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   56406篇
  免费   3104篇
  国内免费   91篇
林业   2919篇
农学   1856篇
基础科学   309篇
  7064篇
综合类   8835篇
农作物   1767篇
水产渔业   3071篇
畜牧兽医   29678篇
园艺   783篇
植物保护   3319篇
  2020年   465篇
  2019年   487篇
  2018年   975篇
  2017年   977篇
  2016年   933篇
  2015年   780篇
  2014年   901篇
  2013年   2126篇
  2012年   1707篇
  2011年   2043篇
  2010年   1364篇
  2009年   1249篇
  2008年   1992篇
  2007年   1935篇
  2006年   1816篇
  2005年   1645篇
  2004年   1631篇
  2003年   1588篇
  2002年   1387篇
  2001年   2031篇
  2000年   2110篇
  1999年   1591篇
  1998年   606篇
  1997年   520篇
  1996年   483篇
  1995年   646篇
  1994年   549篇
  1993年   491篇
  1992年   1236篇
  1991年   1311篇
  1990年   1298篇
  1989年   1220篇
  1988年   1167篇
  1987年   1175篇
  1986年   1127篇
  1985年   1050篇
  1984年   889篇
  1983年   753篇
  1982年   465篇
  1979年   725篇
  1978年   569篇
  1977年   500篇
  1976年   451篇
  1975年   515篇
  1974年   605篇
  1973年   597篇
  1972年   598篇
  1971年   527篇
  1970年   497篇
  1969年   545篇
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
991.
The objective of this study was to assess fermentability by canine gut microflora to include shortchain fatty acid (SCFA) production, organic matter (OM) disappearance, and gas production of vegetable and fruit fiber sources compared to fiber standards (psyllium, citrus pectin, and Solka Floc). Fiber sources included apple pomace, carrot pomace, flaxseed, fruit blend (mixture of peach, almond, nectarine, and plum), grape pomace, pea hulls, pistachio, and tomato pomace. Substrates were fermented in vitro for 4, 12, and 24 h with fecal flora obtained from three healthy dogs. Citrus pectin had the highest OM disappearance, SCFA production, and gas production at all times of fermentation; psyllium was intermediate and Solka Floc was lowest. A wide variation in fermentability was noted among the vegetable and fruit fiber sources. Apple pomace, carrot pomace, and flaxseed had the greatest fermentability as assessed by OM disappearance. Pea hulls and tomato pomace had intermediate OM disappearances, and fruit blend, grape pomace, and pistachio were poorly fermented. Carrot pomace produced the largest amounts of gas and SCFA. Apple pomace produced high concentrations of gas but intermediate concentrations of SCFA. Pea hulls and tomato pomace produced intermediate concentrations of gas and SCFA, whereas flaxseed, fruit blend, grape pomace, and pistachio produced low amounts of these fermentation products. For all substrates collectively, OM disappearance was highly correlated with both gas production (r2 = 0.782 and 0.723 for 12- and 24-h values, respectively) and SCFA production (r2 = 0.737 and 0.738 for 12- and 24-h values, respectively). In general, OM disappearance, gas production, and SCFA production were related to the insoluble:soluble fiber ratio in the samples; as the insoluble:soluble ratio decreased (increased soluble fiber), the OM disappearance, gas production, and SCFA production increased.  相似文献   
992.
