Well, they say they've done it.
For years researchers in China, with the backing of a "major biotechnology company" have been working to genetically modify cows to produce human milk, and The Telegraph says that they've done it:
The scientists have successfully introduced human genes into 300 dairy cows to produce milk with the same properties as human breast milk.
The scientists behind the research believe milk from herds of genetically modified cows could provide an alternative to human breast milk and formula milk for babies, which is often criticised as being an inferior substitute.
They hope genetically modified dairy products from herds of similar cows could be sold in supermarkets. The research has the backing of a major biotechnology company.
You already know that a cow can't replace you, but I thought I'd make it clear that, no matter how much effort is expended, no matter how much money is invested, science will never, ever be able to create a cow that can produce breastmilk or confer the health effects of breastfeeding.
1) Not even close to a moving target. The article says that this genetically modified (GM) cow's milk contains a few of the components of human milk. They're important ones - lysosyme, lactoferrin, and alpha-lactalbumin - but human milk contains over 200 components, and some constituents haven't even been identified yet. And of the ones that have been named, research is still ongoing into the role they play in the development of human babies. It's only in the last few years, for example, that research has attributed a major protective function to glycans, a significant component of human milk that was once thought to be without purpose. So, these cows are producing a milk that is a far cry from human milk, and since we're just beginning to understand how human milk works in the first place, this research is chasing a moving target.
2) Your milk is you. Even if the scientists working on this concept created a milk that was very close to human milk, it could never match the milk you make. Why?
Because the milk you make for your baby contains antibodies against pathogens you have been exposed to. When you get exposed to Virus A at the supermarket, you make milk that contains antibodies against Virus A, which you give to your baby. Same with bacteria. So unless these cows are built to be carried around with you so they can get exposed to viruses and bacteria in your daily life, they won't make antibodies against those germs.
Because human milk constantly changes to meet the nutritional needs of your baby. The milk you make on day 1 of your baby's life is different than on day 10 or day 100. The milk you make at 8 am is different than the milk you make at 8 pm, and the milk your baby gets at the beginning of a feeding is different than the milk he gets at the end. And have you ever pumped milk after not feeding for a while and seen how thin the layer of cream is after it separates? That's because your body knows that it's been a long time in between feedings and knows that the first priority is to hydrate your baby - hence the higher water content. If you feed a baby frequently your body says, "this baby's hungry!" and makes a higher fat milk. Can a bottle of GM cow's milk do that?
Because there are other ways in which a cow's milk can never match human milk: Human milk contains oligosaccharides which are specific to the mother's blood type. The flavors of your milk reflect the flavors of your diet. The milk you make for your newborn is tailored to the gestational age of your baby. Your milk matches more than 50% of your baby's genetic material. And don't forget that your milk is a "live" fluid, containing millions of live cells carrying out their respective duties in your baby's system. This list could go on and on.
3) It's not all about the milk. While breastmilk is certainly magical, some of the power of breastfeeding doesn't derive from the milk itself, but from the act of breastfeeding. There are many examples of this (one I like is that breastfeeding creates a normal palate, resulting in less dental problems), but a nice way to show this is with some of the obesity research. Breastfeeding, you probably know, is associated with lowered risk of obesity. One study found that, when comparing breastfed, formula fed, and breastmilk-by-bottle-fed babies, the formula fed babies and the breastmilk-by-bottle fed babies were at elevated risk. Why? Possibly because it's not the milk but the method of feeding: breastfed babies control their intake, bottle fed babies' intake is controlled by the person feeding them.
Of course, there are many other issues with the concept of genetically altering cows to produce "human" milk, from animal welfare to the safety of consuming GM foods. But since the big money is on the ability to sell this milk as a substitute for breastfeeding, I think it's worth showing that this proposition is a categorical mistake and truly a fool's errand.