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   189篇
  免费   27篇
林业   12篇
农学   10篇
基础科学   2篇
  38篇
综合类   7篇
农作物   16篇
水产渔业   38篇
畜牧兽医   81篇
园艺   3篇
植物保护   9篇
  2023年   2篇
  2022年   2篇
  2021年   8篇
  2020年   16篇
  2019年   7篇
  2018年   20篇
  2017年   7篇
  2016年   7篇
  2015年   9篇
  2014年   9篇
  2013年   7篇
  2012年   11篇
  2011年   12篇
  2010年   9篇
  2009年   8篇
  2008年   9篇
  2007年   11篇
  2006年   9篇
  2005年   11篇
  2004年   6篇
  2003年   2篇
  2002年   4篇
  2001年   4篇
  2000年   5篇
  1999年   2篇
  1998年   3篇
  1995年   3篇
  1993年   1篇
  1992年   1篇
  1991年   2篇
  1989年   1篇
  1988年   1篇
  1987年   1篇
  1985年   1篇
  1982年   2篇
  1973年   1篇
  1972年   1篇
  1945年   1篇
排序方式: 共有216条查询结果,搜索用时 0 毫秒
1.
2.
The effects of frame size (FS) and body condition score (BCS) on performance of Brahman cows were evaluated using records collected from 1984 to 1994 at the Subtropical Agricultural Research Station, Brooksville, Florida. Age at puberty (AP), calving rate (CR), calving date (CD), survival rate (SR), weaning rate (WR), birth weight (BWT), weaning weight (WWT), preweaning ADG, and kilograms of calf produced per cow exposed (PPC) were obtained from first- (n = 215), second- (n = 130), and third or greater-parity (n = 267) dams. Based on hip height at 18 mo of age, heifers were assigned to three FS groups: small (115 to 126 cm), medium (127 to 133 cm), or large (134 to 145 cm). Small and medium FS heifers attained puberty at younger (P<.05) ages (633.2+/-12.3 and 626.4+/-12.0 d) than large FS heifers (672.3+/-17.1 d). Calving rate in large FS second-parity dams was 27% less (P<.05) than in small and medium FS dams. In third or greater-parity dams, CR was greater (P<.05) for small FS cows than for medium and large FS cows. Across the three parity groups, CR improved with increasing BCS. Except for the first-parity dams, animals with better fall BCS calved earlier (P<.05). In first-parity dams, SR was less (P<.01) in large (47.9+/-11.0%) than in small (80.7 +/-5.2%) and medium (83.4+/-4.7%) FS groups. Weaning rates of large FS first- and second-parity dams were less (P<.05) than those of small and medium FS dams. Second-parity dams with BCS 3 had lower (P<.05) WR than dams with BCS 4 and 5. Within first- and third or greater-parity dams, BWT of calves born to small FS cows were the lightest, and those born to large FS dams were the heaviest; those born to medium FS dams were intermediate (P<.05). In second-parity dams, BWT of calves of large FS dams were greater (P<.05) than those of small and medium FS dams. In first-parity dams, calves weaned by small FS cows had lower (P<.05) WWT than those weaned by higher FS cows. In the third or greater-parity group, large FS dams weaned heavier calves (P<.05) than other dams. In all parity groups of dams, calves out of large FS cows had greater ADG (P<.05) than those from small and medium FS cows. In first-parity dams, PPC was comparable between small and medium FS dams, but both tended to be greater (P<.10) than PPC of large FS dams. Small and medium FS females reached puberty at an earlier age, calved earlier, and had greater calving, survival, and weaning rates, as well as greater kilograms of calf produced per cow exposed than the large FS females. As the large FS cows matured, they seemed to have overcome the negative effects imposed by FS that were observed at younger ages. Their performance traits were generally all comparable to those of smaller cows once they had reached maturity.  相似文献   
3.
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   
4.