A study involving nine research stations from the NCR-42 Swine Nutrition Committee used a total of 1,978 crossbred pigs to evaluate the effects of dietary ZnO concentrations with or without an antibacterial agent on postweaning pig performance. In Exp. 1, seven stations (IA, MI, MN, MO, NE, ND, and OH) evaluated the efficacy of ZnO when fed to nursery pigs at 0, 500, 1,000, 2,000, or 3,000 mg Zn/kg for a 28-d postweaning period. A randomized complete block experiment was conducted in 24 replicates using a total of 1,060 pigs. Pigs were bled at the 28-d period and plasma was analyzed for Zn and Cu. Because two stations weaned pigs at < 15 d (six replicates) and five stations at > 20 d (18 replicates) of age, the two sets of data were analyzed separately. The early-weaned pig group had greater (P < 0.05) gains, feed intakes, and gain:feed ratios for the 28-d postweaning period as dietary ZnO concentration increased. Later-weaned pigs also had increased (P < 0.01) gains and feed intakes as the dietary ZnO concentration increased. Responses for both weanling pig groups seemed to reach a plateau at 2,000 mg Zn/kg. Plasma Zn concentrations quadratically increased (P < 0.01) and plasma Cu concentrations quadratically decreased (P < 0.01) when ZnO concentrations were > 1,000 mg Zn/kg. Experiment 2 was conducted at seven stations (KY, MI, MO, NE, ND, OH, and OK) and evaluated the efficacy of an antibacterial agent (carbadox) in combination with added ZnO. The experiment was a 2 x 3 factorial arrangement in a randomized complete block design conducted in a total of 20 replicates. Carbadox was added at 0 or 55 mg/kg diet, and ZnO was added at 0, 1,500, or 3,000 mg Zn/ kg. A total of 918 pigs were weaned at an average 19.7 d of age. For the 28-d postweaning period, gains (P < 0.01), feed intakes (P < 0.05), and gain:feed ratios (P < 0.05) increased when dietary ZnO concentrations increased and when carbadox was added. These responses occurred in an additive manner. The results of these studies suggest that supplemental ZnO at 1,500 to 2,000 mg Zn/kg Zn improved postweaning pig performance, and its combination with an antibacterial agent resulted in additional performance improvements.  相似文献   
993.
The objectives of this study were to evaluate the effects of pre- and postpartum undegraded intake protein (UIP) supplementation on body condition score (BCS), BW, calf weight, milk production, serum IGF-I concentrations, and postpartum interval in primiparous beef heifers (n = 44). Heifers were maintained on endophyte-free stockpiled tall fescue (11.7% CP, 38% ADF) and individually fed supplement daily beginning 60 d prepartum. Pre- and postpartum supplements provided 19.3% CP, 83.4% TDN (UIP); 14.1% CP, 84.1% TDN (Control); 21.5% CP, 81.5% TDN (UIP); and 14.6% CP, 81.4% TDN (Control); respectively. Blood meal (146 g/d) was the source of UIP. Six heifers were removed from the study due to calf loss unrelated to treatment; therefore, postpartum measurements are based on 19 animals per treatment. Statistical analyses using ANOVA and a split-plot design revealed no effects of treatment (P > 0.2) on BCS, BW, calf weight, milk production, or postpartum interval. There tended to be a treatment x time interaction on BCS (P < 0.09) with UIP heifers having higher BCS than Control at wk 5, 7, and 9 postpartum. There was a treatment x time interaction on serum IGF-I (P < 0.06) during the first 35 d postpartum. In UIP heifers, serum IGF-I was greater at calving compared with Control heifers (117.5 vs 92.4 ng/mL, respectively); however, these differences were not related to changes in BCS or BW. Although serum IGF-I concentrations were increased at calving in heifers receiving UIP, there were no treatment effects on postpartum interval (P > 0.7). During the first 30 d postpartum, IGF-I differed (P < 0.01) among heifers with postpartum intervals defined as short, < 50 d (128.9 ng/mL); medium, 51 to 65 d (115.2 ng/mL); and long, 66 to 130 d (52.9 ng/mL). When analyzed as a regression, a 1 ng/mL increase in IGF-I (UIP and Control heifers) at calving (P < 0.05) and throughout the postpartum period (P < 0.01) corresponded to a decrease in postpartum interval of 0.13 d. Based on the results of this study, the inclusion of UIP in diets for primiparous heifers and its effects on postpartum interval warrant further evaluation.  相似文献   
994.
To assess the effects of feeding high-oil corn on carcass characteristics and meat quality, 60 yearling steers were fed high concentrate diets containing either control corn (82% of diet), high-oil corn (82% of diet), or high-oil corn at a concentration that was isocaloric with the control diet (74% of diet). After being fed for 84 d, steers were slaughtered. At 72 h postmortem, carcass data were collected and rib sections from five steers grading U.S. Choice and five steers grading U.S. Select from each treatment were collected, vacuum packaged, and aged for 14 d. Three steaks (2.54 cm thick) were removed from each rib for Warner-Bratzler shear force measurement, sensory appraisal, and fatty acid composition analyses. Data were analyzed with treatment as the main effect for the carcass data and treatment, quality grade, and two-way interaction in the model for the longissimus data. The two-way interaction was nonsignificant (P > 0.05) for all variables tested. No differences were detected (P > 0.05) in carcass measurements except for marbling scores and quality grades, both of which were greater (P < 0.05) for carcasses from steers fed the high-oil corn. Overall, 78% of steers fed the high-oil corn graded U.S. Choice compared with 47% for the control and 67% for isocaloric group. Shear force and sensory properties of the longissimus were not different (P > 0.05) among treatments. Steaks from U.S. Choice carcasses rated higher (P < 0.05) for tenderness and tended to rate higher (P < 0.10) for juiciness. Feeding the isocaloric and high-oil diets increased (P < 0.05) linoleic acid, arachidonic acid, and the total PUFA content of lipid extracted from the longissimus. Saturated fatty acid percentage was lowest (P < 0.05) for high-oil corn and highest (P < 0.05) for control, with isocaloric being intermediate. Feeding high-oil corn increased (P < 0.05) pentadecyclic acid, margaric acid, and total odd-chain fatty acid content. Feeding high-oil corn in finishing beef cattle diets enhanced intramuscular lipid deposition and increased unsaturation of fatty acids of the longissimus.  相似文献   
995.
An experiment was conducted to determine the effect of dietary betaine (0, 0.125, 0.250, or 0.500%) on growth, carcass traits, pork quality, plasma metabolites, and tissue betaine concentrations of cross-bred finishing pigs. Four replications of three pigs (two barrows and one gilt) each were used for each treatment. The basal diet contained 0.85 (69 to 88 kg BW) or 0.65% Lys (88 to 115 kg BW). Overall ADG and gain:feed were not affected (P > 0.10) by betaine, but overall ADFI was decreased (quadratic, P < 0.05; 0 vs betaine, P < 0.01) by betaine; pigs fed 0.250% betaine had the lowest ADFI. Loin muscle area, average back-fat, dressing percentage, percentage lean, total fat, lean:fat, and leaf fat weight were not affected (P > 0.10) by betaine. Tenth-rib backfat thickness was decreased (quadratic, P < 0.05; 0 vs betaine, P < 0.05); pigs fed 0.250% betaine had the lowest 10th-rib backfat thickness. Carcass length was increased (linear, P < 0.05; 0 vs betaine, P < 0.10) as the level of betaine was increased. Fat-free lean, lean gain per day, ham weight, ham fat-free lean, and ham percentage lean were increased (quadratic, P < 0.10), but percentage fat, total ham fat, percentage ham fat, and butt-fat thickness were decreased (quadratic, P < 0.10); these traits were respectively highest or lowest in pigs fed 0.250% betaine. Thaw loss and 24-h pH were increased (quadratic, P < 0.10; 0 vs betaine, P < 0.05) and cook loss was decreased (linear, P < 0.05) in pigs fed betaine. The CIE L* value for the biceps femoris was decreased (quadratic, P < 0.10; 0 vs betaine, P < 0.10); pigs fed 0.250% betaine had the lowest CIE L* value. Subjective color, firmness-wetness, marbling, percentage moisture and bound water of the loin muscle, and shear force were not affected (P > 0.10) by betaine. Betaine was not detectable (< 0.07 mg/g) in the loin muscle of pigs fed 0% betaine, but betaine was detectable and relatively constant in pigs fed 0.125, 0.250, or 0.500% betaine (0.22, 0.17, and 0.21 mg/g, respectively). Plasma urea N, total protein, albumin, triglycerides, and HDL cholesterol concentrations were not affected (P > 0.10). Plasma total cholesterol (linear, P < 0.10) and NEFA (quadratic, P < 0.10) were increased in pigs fed betaine. Betaine improved carcass traits when provided at 0.250% of the diet and improved some aspects of pork quality.  相似文献   
996.
