The artist was Erik Krause, who did a number of other posters in this same propaganda style. The others carry slightly more socialistic messages, but similar artwork. This one reads, "Nurse the baby: Your protection against trouble. Inform yourself through the Health Bureau publications and consult your doctor."
Before 1930 most mothers breastfed their babies. Artificial formulas were expensive and still in development. Apparently some mothers thought it was more sophisticated to feed their babies from a bottle, so they would give their babies cow's milk in glass bottles. But this would've had to have been transported and refrigerated and couldn't have been convenient or affordable for average families.
According to this article (linked below), by the late 1930s, formulas had become safer and were viewed as more scientific by mothers who could afford them. And then it became a sign of affluence, or for immigrants - assimilation, and quickly became the norm.
I found some very interesting and relevant information about the history of breastfeeding in a preview of this book: http://books.google.com/books?id=hHbTDVfWNU4C&lpg=PP1&pg=PR17#v=onepage&q=&f=false
You can see this WPA poster in the Library of Congress here: