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What Size Fits “Best”

With high input costs, cattle producers continually refine and evaluate how resources are being allocated.

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’s 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.

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