Cow weight is probably easier to wrap your mind around than milk production, but research has shown that cows with the genetic propensity to milk heavily require more nutrients year round, not just when they are milking. The National Research Council (NRC) data shows that a cow who produces 25 lbs. of milk at peak lactation requires 10% more feed energy than a cow producing 15 lbs. of milk at peak lactation. To see a 10% difference in feed energy with regards to mature weight it would require moving from a 1,000 lb. cow to a 1,200 lb. cow, or a change of 200 lbs. of body weight. There are breed differences in lactation yields so breed selection is critical in match genetics to your environment. These breed differences can be found in literature from research at the Meat Animal Research Center (MARC). We know that selection for increased yearling weights has led to larger mature cows that are more expensive to maintain. Moderating mature cow size and selecting for an optimal window of milk production is beneficial when it comes to cutting costs regardless of your production environment. However, in limited feed environments females with high maintenance energy requirements may also have difficulty maintaining an acceptable body condition score and rebreeding. Nugent et al. (1993) determined that with limited nutrient availability, breeds with a high genetic potential for milk production had longer anestrous periods which lead to lower conception rates during a fixed breeding season. Other researchers have concluded that selection for increased milk production past an adequate threshold is not economically or biologically efficient (Oijen et al., 1993).
Some breed associations have developed genetic selection tools to aid in decreasing cow costs. The American Angus Association has a Cow Energy Value Index ($EN), which is measured in dollars of savings per head due to decreased energy requirements for maintenance (www.angus.org). In low input environments, a high $EN would be more desirable (Angus Journal, February 2005). The Red Angus Association of America calculates a Maintenance Energy (ME) Expected Progeny Difference (EPD) that indicates differences in the Mcal/month needed for maintenance due to mature size (corrected for body condition score) and milking ability (The Rancher’s Guide to EPDs available at www.redangus.org). A much simpler way to think of it is that a bull with a ME EPD of +10 compared to one that is +0 will produce daughters that will require approximately 11 more pounds of average quality forage per month (assuming average quality forage = .86Mcal/lb.) Clearly identifying your production environment and realistic production goals given that environment is critical. Profit lies in the optimization of expense and revenue and optimization is always more challenging than maximizing outputs or minimizing inputs. It will require more effort, detailed financial records, and a structured breeding objective that builds a cow herd based on optimum values and not extremes.