What Size Fits “Best”

July 2009

With high input costs, cattle producers continually refine and evaluate how resources are being allocated. Because feed cost make up the largest portion of annual cow costs, they look to attack feed cost to make their livestock enterprise competitive. As producers re-evaluate feed inputs, revisiting items that drive nutrient needs of beef cows is a good first step. Beef cattle geneticists make the statement that cow type needs to fit the feed resource of your operation and that different biological types fit "better" in high stress and low stress environments. Feed resources differ from ranch to ranch and not all cow types will excel in a particular production system.

The 1996 National Research Council (NRC) Nutrient Requirements for Beef Cattle is considered the bible for nutrient needs. Nutrient requirements for beef cows are impacted by cow age (1st-calf-cows versus mature cows), weight/size, lactation ability (breed type), stage of production (gestating versus lactating), environmental conditions, and body condition of the female. The 1996 NRC for Beef Cattle allows for adjustments for cows that are in different body condition. The body condition scoring system used in the 1 to 9 scoring system (1 to 9 scale; 1 = thin and emaciated, 9 = fat and obese). The 1996 NRC will adjust nutrient needs for different environmental temperatures. For beef cows, it's how do you manage a cow herd when a sustained number of days are well below their lower critical temperature. In some situations, not enough feed can be fed or intake is not high enough to really meet their nutrient needs. In these situations is why managing body condition is so critical and is used as a risk management tool. Finally, the increase in nutrient needs of young cows versus mature cows is primarily due to the growth requirement of young cows.

Maintenance feed intake is proportional to metabolic body weight which is described as body weight to the 3/4 power (body wt.3/4). Metabolic body weight isn't just weight of the animal but also describes the surface area of the animal. Cows that weigh 1,300 pounds have a greater nutrient (pounds of protein, pounds of energy (TDN), ounces of mineral, etc) requirement compared to cows that weigh 1,000 pounds. If a 1,000 pound cow and a 1,300 pound cow are grazing the same forage resource, the 1,300 pound cow will need to consume more of that forage to meet her requirements. Heavier cows eat more feed to meet their requirements. The question is: how much more do heavier cows eat? Data suggests that for each 10% increase in body weight, there is not a 10% increase in maintenance feed intake. The data suggests about a 7% increase in feed intake for each 10% increase in live weight. A 1,300 pound cow is will consume 22% more feed than the 1,000 pound female although there is a 30% difference in body weight. To present this concept a little differently, energy needed for cows that weigh 1400, 1200, and 1,000 pounds as they progress from calving to their next calving. The energy needs would be of cows of similar milk potential. Peak lactation would occur about 60 to 80 days post-calving. The difference between the 1,400 lb cow and the 1,000 lb cow is about 5.1 Mcal a day. Native hay is about 0.78 Mcal/lb so that's about 6.5 pounds difference in hay intake between the 1000 lb and 1,400 lb cow on a dry matter basis. If the hay is 88% dry matter and costs $65/ton that calculates to $0.24 per day difference. Illustrated another way, if a 1,000 lb cow with a calf at her side is 1 AUM (consumes 26 lb/da dry matter of forage), then the 1,400 cow is 1.4 AUMs, then the difference is 0.4 AUMs and you can calculate the number of 1,000 cow/calf pairs you can graze on the same pasture resource.

Managing for milk level in your cow herd is kind-of like determining whether the porridge is too cold, too hot, or just right. Too little milk in the cow herd equates to lighter weaning weights which impacts dollars generated in the cow/calf enterprise. However, low milk level in a cow herd should result in lower feed costs. High milk level equates to heavier weaning weights, but also has the potential to increase feed inputs and therefore cow costs. As milk potential increases so does nutrient needs. Cows that have a high milk level have a greater need for pounds of protein, pounds of energy (TDN), ounces mineral, etc. to be consumed daily to meet those needs compared to cows with a low level of milk potential. A number of years ago at the University of Nebraska, three groups of cows were developed that were similar in weight, but differed in level of milk produced. The cows in the moderate level of milk group gave 28% more milk than the cows in the low level of milk group. Likewise, the cows in the high milk level gave 46% more milk than cows in the low milk level group. As one would expect, the feed for lactation between the three groups differed, with cows in the moderate and high level milk groups needing more feed to stay in similar weight and body condition as compared to cows in the low level of milk group. Even more interesting is that feed during gestation was greater for cows in the moderate and high level of milk groups compared to cows in the low level of milk group. Indicating that even when the cows with different milk producing abilities were not lactating, milk potential increased nutrient needs. In addition, feed needed in the feedlot was greater for the offspring from dams that had higher milk potential. Data from the U.S. Meat Animal Research Center in Clay Center, NE indicates that cows with greater potential to produce milk also have a greater percent of their body weight as heart, liver, and lungs. So the greater nutrient need for non-lactating cows and for their offspring in the feedlot may be a result of having to maintain more visceral organ weight that are highly active tissues.

Metabolizable protein between cows the have the potential to produce different levels of milk is different. Early in the lactation curve, the is about 0.5 lb difference in MP requirement. That 0.5 difference in MP narrows toward the end of the lactation curve. If cattle are asked to produce in a lush environment, then cow mature weight and level of milk is less of a concern except that carrying capacity will be reduced. If feed resources are limited, cow weight and milk production need to be carefully scrutinized.

Cow weight and milk level impact not only nutrient requirements but also impact feed intake. The focus of the article is not that we need smaller cows, but to point out items that have a big impact of the nutrient needs of beef cows, mainly cow weight and level of milk production. From an economic standpoint, the greater number of cows that can be grazed on a given forage base and meet their nutrient needs from the grazed resource base, the greater the profit potential of the enterprise. From a commercial cow/calf perspective it may be wise to avoid extremes in size and milk production, include cow maintenance EPDs as a part of your bull selection criteria, and adjust calving and weaning dates to manage body condition.

Rick Rasby Dr. Rick Rasby, Professor of Animal Science
Animal Science, University of Nebraska–Lincoln, Lincoln, NE