GB...... I could not agree with you more! Yes there is a definite correlation between selection for low birth weights and smaller pelvic areas. My vet says the same thiing, and he says they are doing more C- sections on heifers with75 - 80 lb calves than he has ever seen in his years of practice. He was telling me about a research study that showed that constant selection for low birth weight will result in smaller pelvic areas. I am going to have to ask him again about this and see if I can find a copy of the study. I also know of a large commercial herd who has been using low birth weights as a selection tool for buying their herd bulls. After several years of doing this, they are pulling more calves than ever before. Some of these heifers would have a problem calving a small cat.
As I get older, it is becoming very apparent to me, that most everything in life is best in optimum amounts, and this goes for most everything from the food we eat, to the amount of money we make, to how we breed our cattle, etc etc etc. Just look at most anything in your life... and too much or too little of anything almost always results in problems of some kind. In regards to birth weights, I think we all should be considering calving ease in conjunction with birth weights. Personally, I think that if we try to select for only low birth weight, we are also selecting for shorter made animals. It stands to reason that a calf that is 1 inch longer than another calf, is going to weigh more.
I also feel that selection of low birth weight sires also can result in calves that have less performance. Occasionally you will get a high performance animal that had a very low birth weight. This is usually the exception rather than the rule. We have just completed our bull test for our upcoming bull sale. Not one of my bulls that indexed over 100 for ADG and WPDA , are in the lowest 50% for birth weights. Again I say NOT ONE. I find this perplexing as almost every inquiry I have on the bulls, is concerned about birth weights... and virtually no one is concerned about the performance of the bull. In times like we have now, with high grain prices, I happen to think that performance is extremely important. I also agree that no one likes having to assist a birth, so there has to be some optimum point where we can achieve both. I think this almost obsession about birth weights in the beef industry is way over blown, and I am afraid that if we continue down this trail too far, that it is only going to bring problems of other kinds.
Eight years ago, I made a decision that I would band any bull calf that I had to assist at birth, unless it was a malpresentation.If the calf still needed to be assisted after the malpresentation was corrected, it would also be banded. I also started banding every bull calf with a BW of over 110 lb, regardless if it was born unassisted or not.... and regardless of it's pedigree. It has really been hard to band a few excellent bull calves that looked great, but happened to have BWs over 110 lb, but I have bit my lip and done it. Maybe I have been lucky, but I have not had any bull buyer complain to me about having calving issues from our bulls since I started doing this. Some of you will say that 110 lb is too big a BW but I decided to keep bulls with these birth weights providing they were born unassisted. I felt that I also wanted to maintain some performance in the bulls. I do not know if this is the proper cut off weight to use but it seems to have been working here anyways. I always ask a potential bull buyer if they is planning to breed heifers. If they are, I steer them away from the 110 lb BW bulls and suggest others to them. If they are breeding cows and are concerned about weaning weights, or are feeding out their calves, I recommend the larger birth weight calves.
Selection using only performance is also not the best idea. Several years ago, one of the best herds in this part of the world, based all their selection on performance. They kept all their heifers and the replacements were selected from the highest performing end. They were selected from the top 20% based on yearling weight. After a few years of doing this, they found that they were seeing more reproductive problems in their replacements. They changed the selection criteria, and the top 10 % were finished and slaughtered. Their replacements came from the next 20%. They then sold the next 20 % as breeding heifers for others to purchase and the bottom 50% were fed out and slaughtered as well. Once they figured out that Optimum was the best, their fertility in their herd improved very signifigantly.
Back to the topic of this thread. No cow should ever be bred to a bull that could result in a 160 lb calf. That is simply retarded. I don't care how good his calves are, I would simply NOT use this sire. I would never use a bull that consistently threw some calves that were 125 lb or more. I will agree that there is more to having a high birth weight calf than just the sire used. The bull seems to get blamed for everything. Part of the genetics for birth weight comes from the dam as well. If you constantly keep a bunch of heifers that have high birth weight genetics stacked into their pedigree, you are probably going to have some big calves from any sire you use... even a low birth weight sire. There is another selection tool that is often forgotten... and that is COMMON SENSE. There is no element of common sense in using a bull that could kill your cows or result in dead calves, injured calves or injured cows.... or higher vet bills. Period!!!