The aim of the study was to evaluate the performance of Litopenaeus vannamei in an intensive photo‐heterotrophic hypersaline system with minimal seawater replacement, and establish relationships between parameters of a stochastic production model and relevant water quality variables. Six experimental 1000 m2 lined ponds were stocked at a density of 120 shrimp m?2 for a 105‐day trial. Salinity increased from 37 to 45 ± 2 g/L, and the water level was maintained with the weekly addition of filtered seawater, equivalent to 1.6% per day. The stochastic model predicted that, at harvest, there is 95% confidence that the system produces between 12.1 and 14.7 t/ha with a mean final individual weight of 13.1 g and a mean survival of 84.2%. Sensitivity analyses showed that dissolved oxygen and individual final weight of shrimp were the main variables influencing yield variance. Nitrogenous compounds were maintained between optimal cultivation levels (NH3–NH4+ = 0.73 ± 0.43 mg/L, N–NO2? = 0.09 ± 0.05 mg/L, N–NO3? = 3.22 ± 0.11 mg/L). Heterotrophic bacteria (6.6 ± 3.4 × 105 CFU/ml) and chlorophyll‐α concentration (108.5 ± 80.2 μg/L) showed a similar development pattern, indicating a strong relationship between bacteria and microalgae during cultivation. Vibrio spp. concentrations were low (1.24 ± 1.42 × 103 CFU/ml). It was shown that the photo‐heterotrophic system could be used in hypersaline conditions, typical of semi‐arid regions, to consistently produce between 12.1 and 14.7 t/ha in 15 weeks.  相似文献   
5.
The clinical and pathological aspects of a neurological disease observed in 16 horses in Pará, Amazonia, Brazil, are presented. The symptoms were mainly motor incoordination, paresis of the tongue, somnolence, difficulties in apprehension, chewing and swallowing of food, as well as instability and standing with abducted members. The clinical course was subacute or chronic and in most cases was not fatal. Postmortem examination performed in one already very sick, euthanized animal, did not show significant macroscopic lesions; histopathological examination revealed slight edema and degenerative alterations of a few axons, mainly in the medulla oblongata. In all pastures where horses were affected, plenty of bamboo had been eaten, probably because of scarcity of pasture. By feeding large amounts of fresh bamboo leaves of this region, in different growing stages, to three horses (horse 1, 47 g/kg/d for 30 days; horse 2, 10 g/kg/d for 60 days; horse 3, 18 g/kg on the first day, and 31 g/kg/d for 6 more days)—the animals ate the leaves unassisted—it was possible to reproduce nervous symptoms essentially identical to those observed in the natural disease 24 to 72 hours after the first feeding of the plant. In spite of continuous administration of the plant, intensity of the clinical signs did not increase. Based on field observations and comparison of the clinical and pathological pictures seen in the natural and experimental disease, the described illness can be concluded to be caused by the ingestion of large amounts of the leaves of Bambusa vulgaris f. vulgaris.  相似文献   
6.
Cyanobacteria are important for global nitrogen cycle and often form complex associations referred to as cyanobacterial mats or periphyton that are common in tropical, limestone-based wetlands. The objective of this study was to monitor the nitrogen fixation rate using the acetylene reduction assay of these cyanobacterial mats in a tropical, unfertilized, and protected wetland. To account for temporal and spatial variation of nitrogenase activity, we were interested in seasons in a hydrological cycle (dry, rains, and end of rains), sites with different vascular vegetation, and rates of nitrogenase activity in a 24-h cycle. The annual average of nitrogenase activity was 22 nmol C2H4 cm−2 h−1, with a range of <6 to 35 nmol C2H4 cm−2 h−1, and the annual nitrogen fixation rate of our study site (9.0 g N m−2 year−1) is higher than similar estimates from other freshwater wetlands. There was a clear temporal pattern in nitrogenase activity with a maximum rate occurring during the rainy season (August) and a maximum nitrogenase activity occurring between 0600 and 1200 hours. We found spatial differences in nitrogenase activity among the four sites that could be attributed to variations in species composition within the periphyton.  相似文献   
7.
This study investigated the acute toxicity (LC50-24 hr) effects of the essential oil of Cymbopogon citratus for adult Argulus sp. and Dolops discoidalis, before and during oviposition. In vitro acute toxicity (LC50-24 hr) was tested using 20, 40, 60, 80, 100, 120, 140 and 160 μg/L of C. citratus essential oil, and two control groups (one with cultivation tank water and one with cultivation tank water + alcohol) were used. Specimens of Argulus sp. and D. discoidalis submitted to acute toxicity were evaluated using histological procedures. The major chemical compounds of C. citratus essential oil were geranial (47.5%), neral (35.6%) and myrcene (6.7%). The LC50-24 hr for Argulus sp. adults was 67.97 μg/L, while for D. discoidalis it was 59.55 µg/L. In the oviposition of both species of argulids, maximum mortality began with treatments of 140 μg/L, while the LC50-24 hr for Argulus sp. and D. discoidalis was 83.98 μg/L and 82.48 μg/L, respectively. In both argulid species exposed to C. citratus essential oil, morphological alterations were observed only in the eyes, and they occurred in the ommatidium and rhabdomeres and were dependent on the concentration of C. citratus essential oil and the parasite species.  相似文献   
8.
