Reader Mailbag: Question about cow size and nutrient requirements
A reader responded to Ken Olson’s March 5, 2011 column, “Relationship between cow size and nutrient requirements” by asking:
“Can larger cows still consume the same percentage of their body weight in feed? If they can, since they require a decreasing proportion of nutrients as they get larger, can a larger cow do better than a smaller cow on poor quality forage? Can a larger cow meet her nutritional requirements with poorer quality forage than a smaller cow?”
We passed along that’s reader’s question directly to Olson, who responded:
These are excellent questions. Theoretically, larger cows should be able to eat the same percentage of body weight as smaller cows. Thus, if a feed has the quality to be consumed at 2 percent of body weight, a 1400-lb. cow should eat 28 lbs., and a 1200-lb. cow should consume 24 lbs. However, I can think of at least two things that will influence this in relation to your questions about the larger cow getting by on lower quality feed since she needs less nutrients per pound of body weight, and therefore also less nutrients per pound of forage.
The first problem relates to the nature of the forage. As forage quality declines, a ruminant will not be able to consume as much of it. This is because passage from the rumen to the lower digestive tract is restricted. Feed particles have to be digested in the rumen to a small enough size to fit through the orifice (opening) out of the rumen. Lower-quality forages digest more slowly, so they must remain in the rumen longer to be reduced to a size that will pass beyond. Once the rumen is full of these slower-digesting feed particles, intake is restricted. Thus, if we lower the quality of the feed to the larger cow, we will no longer be able to maintain the same level of intake. This will lead to both lower nutrient content and lower intake, which may not allow this cow to meet her nutrient requirements.
The second possible problem relates to the nature of the cow herself. Does she weigh more because she has a larger frame, or because she is deeper-bodied with greater rumen capacity to digest that lower-quality feed? If the bigger cow is several frame scores taller than the smaller cow, but is not any bigger around at her heart girth, she will have limited capacity to utilize the nutrients from lower-quality forages because she won’t have the rumen capacity to digest it.
A number of years ago, the manager of a large ranch told me that they sorted all of the 10-year-old and older cows from the herd at weaning time. These were cows that had consecutively raised eight or more calves in their lives. This was a ranch that expected mature cows to take care of themselves on range forage with little to no supplemental nutrients. As a result, these cows had to be able to extract enough nutrients from range forage to meet their requirements to maintain themselves, nurse each calf, and be fertile enough to get pregnant every year during a defined breeding season. The manager said that once they were sorted, this group of cows was surprisingly uniform in appearance. They were all short-legged, relatively small-framed cows that were very deep-bodied with a lot of gut capacity. They were not the tall and elegant sort of cattle that are considered pretty to look at.
My point is that I think there are bigger cattle that will be functional consuming low-quality forages in the manner you are wondering about, but they have to have the right conformation to do it. They have to have the rumen capacity to maintain intake and extract those limited nutrients when confronted with low-quality feed.
editor’s note: find olson’s original column at: http://www.tsln.com/article/20110307/tsln01/110309955, or in the march 5, 2011 edition of tri-state livestock news.
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