I always get this question.
A lot of people even think Wagyu is fully grass-fed. (Crazy!)
And then many people think that grass-fed, since you know, it's grass, MUST be better than grain-fed. So what's the real deal? Which one is better? and most importantly, which should you buy?
Let us explain a little bit..
Most, if not all, Australian premium meat is grass-fed. When Australian producers mention Wagyu or grain-fed, they are not fully grain-fed all their lives, but actually their feeding was finished with grain. Therefore this beef is called grain-finished. The premier beef producers in the world in Wagyu beef production specialize in long-term grain feeding of cattle to get that amazing marbling (i.e. Rangers Valley). Marbling in meat means that it has a had a good life, and that its growth has reached full potential. (By the way, Japanese Wagyu is fully grain-fed!)
When cattle is fed with grass they are free to roam and use their muscles more freely, therefore there is less fat distributed in the meat. Fun fact: due to the chlorophyll in the grass, the fat in grass-fed beef is actually more yellow in color than the fat in grain-finished beef. Also, grass-fed beef looks darker red in color than grain-finished which is a brighter red.
So what's the answer? Neither!
According to a 2014 study carried out by Dr. Stephen Smith in the Department of Animal Science in Texas, he found that there is no scientific evidence to support the claim that ground beef from grass-fed cattle is a healthier alternative to ground beef from conventionally raised, grain-fed cattle.
Grass-fed meat does have a high level of a-linolenic-acid (ALA) but it also has a high level of saturated and trans fat compared to grain-fed meat. On the other hand grain-fed meat also has a higher level of oleic acid. Researchers have known for decades that oleic acid has positive health benefits, such as reducing LDL-cholesterol (the bad cholesterol) and perhaps increasing HDL-cholesterol (the good cholesterol). The World Health Organization has recommended that intake of oleic acid should be 15%-30% of daily energy intake.
So long story short, both types of meat have different types of fatty acids which are beneficial for health.
Ideally you could take off all the fat possible on meat, but without that goes the flavor too. Better eat some chicken breast instead.
Notes and sources:
Grass-Fed Vs. Grain-Fed Ground Beef -- No Difference In Healthfulness (http://www.beefmagazine.com/beef-quality/grass-fed-vs-grain-fed-ground-beef-no-difference-healthfulness)
Rangers Valley website (http://www.rangersvalley.com.au/)
Stephen B. Smith is a Regents Professor and Texas A&M AgriLife Research meat scientist in the Department of Animal Science at Texas A&M University.