Lippia alba hydrolate (LAH) is produced as a secondary product in essential oil extraction. The objective of this study was to evaluate the use of LAH in a simulation of the transport of tambaqui (Colossoma macropomum) juveniles. A series of 17 hr simulations were performed using load densities of 30, 60 and 90 juveniles per L and four LAH concentrations of 0.4%, 1%, 2% and 5%, with a control of just water. Fish survival, temperature, dissolved oxygen, conductivity, pH, ammonia excretion, alkalinity water hardness and net Na+, K+, Cl fluxes, were all evaluated before and after the transport simulations. Mucous cell densities in the gills were also determined microscopically, post simulation. The results showed that addition of 5% LAH achieved better survival during the simulation than the water control. Dissolved oxygen was significantly lower with an increased load density. Also, conductivity, NH4 excretion and net ionic fluxes (K+ and Cl?) were lower with 5% LAH, although mainly where higher load densities were tested. Mucous cell density was lower in LAH concentrations of 2% and 5%, again with higher load densities. In view of these findings, it is suggested that 5% LAH is an effective sedative for use in the transport of tambaqui, mainly with high load densities. This work is the first study of LAH and highlights its potential applications in aquaculture management.  相似文献   
9.
Edible coatings based on high molecular weight chitosan, pure or combined with methylcellulose or oleic acid, were applied to fresh-cut carrots cv. Nantesa by simple immersion and by applying a vacuum pulse (5 kPa for 4 min). Water vapour resistance, colour, mechanical properties and respiration rates of non-coated and coated samples were determined. Coatings improved sample appearance, since they diminished the occurrence of the white blush during storage. When applied by simple immersion, they neither conferred significant barrier properties nor the preservation of the mechanical properties of fresh-cut carrot samples. In contrast, coating application with a vacuum pulse enhanced all the positive effects, since the resistance of water vapour transmission of the samples was significantly improved, and better preservation of the sample colour and mechanical response during cold storage was obtained. Differences in film composition did not significantly affect the coating behaviour, probably due to the variability induced by different factors when coatings were applied to the carrot surface.  相似文献   
10.
In this paper we report the results of a study on the suitability of the dehesa (Mediterranean woodlands) as habitat of red deer (Cervus elaphus hispanicus) in a closed country estate of about 4300 ha total surface situated in the San Pedro Mountain, in Central Extremadura, Spain. We have described the deer management with respect to habitat, exploitation mode, deer feeding, population control and production of goods and services. It is concluded that the dehesa system is a rich natural resource that preserves and improves the degraded environment.
Resumen En el presente trabajo se estudia la dehesa como hábitat del ciervo (Cervus elaphus hispanicus). Se ha tomado como referencia una finca cerrada de 4300 ha de ST situada en la Sierra de San Pedro, en Extremadura, España. Se describen los sistemas de gestión actuales del ciervo en cuanto a hábitat, modo de explotación, alimentación, control de población y producción de bienes y servicios. Concluyendo con la necesidad de fomentar este tipo de explotaciones cinegéticas de dehesa que ayudan a conservar y potenciar el medio ambiente en zonas despobladas y deprimidas

Abbreviations CS collected surface - DMFCU deedmixed feed cattle units - Fi final inventory - FS fertilized surface - Ii Initial inventory - K Potassium - N Nitrogen - NBF number of breeding females - NBFIi number of breeding females of initial inventory - NBFR number of breeding females released - NBM number of breeding males - NBMIi number of breeding males of initial inventory - NBMR number of breeding males released - NCCR number of captive calves released - NCFi number of calves of final inventory - NCR number of calves released - NFR number of female renovation - NYMR number of young males released - NYFR number of young females released - P phosphorus - PP permanent Pastures - pts pesetas - QS Quercus stalks - RDED resources directly eaten by deer - SPOM selected hunted down poor quality males - SS sowed surface - TN total needs of deer - TNR total number released - UAS useable agrarian surface - WB wooded brush - WQS wooded Quercus surface - WS wooded surface  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