收费全文 | 4924篇 |
免费 | 346篇 |
国内免费 | 2篇 |
林业 | 322篇 |
农学 | 132篇 |
基础科学 | 38篇 |
867篇 | |
综合类 | 889篇 |
农作物 | 227篇 |
水产渔业 | 246篇 |
畜牧兽医 | 2078篇 |
园艺 | 119篇 |
植物保护 | 354篇 |
2023年 | 34篇 |
2022年 | 38篇 |
2021年 | 78篇 |
2020年 | 88篇 |
2019年 | 92篇 |
2018年 | 112篇 |
2017年 | 106篇 |
2016年 | 121篇 |
2015年 | 89篇 |
2014年 | 129篇 |
2013年 | 215篇 |
2012年 | 248篇 |
2011年 | 309篇 |
2010年 | 200篇 |
2009年 | 161篇 |
2008年 | 282篇 |
2007年 | 259篇 |
2006年 | 294篇 |
2005年 | 271篇 |
2004年 | 204篇 |
2003年 | 241篇 |
2002年 | 200篇 |
2001年 | 75篇 |
2000年 | 82篇 |
1999年 | 90篇 |
1998年 | 45篇 |
1997年 | 41篇 |
1996年 | 43篇 |
1995年 | 35篇 |
1994年 | 30篇 |
1993年 | 38篇 |
1992年 | 40篇 |
1991年 | 55篇 |
1990年 | 50篇 |
1989年 | 48篇 |
1988年 | 36篇 |
1987年 | 62篇 |
1986年 | 43篇 |
1985年 | 61篇 |
1984年 | 40篇 |
1983年 | 48篇 |
1982年 | 29篇 |
1981年 | 27篇 |
1980年 | 25篇 |
1979年 | 36篇 |
1978年 | 27篇 |
1976年 | 26篇 |
1973年 | 27篇 |
1970年 | 25篇 |
1969年 | 31篇 |
2. In this experiment, carcase analyses of each of three breeds of pullets were conducted at weekly intervals throughout the growth of the pullets, to 18 weeks of age. Measurements were made of body weight, gut‐fill and feather weight, and chemical analyses consisted of water, protein, lipid and ash measurements of both the body and the feathers. Each age group comprised 10 birds of each breed.
3. Gompertz functions accurately estimated the growth of both body protein and feather protein, to 18 weeks of age, from which the rate of growth of these two components of the body could be estimated. The mature weight of pullets was overestimated by the Gompertz growth curve, which may indicate that a pullet ceases to increase in body protein content once sexual maturity has been reached.
4. Using allometric relationships between the chemical components of the body and of feathers, all the components of growth could be estimated from the growth of body protein and feather protein. These components were then added together to determine the growth rate of the body as a whole.
5. The daily amino acid requirements for 4 functions were calculated, namely, those for the maintenance of body protein and feather protein, and for the gain in body protein and feather protein. These requirements were then summed to determine the requirement of pullets on each day of the growing period.
6. Using the ‘effective energy’ system, the amount of energy required by these pullets was calculated for each day of the growing period, from which the desired daily food intake of the pullets could be predicted. By dividing the amino acid requirement by this daily food intake it was possible to determine the concentration of amino acids that would be needed in the diet in order to meet the requirements of a pullet.
7. The results indicate that the ratio between the requirement for lysine and for methionine and cysteine changes dramatically during the growing period, negating the concept of a fixed ratio between all the amino acids during growth.
8. The above process is the first step in determining the optimal feeding programme for a population of pullets of a given genotype. The constraining effects, of the diet being offered and of the environment in which the pullets are housed, on the food intake and growth rate of each pullet have to be estimated, and such a theory can then be expanded to include all the individuals in the population. Only by the use: of simulation models can all these constraining effects be considered simultaneously. 相似文献
2. Age at first egg, body‐weight gain and egg production were affected by energy allowance. Birds on the lower energy allowances came into lay later than birds on the higher energy allowances and at a lower body weight.
3. Body‐weight gain decreased with decreasing energy allowance. The decrease in egg output in response to decreasing energy allowance resulted from more birds ceasing to lay and fewer birds laying on more than 3 d per week. Similar changes in the distribution of rates of lay were observed on each treatment as the flock aged.
4. The relationship between body‐weight gain and egg number on each treatment was negative from 21 to 36 weeks, but became less consistent with age.
5. Protein intake had little effect on body weight. At the lowest energy allowance, egg number and egg weight decreased with increasing protein allowance. This effect was not observed on the higher energy allowances. 相似文献
2. Body‐weight gain and carcass fat and water content increased and fertility decreased with increasing energy allowance. Maximum egg production occurred at an energy intake of 1.73 MJ AME/bird d.
3. Differences in egg weight and hatchability were related to differences in both energy and protein intake. The highest egg weight occurred at the highest allowance of energy and protein. Hatchability was depressed where the daily allowances of protein and energy were in a ratio of more than 15 g protein: 1 MJ AME.
4. Apart from egg size no significant effects on reproductive performance were observed when dietary protein intake was varied from 27 to 19.5 g/bird d.
5. Requirements of broiler breeder hens for protein during lay may be lower than previously thought. For the strain used a protein intake of 19.5 g/bird d appeared adequate provided essential amino acid concentrations were maintained.
6. The close relationships between body weight and energy allowance and the latter and egg production make body‐weight gain a useful guide to management. A body‐weight gain of about 1.1 kg from 21 to 36 weeks of age was associated with optimum performance in this study. 相似文献