Just because they're technically from the same plant does not mean they're identical. Just as eating an apple isn't the same thing as eating an apple leaf or the bark of an apple tree, grass and grains have different nutritional values, and affect the quality and flavour of the beef. It's not just marketing.

>Is there a scientific explanation for why the two are separated in lifestock feed advertising or is it just marketing?

Because they're wildly different in appearance and nutritional content

Grass and Hay are both very bulky but not very nutrient rich, that's why cows have 4 stomachs to help them break down grass and get the nutrients

Grain is wayyy denser in nutrients. A pound of grass hay has about 900 calories for the cow but a pound of feed corn has about 1500 calories, and corn is about 3x denser than hay so the same volume of corn contains about 5x the amount of calories making it a lot easier to fatten up a cow off grains like corn than just grass

The cow is going to eat until its stomach feels full. If you're feeding it grain then its going to eat a lot more calories and pack on a lot more fat than if you were just feeding it low density hay

My family has been ranching for four generations.

Food and agriculture is filled is myths, half truths, and deceptive marketing. For example, the vast majority of foods labeled as "GMO free" don't even have GMO variants. Here is the list of available bioengineered foods in America: https://www.ams.usda.gov/rules-regulations/be/bioengineered-foods-list

When it comes to steers, every steer is grass fed. The majority of a steer's life is spent grazing on grass or munching on hay (grass) during the winter months. The difference is how they are finished. Normally, steers are sent to a feedlot for the last portion of their lives and are fed grain of some sort along with hay and things like beets. The high energy concentration in grain allows them to both pack on the pounds and marble their meat. Marbling is what makes a steak taste good.

Generally the difference is going to be: grain finished beef will taste milder while grass finished beef will taste gamier due to how they marble.

Often, the supposed draw of grass finished beef is it avoids feedlots and "factory" farming. But this is not true. A feedlot can feed steers grass pellets and voila the beef is labeled "grass fed". Here is a helpful source that details the various kinds of beef and the processes behind the labels:

https://www.beefitswhatsfordinner.com/cuts/grass-vs-grain

There are mild nutritional differences but it's not right to say one is "healthier" than the other. Ironically enough, grain finished cattle have a lower environmental impact. So, as I said, the only real, meaningful difference for the consumer is the taste.

https://hagancattleco.com/blogs/news/which-beef-is-better-grass-fed-or-grass-fed-gran-finished

When people talk about grain fed, I think they really mean corn fed. There is a difference in grass fed vs corn fed. Corn fed is actually less healthy for the cow because they are not supposed to eat corn, they can have bacteria in their body and that bacteria gets on the meat during slaughter/butchering. This is why some livestock gets antibiotics, to counteract the bacteria. So they say grass fed is better, but there is a change in the way fat is stored / distributed in the body when they eat corn vs grass, and this has an impact on how the food tastes / the cut of meat.

Grain contains starch, protein and a little oil. Grass contains mostly cellulose and a mix of many substances like for example sugars. They are indeed different from another.

Grass is primarily cellulose. It's difficult for most animals to break down into usable nutrients, but ruminants, like cows, have multiple stomachs that act as fermentors/bioreactors, where specialized bacteria break down cellulose in a way that provides nutrition for the cow.

Grain is primarily carbohydrate and protein, which is why it's sought after by animals that lack ruminant guts.

When cows are fed on primarily grass, it means a) they're usually out wandering around a pasture and b) they're eating their typical diet. This tends to produce lean meat and mean that the cows weren't primarily crammed in a feedlot.

When cows are fed on primarily grain, the energy-density of their diet is much higher, and it takes much less effort for them to break down, so they tend to put on more fat. "Grain-fed" typically means they were raised on a mixed diet and "finished" on grain (fed a primarily grain-based, energy-dense diet toward the end of their lives), which results in fattier, marbled cuts of meat that may be more flavorful/tender.

Grass fed typically means they are grazing I pastures, or being fed hay or silage. Grain fed means they are eating corn and stuff from troughs. The reason they are separated is because the fat content and flavor of the beef will be very different between the too. Grass fed beef will be leaner and earthier tasting. Grain fed will be fattier, and less earthy/minerals.

You will also see stuff like Grass fed, grain finished. Where they spend time in pastures, but then spend the last few months eating grains and other stuff. This builds a healthy beef but then cleans out the earthy flavor, which some don't like.

Just marketing , grass fed animals can walk , run ? Roll around , gallop if they want - all of which improves muscle mass and tone but this takes land and good pasture which can be expensive , grain fed or the other hand - cattle are kept in feed lots with minimal room to walk , run , in general exercise so muscles atrophy become flabby and cattle just put on fat which you pay for in the weight , you think you are buying tasty meat but end up getting flabby loose meat and lots of fat. Fat can be tasty if it’s built on varied muscle and variety of grasses but corn just makes a yellowish tasteless fat Feed lots are good for profits but useless for anything else PROFITS FIRST! ITS THE AMERICAN WAY !!