin, neomycin, enrofloxacin and marbofloxacin. Predominating in this study, Pseudomonas spp. maintained overall aminoglycoside susceptibility despite some emerging resistance, and good fluoroquinolone susceptibility. High resistance to Polymyxin B could have arisen from its common use as first-line therapy for bacterial corneal ulcers. Although further research is required, these new findings about predominant bacteria in equine corneal ulceration in the Camden region and their antimicrobial susceptibility patterns can be used to guide the empirical treatment of bacterial corneal ulcers in horses.  相似文献   
997.
    
A 17-month-old Warmblood filly was referred to our clinic for evaluation of congenital facial nerve (FN) paralysis. Clinical examination revealed a right-sided facial paralysis with mild masticatory muscle atrophy, mild dysphagia and exposure keratitis. Apart from the FN deficits, neurological examination of the remaining cranial nerves showed no abnormalities. Magnetic resonance imaging (MRI) examination using a 3.0 Tesla scanner showed that in comparison to the left FN, the intracranial section of the right FN between the pons and internal acoustic canal was thinner, whereas it appeared indistinct and thickened within the internal acoustic canal and facial canal. Signs of meningitis or encephalitis were not present on MRI. Cerebrospinal fluid analysis showed mild pleocytosis. The owner of the filly requested euthanasia due to the guarded prognosis. At necropsy, the intracranial section of the right FN was macroscopically thinner than the left side and within the facial canal, a 5 mm tissue stump could be identified with an absent extracranial part of the right FN. Histological examination of the brain stem showed different architecture of the left and right motor nuclei of the FN: in the left nucleus, motor neurons of a normal size and well stainable Nissl bodies were present, whereas in the right nucleus, neurons with Nissl bodies were decreased in number and size. Further, a cytoplasmic rich cell population with a nucleus size compatible with normal neurons was present. These cells were suspected to be atrophic neurons. The tissue stump within the facial canal was histologically identified as connective tissue. Unilateral malformation of the FN has not previously been described in the horse. This filly showed a right-sided, intracranial hypoplasia accompanied by an extracranial aplasia of the FN causing complete, congenital facial nerve paralysis, which corresponded to a difference in the architecture of the affected motor nucleus of the FN.  相似文献   
998.
    
When racehorses fail to thermoregulate effectively, whether due to intrinsic or extrinsic factors, a condition called exertional heat illness (EHI) may occur, which can be life-threatening and represents a significant welfare issue. Horses usually become affected in the recovery phase soon after racing, so that a knowledge of the ‘normal’ thermoregulatory processes which operate at this time and their variations is essential to enable early detection of EHI. Racing in hot or warm and humid weather conditions represents the upper extreme of thermoregulatory variations, and observations on how horses normally recover at the racetrack may provide a clearer perspective on the physiology of that process for all sports horses. This review focuses on the post-exercise thermoregulatory capacity of the Thoroughbred racehorse, with a view to understanding the physiological mechanisms that should efficiently dissipate excess heat. The clinical manifestations of exertional heat illness are due to inadequate or harmful physiological responses, and if recognised and treated early can be effectively curtailed.  相似文献   
999.
Enteral nutrition coupled with bacterial colonization has been shown to have major functional and developmental effects during the postnatal period of neonates. In this study, we developed a highly sensitive premature pig model to elucidate the specific developmental impact of initial bacterial colonization on premature neonates by comparing germ-free and conventionally reared pigs. Thirty-eight preterm pigs (93% gestation) were delivered via caesarean section and reared in either germ-free or conventional isolators for 40–48 h. Pigs were fed either infant milk formula or sow's colostrum. Enteral feeding for two days had trophic effects on gastrointestinal weights, particularly for pancreas, stomach and small intestine (SI). The absence of bacteria in formula fed pigs resulted in a mucosa that appeared more robust and had higher weights for both SI and pancreas, compared to conventional formula fed pigs (P < 0.05). Colostrum fed pigs also had markedly increased mucosal SI proportions, lung and spleen weights compared to conventional formula fed pigs. Colostrum fed pigs and germ-free pigs were similar although stomach, pancreas and distal SI weights were highest for germ-free pigs. The results demonstrate that the initial bacterial colonization interacts with diet to modulate the early neonatal organ development, particularly of the GIT.  相似文献   
1000.
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