Connecting Gate to Plate Blog

Invited Guest Blogger

Cows need nutritionists? Don’t they just eat grass?


~ guest blog post by Robin Rastani, Ph.D

This is the response that I frequently get from family members and fellow travelers in airports, when they ask about my work. Many people still think that dairy cows consume grass and grass alone. They have that idyllic image of black and white cows out in a green pasture next to a red barn. While some cows can sustain many of their needs on grass alone, they are usually the non-lactating cows (i.e., cows that aren’t producing milk). A lactating dairy cow has a high metabolism, and is very similar to a marathon runner or high performance athlete.

Dietitians for Milk Cows

A modern dairy cow consuming grass alone would be equivalent to a marathon runner or Olympic athlete consuming only lettuce with a few sprigs of broccoli. In the old days, everyone had a couple cows, and they only needed to make enough milk for their family. The modern dairy cow now makes about 10 gallons of milk every day. On grass alone, a modern average producing lactating dairy cow would eventually lose tremendous amounts of weight and be unhealthy. As a dairy cow nutritionist, I make sure cows have all the needed nutrients to perform and remain healthy while producing healthy nutritious milk.

A typical dairy cow’s diet consists of around  about 50% forage and 50% grains. Most of the forages are plant material that is fed as hay or fermented forage, known as silage. This allows farmers to feed grass, legume and corn-based forages year round. The most common concentrates fed are corn and soybeans, along with by-product feeds like whole cottonseeds, citrus pulp, almond hulls or soy hulls. Cows enjoy variety in their diets, and having a mix of both forage and concentrates allows this. Just like with human nutrition, we must provide the correct amounts and balance of nutrients.

Cows are different from humans in that:

  1. Cows have a four compartment stomach with a large fermentation vat. This fermentation vat is known as the rumen. In the rumen, bacteria help to digest the feed. This allows cows to obtain nutrition from feedstuffs that contain cellulose and fibrous material that humans and other animals cannot. This is one reason why cows can consume many by-product feeds.
  2. Cows are limited in the selection of feed that is offered to them. Nutritionists formulate their diet, and it’s offered to them in one mixed up casserole, called a total mixed ration or TMR. However, cows can be picky, and they will try to sort through the feed offered to them. Just like humans, they prefer some feedstuffs to others.
  3. Cows also have the ability to ruminate. They eat their meals rather quickly, and then while resting they will further digest the feed that was consumed. They regurgitate a ball of feed, known as a cud, and then they chew on that cud. This allows them to break up the feed into smaller particles. It also produces saliva, which helps to keep the material in the rumen from becoming too acidic and cause indigestion.

As a cow nutritionist, I do have some advantages that I’m sure dietitians would appreciate. I can check diets based on records of what’s been consumed, chemically analyze that diet, and modify it accordingly… and my clients (the cows) will usually accept my recommendations. Frankly, cows eat better diets than humans!

There are many similarities and a few key differences between the nutritional demands of a cow and a human, as you can see. Cow nutritionists have a common goal with human dietitians – providing a healthy, balanced diet within a budget for our clients.

Nutrionist on corn vs grass

While holding a Ph.D in nutrition, Robin enjoys interacting with consumers about practices on dairy farms, as well as the benefits of dairy products.

If you’re a farmer or agricultural professional, this knowledge may see mundane or commonplace. However, to many of your neighbors and consumers of your products, it is new and intriguing to them. How can you explain the science of food production to them?

Robin R. Rastani, Ph.D., is a Dairy Technical Manager for NOVUS International, Inc. She works with dairy producers and nutritionists to provide optimal nutrition for dairy cows to ensure the cows are healthy and produce a great product for consumers. Feel free to follow her on Twitter (@cownutritionist).


  1. Robin on March 13, 2011 at 2:22 am

    This question appeared on my Facebook page, and it may be useful to dairy farmers that graze their cows on pasture, so I’m reposting it here.

    Q posed- Robin this blog was great for the public! The Collins cows have always been supplemented with pasture in the summer but John gets really upset over the big drops in milk. Jack agrees they slug feed, mavis insists on grass, yet they all agree it is helpful to get them off the concrete. Any suggestions? Have you seen herds that accomplish it successfully?

    Response- It’s a tough balance. I think you have to balance the milk production, saving in feed costs, added labor, and improved lameness status by getting them off the concrete. It’s very hard to estimate feed intakes on pasture too. I’ve seen some herds come to a happy medium where they keep the cows inside and feed them a TMR during the daytime hours, or for a couple hours after milking, and then supplement with the pasture.

    I have a colleague in WI that keeps Fresh & High cows closest to the barn on the lushest pasture. This allows them the most time on feed and least amount to walk…. and the heifers and dry cows are kept in the pastures farthest from the barn, as they don’t need to come to the barn for milkings. I’d be happy to put her in touch with you, if you want to discuss options. She’s a nutritionist and has a herd of about 250 Jerseys.

  2. Emily on March 14, 2011 at 6:38 pm

    Great post. As an organic, pasture based dairy, people often assume that is ALL we feed our cattle. Instead, we use pasture/grass/hay for about 60%-70% of their dry matter in take and then supplement with corn and/or barley silage. Even when they are a on pasture in the warm months they get a cow “salad” twice a day (though much less than winter feeding).
    Could we go all grass and pasture? Maybe, but in MN when the weather in cold and extra calories are needed grass is not enough. Also, in a perfect world: soil nutrients would be perfect in turn making pasture nutrients perfect. Since we don’t live in a perfect world our supplemental feeding is like taking a multi-vitamin to cover those nutrients that are not provided in the grasses.
    Robin, we wish you lived closer, so we could pick your brain more often (not just on Twitter) 🙂

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