Anything that you put into your body, wheatgrass or any other kind of juice breaks your water fast. I don't know how many people drink juice, but from what I've read on this forum, if you drink anything other than water it is called a diet and not a fast. The purpose of drinking water only is that your digestive system doesn't have to work digesting food, it is able to rest and start repairing itself. Also, when you put something into your system other than water it triggers not only your hunger again but it starts up your digestive process again, which stops the cleansing of your body, even with just drinking wheatgrass or fruit juice.
This is what Chrisb1 says in many of his posts, paraphrased (Chris, please correct me if I'm wrong): Taking anything apart from water according to thirst on a fast is in fact dieting, or what some of the most experienced fasters amongst us here here call a starvation diet. The benefits of fasting are severely reduced by taking any nourishment from outside, rather than living exclusively on food reserves from within via ketosis; full ketosis (and therefore full healing and cleansing) is not achieved if you consume anything other than water. Water only though has been described as "The Ultimate Diet" for weightloss, healing & cleansing.
I hope this helps you until Chris sees this post and responds. He really is the expert on fasting in this forum and could answer such questions far better than I. Also Fonty is very knowledgeable. It has helped me to no end to read previous posts on this forum, going back several pages, you would learn a lot.
Good luck, this a learning journey for me as well as it is for you. I'm so glad that I found this forum. Stay strong.