When an infant is involved in a custody dispute and the mother wants to limit the father's time with the child, she or her attorney may will trot out the old "breastfeeding argument".
Claiming that an infant "can't be away from the mother" for any length of time because of the need to breastfeed the child is not a valid reason to limit or deny contact with the father. The mother or her attorney may also say that the child won't drink formula and/or should not be fed formula. This is a classic way to severely limit or even eliminate a father's contact with the child, and it often works. Here, however, is one way to counter this kind of bogus argument.
If your ex tries to use the "breastfeeding argument" to limit your access to your child, submit to the court a declaration of your willingness to purchase her the breast pump of her choice so that she can send along the dairy product of her choice for when the child is in your custody. If the mother is denying the father parenting time and claiming that it is due to the need to breastfeed the child, she is implicitly agreeing to pump if the court finds that visitation with dad is in the best interest of the child.
If she resists or refuses to provide pumped milk and objects to the use of formula, it becomes obvious that she is denying the father parenting time for reasons other than what she claims. If it is truly important to the mother that the child receive breast milk, she should be willing to pump- after all, her duty to the child shouldn't change simply because it is going to go spend some time with the father.
You might also consider submitting video tapes of your child happily eating prepared baby food to contest and disprove her almost certain assertion that the child has "only been breast fed from birth" or that he or she "won't eat commercial baby food".
Most judges will see this "argument" for what it really is- a phony excuse to limit the father's access to the child. At this time you must be polite but firm. State that you desire to spend time with your child and that you are every bit as capable of taking care of your child as is the mother. If she balks, refer to the video tapes as proof that the child can and will eat food(s) other than breast milk, and reiterate your offer to purchase whichever breast pump she prefers.