Cow size

[DiamondSCattleCo]

In the midst of your little rodeos Kaiser, have you
remembered to keep the nuts in a couple of heavier
BW bulls for me? Some of you guys may like to see
those jackrabbits hitting the ground, but I'll take a
hunnert pounder any day

Rod

[smcgrath76]

Again, this relates heavily to what your resource base is and what you plan to do. An AUM is defined as a 100 pound cow (eating 2.5% of Bodyweight per day). The new technical definition raises BW to the 3/4 power, giving an advantage to the larger cow. A 1500 pound cow by this definition is not 1.5 AU, they are slightly less in fact. As well, as milk increases, intake increases just to support the gut mass for milking.
If you have a lot of aftermath grazing, ralatively flat land, decent moisture, etc. big cows probably make more money. If you graze year round, smaller cows are probably better.
High milk production can partially be overcome by calving later and weaning earlier, but it still may impact fertility given that there are only so many biological resources to go around.
At our place the math on 1200 pound cows, vs 1500 pound cows (based on metabolic body weight and similar milking ability) looks something like this...
100 1500 pound cows = 136 AU
136 AU = 118 1200 pound cows
If every cow weans 50% of her body weight this works out to...
750 * 100 = 75000 pounds saleable
600 * 118 = 70800 pounds saleable
It looks like the advantage goes to bigger cows, especially if you look at salvage value, until you consider that in our system these larger cows can't maintain condition, have a higher degree of opens and don't live as long. This is not right or wrong, just the fact of the matter is in our winter feeding system, these larger cows can't meet their metabolic needs, even though they are proportionally lower than a smaller cow per pound of cow. It is also easier for a 1200 pound cow to wean a 650 pound calf (54% of body weight) than a 1500 pound cow weaning an 810 pound calf. Reality is most producers weaning weights would be significantly less in a pounds weaned per cow exposed scenario.
Basically every producer needs to do their own math.
The high birth weight thing is OK for terminal cattle, but BWT is highly related to mature weight. We really work on finding curve benders to produce replacements as they grow fast and then quit at a reasonable size.

[per]

Not arguing with your math or reasoning but we must also consider that up until recently the 600lb calf brought as much per head as the 750lb calf. Going back to pounds per head to cows expose separates the pack even more. Many pounds per head weaned is important but few cows empty will result in more total pounds to sell. I guess I'm just repeating what has already been said. This thing still boils down to longevity and fertility. Only a cow that fits your environment will do that. Can't argue that those big old cows bring better salvage money but on our ranch the more moderate sized cows last longer.

[smcgrath76]

yep. It boils down to resources, marketing strategy, personal economics, etc. Larger cows given good resources may have a long lifespan. Sometimes limiting resources will reduce this.
Marketing into a carcass program or otherwise can change the economics again.

[rkaiser]

finally saw a heifer calve this year and had to help out a bit. She had a foot back so I laid down behind her and got things straightened out and gave it a pull. Funny thing about this heifer was that she is only 13 1/2 months old. Weighed her the day after she calved and she was 630 pounds. Her calf weighed 67 pounds and momma and baby are doing just fine.