How I can keep my strenght bars without practice?
It is a little bit annoying for me make 20 lessons daily so could I keep them with full barswithout practice?
You can't. But if you feel like you know the lessons, you can ignore them not being gold. The vocabulary will appear in later lessons and if you get them right in later lessons it can make them gold again. For that reason you should always practice from the bottom up not with the strengthen skills button, that goes from the top down, you can turn more skills gold that way.
Here's a guide to get a golden tree that stays that way: https://www.duolingo.com/comment/6176795/Best-Way-to-Make-Your-Tree-Turn-Gold-and-Stay-Gold
The easiest way for me to keep my strength full is to do general strengthening exercises. Not going to the weak lessons, but just the general strengthening. I use both the computer and my phone for this. I have kept both my French and Reverse trees golden this way and am trying now to get all of my word strengths to full strength. When I click on the Words button, I can see that slowly, all my words are being strengthened. Best of luck to you.
The others above have all said no, but that's only true in the short term. Duo's strength algorithm takes into account how well you know a lesson and leaves it strong for longer each time you practice it.
Therefore if you drop to only doing 4 lessons per day, initially you will get 20 skills drop from gold, but eventually it will sort itself out and everything will be gold again. It will take some time, but will eventually happen.