I agree a even a pot smoking mom has better milk than any formula....

"Human milk expressed by mothers who have been lactating for >1 year has significantly increased fat and energy contents, compared with milk expressed by women who have been lactating for shorter periods. During prolonged lactation, the fat energy contribution of breast milk to the infant diet might be significant."
-- Mandel 2005

"Breast milk continues to provide substantial amounts of key nutrients well beyond the first year of life, especially protein, fat, and most vitamins."
-- Dewey 2001

In the second year (12-23 months), 448 mL of breastmilk provides:
29% of energy requirements
43% of protein requirements
36% of calcium requirements
75% of vitamin A requirements
76% of folate requirements
94% of vitamin B12 requirements
60% of vitamin C requirements
-- Dewey 2001

Nursing toddlers between the ages of 16 and 30 months have been found to have fewer illnesses and illnesses of shorter duration than their non-nursing peers (Gulick 1986).

"Antibodies are abundant in human milk throughout lactation" (Nutrition During Lactation 1991; p. 134). In fact, some of the immune factors in breastmilk increase in concentration during the second year and also during the weaning process. (Goldman 1983, Goldman & Goldblum 1983, Institute of Medicine 1991).

Per the World Health Organization, "a modest increase in breastfeeding rates could prevent up to 10% of all deaths of children under five: Breastfeeding plays an essential and sometimes underestimated role in the treatment and prevention of childhood illness."

Extensive research on the relationship between cognitive achievement (IQ scores, grades in school) and breastfeeding has shown the greatest gains for those children breastfed the longest.

A US Surgeon General has stated that it is a lucky baby who continues to nurse until age two. (Novello 1990)

The World Health Organization emphasizes the importance of nursing up to two years of age or beyond (WHO 1993, WHO 2002).

Extended nursing delays the return of fertility in some women by suppressing ovulation (References).

Breastfeeding reduces the risk of breast cancer For the Mother. Studies have found a significant inverse association between duration of lactation and breast cancer risk.

Breastfeeding reduces the risk of ovarian cancer for the mom(References).

Breastfeeding reduces the risk of uterine cancer for the mother(References).

Breastfeeding reduces the risk of endometrial cancer for the mother(References).

FAT CONTENT OF INFANT/TODDLER FOOD
Calories (per ounce) / Fat (per ounce)
human milk 22 (average)â?*/ 1.2 (average)â?*
infant formula 20 / 1.06
cow milk (whole) 19 / 1.00

Refrences
KellyMom
LLLI