997.
Development of gilts that conceive early and continue to produce offspring is an objective of swine production. We investigated different patterns of growth on reproductive development and performance of gilts through the first farrowing. At 13 wk of age and 43 kg BW, 286 white crossbred gilts were penned individually and assigned to treatments: Ad lib, ad libitum intake from 13 to 25 wk of age; Control, ad libitum intake from 13 wk of age until 100 kg BW and then 90% of ad libitum intake until 25 wk of age; and Restricted, 74% of ad libitum intake from 13 wk to 25 wk of age. Feed was formulated to restrict energy intake. The study was replicated in three seasons. At 25 wk of age, gilts were moved by treatment to group pens, fed for ad libitum consumption, and estrus detection was initiated. Gilts were inseminated at first estrus, and those recycling were remated. Postmating gilts were fed 1.5x maintenance until 105 to 110 d of pregnancy. Gilts were moved either to the farrowing facility or the abattoir at 105 to 110 d of pregnancy. Those taken to the abattoir were slaughtered and number, weight, and condition of the fetuses were recorded. Gilts moved to the farrowing facility were allowed to farrow, and number, weight, and condition of the piglets were recorded. Daily feed intake during breeding was 3.4 kg/d by Restricted gilts, 2.9 by Control gilts, and 2.7 kg/d by Ad lib gilts. Increased feed intake by Restricted gilts during breeding resulted in compensatory gains that overcame the reduced reproductive performance that resulted from the reduced BW and backfat these gilts carried at the start of breeding. Days to first estrus and pregnancy were not influenced by development period treatment (P < 0.13). Percentage of Ad lib, Control, and Restricted gilts that successfully completed their pregnancies were 61, 74, and 66, respectively (P > 0.19). Total feed fed from 13 wk of age to end of the first pregnancy per gilt assigned did not differ among Ad lib (506 kg) and Control (498 kg) gilts but was less (P < 0.01) in Restricted gilts (451 kg). Number of piglets born per gilt assigned (P > 0.09) and piglets produced per kilogram of feed fed from 13 wk of age to term (P > 0.29) were 6.47 and 0.0134 in Ad lib gilts, 7.26 and 0.0150 in Control gilts, and 6.38 and 0.0149 in Restricted gilts, respectively. Moderate feed restriction, 74% of ad libitum intake, reduced feed consumed from 13 wk of age to end of the first pregnancy with no significant impact on efficiency of piglet production.  相似文献   
998.
Range ewes are commonly evaluated for milking ability by producers to determine the ewe's ability to rear lamb(s). The U.S. Sheep Experiment Station has subjectively scored (low, average, high) a ewe's milking ability within 24 h of lambing for many years. The relationship of subjective milk scores with lamb production was investigated using lambing records of Columbia (n = 1,731), Polypay (n = 1,129), Rambouillet (n = 1,704), and Targhee (n = 1,638) ewes. The incidence of high milk scores increased from less than 10% at first parity to 29 to 40% at second and greater parities. At maturity, Columbia ewes (38%) had the highest percentage of high milk scores. A positive association existed between ewe BW and her milk score at third and later parities. Ewes with high milk scores gave birth to heavier lambs (P < 0.05), whereas ewes with low milk scores were associated with lighter (P < 0.05) lambs at birth. Ewes with low milk scores weaned less (P < 0.05) total weight than ewes with better milk scores across all age groups for all breeds. Lighter weaned litter weights from ewes with low milk scores were linked to lighter birth weights and fewer weaned lambs. Differences for litter weight weaned between ewes with average and high milk scores were generally observed at 2 and 3 yr of age, when litter weights were heavier among ewes with high milk scores (P < 0.05) for all breeds. Between the ages of 1 and 3 yr, Columbia, Polypay, Rambouillet, and Targhee ewes with an average milk score weaned heavier (P < 0.05) litters (average differences of 10, 9, 13, and 12%, respectively) than ewes with low milk scores. For all breeds at all ages, individual lamb weaning weights were heavier (P < 0.05) when they were reared by ewes with high milk scores compared to lambs reared by ewes with low milk scores. Results suggest that milk score is an economically important trait in these four breeds and should be considered in management and breeding objectives; at a minimum, the incidence of low milk scores should be kept as small as possible.  相似文献   
999.
