Why have you trippled the prices for the same product?

In case you haven’t noticed, everything is higher. It’s called inflation and happens when you vote in libtards.

Will this kill bamboo? Anyone tried?

Cut the stalk leaving enough of the hollow portion to pour in 1Tablespoon of the 50% concentrate undiluted. This needs to be done as you cut each stalk (according to Mississippi C. Extension Office within 10 seconds the absorption ceases) I add a little more for larger diameter stalks and little lesser amount for smaller ones. I'm not quick enough for the ten second rule when cutting with a chain saw and stopping to do the application but the alternative to digging up the root ball and root system is labor intensive that requires allot of strength, especially if you have several acres of this nightmare! Spray full strength on any exposed root runners that you may see. Don't expect that this will kill all the rhizomes. A regrowth will occur but a at much smaller size like a small bush. I've been successful in killing these with a much lower concentrate but spray all the leaves, top included. Again, you may have to go back and spray as second maybe a third as another regrowth starts over again. Spring or Summer is best time, but anything you do to harm the plant is better than waiting. I've noticed the chemical absorption does not happen within few minutes, sometimes takes several days for osmosis to occur. Since it rains here almost every day and not wanting the chemical to dilute, I cover the stalks. Be patient, it may take more than seven days for signs of the bushy regrowth to start turning brown and maybe two weeks to die completely. If you can't cut all your bamboo stand, at least step on and damage new larger stalk growth when flushing in the spring. They are easily pulled up or killed when at this stage of growth and is one less stalk to kill later and thins out the stand for the future eradication as some of the older growth dies as well. A sharp pick/hoe is useful to damage the smaller roots that lay just below the surface (Anything that can damage the plant is your friend, just don't use a nuclear bomb)

will this kill thistle weed

Yes and very effectively! From the label: Artichoke Thistle and Russian Thistle use a 1.5% (2oz per gallon) concentration. I'm guessing you have the most common species which is Russian Thistle as do I and I've used Roundup to kill it for years with great success. In fact after two seasons of Roundup use its been eliminated (no recurrence) from all my open areas. If you have thistles growing amongst your desirable plants I suggest you either pull them or cut off the flowering heads before they release their "seed". If you do this then treat the area with a pre-emergent like Surflan-AS you will eliminate recurrence within a season or two.

I have a 4 gal backpack sprayer. how much round up do I need to mix to kill grass? I tried a oz per gallon buy no luck.

I also have a 4 gallon backpack sprayer. I have been using my backpack sprayer with this product every 4 to 6 weeks for about 2 years now. I only add 8oz to my backpack sprayer (2oz per gallon) and I get great results.