If these young cows are pastured with the larger, older cows in the herd, they very likely will be pushed aside when the supplements are being fed in the bunk or on the ground.  The result of these adverse conditions for young cows very often is a lack of feed intake and lowered body condition.  Of course, lowered body condition in turn results in delayed return to heat cycles and a later calf crop or smaller calf crop the following year. 

North Dakota State University data of commercial cow herds recorded over a 21 year period illustrate the differences in size and body condition of very young cows and the very mature (11 year old+) cows.

The North Dakota data clearly show that the average 2 year old is about 20% smaller than her full grown herd mates.  There is little wonder that the younger cows get pushed away from feed bunks, hay racks, or supplements fed on the ground.  The results of the size differences and the need to continue to grow are manifest in the lower body condition scores noted in the very young cows.  The very old cows are experiencing decline in dental soundness that make it difficult for them to maintain feed intake and therefore body condition. 

Consequently, it makes sense to sort very young cows with the very old cows and provide them with a better opportunity to compete for the feed supplies.  By doing so, the rancher can improve the re-breeding percentages in the young cows and keep the very old cows from becoming too thin before culling time.