OBJECTIVE: To investigate the effects of long-term administration of recombinant bovine tumor necrosis factor-alpha (rbTNF) on plasma glucose and growth hormone concentrations, and to determine whether treatment with rbTNF causes insulin resistance in steers. ANIMALS: 5 steers treated with rbTNF and 5 steers treated with saline (0.9% NaCl) solution (control). PROCEDURES: In experiment 1, rbTNF (5.0 microg/kg of body weight) or saline solution (5 ml) was administered SC daily for 12 days. Blood samples were obtained before treatment, and plasma was harvested for determination of glucose, insulin, and growth hormone (GH) concentrations. In experiment 2, insulin, glucose, or growth hormone-releasing hormone (GHRH) was administered IV on days 7, 9, and 11, respectively, after initiation of rbTNF or saline treatment in experiment 1. Plasma glucose and insulin concentrations were measured before and at various times for 4 hours after insulin or glucose administration. Plasma GH concentrations were measured at various times for 3 hours after GHRH administration. RESULTS: In experiment 1, administration of rbTNF resulted in hyperinsulinemia without hypoglycemia and decreased plasma GH concentrations. In experiment 2, plasma glucose concentrations were higher in steers treated with rbTNF and insulin than in controls. Plasma GH concentrations were lower in steers treated with rbTNF and GHRH than in controls. CONCLUSIONS AND CLINICAL RELEVANCE: Prolonged treatment with rbTNF induced insulin resistance and inhibited GHRH-stimulated release of GH in steers. Results indicate that rbTNF is a proximal mediator of insulin resistance and inhibits release of GH during periods of endotoxemia or infection.  相似文献   
1000.
OBJECTIVE: To develop and validate an ex vivo model for study of adherence of Mannheimia haemolytica (formerly Pasteurella haemolytica) to respiratory tract mucosa of cattle and to use this model to confirm adherence of M haemolytica serovar 1 (Mh1) to several relevant respiratory mucosal surfaces. SAMPLE POPULATION: Excised nasal, nasopharyngeal, turbinate, and tonsillar mucosal tissue from the bovine upper respiratory tract. PROCEDURE: Mh1 was radiolabeled by use of tritiated leucine. Various concentrations of labeled bacteria were incubated with bovine upper respiratory tract tissues for various times. Tissue was washed to remove nonadherent bacteria, and percentage of bacteria adhered (percentage of adherence) was estimated using radioactivity. Using an optimal inoculum concentration and incubation time, percentage of Mh1 adherence was compared on nasal, nasopharyngeal, turbinate, and tonsillar mucosal tissue, and adherence to nasopharyngeal tissue was confirmed by scanning and transmission electron microscopy. RESULTS: The optimal Mh1 inoculum concentration was 1 X 10(7) colony forming units/ml and incubation time was 3 hours. Percentage of adherence of Mh1 to nasopharyngeal tissue was greater than adherence to other tissue types. CONCLUSIONS AND CLINICAL RELEVANCE: The ex vivo model maintained the functional and structural integrity of bovine upper respiratory tract mucosa, as confirmed by light and electron microscopy. Electron microscopy revealed participation of epithelial cell cilia and surface mucus in adherence of Mh1 to nasopharyngeal tissue. Adherence of Mh1 was confirmed in repeated assays, indicating that this organism adheres to upper respiratory tract mucosa of cattle.  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