I only give treats when the horse does what is asked and never any other time. That way a treat stays as a reward for positive behaviours and is not "expected".
The positive behaviour you want from your mare is for her to come to you. Start carrying around apples, carrots or whatever her favourite treat is in your pocket at all times for the next little while. [I find anything with molasses works well] Let her nuzzle curiously at your pockets but do not give her any treats instantly.
Talk in low soothing, quiet tones while stroking her face and neck in circular motions as she nuzzles at the treats. When she has stood calm and patiently for a few minutes during this process, give her a single treat, tell her she's a good girl in an exagerated manner and simple walk away completely.
Start this process in the barn at turnout and turnin until she is familar with the routine. Don't give her treats everytime though. Once in awhile just focus on rubbing her neck and face while talking to her in low tones. That way she doesn't "expect" a treat everytime you are in the barn.
Once you have established this routine, start the second step in the process by spending time in the horses' pasture. Do not make any attempts to catch the horses while in the pasture; actually ignore them if you can by reading a book or doing some similar activity but make sure your pockets have the faviourite treats in them.
Let them settle into grazing and find a spot close to them to sit down. Eventually her nose will tell her that your pockets have a desired treat and she will approach you.
Only give the treat after she has nuzzled your pockets and stood calmly while letting you stroke and talk to her for a few minutes. Then walk away to another section of the pasture and sit down. She will follow and you can reward the behaviour by repeating the process.
Follow this process the same way you have done in the barn. After a period of time you can introduce the halter but do not attempt to put it on. Simply leave it on the ground so she sees it and becomes accustomed to its presence.
Eventually you can work up to having it draped over your shoulder while stroking her and giving her a treat. Always remember to walk away after she has received a treat and only give another treat after she has followed you to another section of the pasture, nuzzled your pocket and stood calmly.
As time passes, start stroking her with the halter while talking to her. Then walk away. Repeat after she follows you.
The next and final step is to slip the halter on while you're stroking and rewarding her with treats. Then walk away and wait for her to follow. After she has done that, remove the halter, reward and walk away.
This process is going to take some time and patience. If you're having a bad day, skip it and start fresh the next.
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