Nutrient needs for mature cows and 1st-calvers increase especially during the last trimester and after calving when lactation occurs. When you look at the nutrients required, especially pounds of protein and energy (Total Digestible Nutrients, TDN), there is not a big difference in total pounds of these nutrients needed in the diet for 1st-calf-females compared to mature cows. However, when you compare the dry matter intake of a 1st-calf-heifer compared to a mature cow, there is a considerable difference. With that in mind, to get the same pounds of nutrients into a 1st-calf-heifer compared to a mature cow, the nutrient density of the diet needs to be greater. Put in other words, the quality of the feeds needed in a 1st-calf-heifer diet needs to be high in quality. If a 1st-calver is developed to be 85% of her mature weight at her first calving and her mature weight is 1,250 pounds, her weight at her first calving is about 1,065 pounds. The actual weight of the female will be more than that because of the weight of the fetus and fetal fluids and membranes and will be closer to 1,190 pounds (1,065 + 125 pounds of fetus and fetal membranes and fluids). If she is consuming an average quality forage diet, her intake is about 2.1% of her body weight on a dry matter basis, then her daily intake is about 22 lb/da (1,065 x .021 = 22.4 lb D.M.). Compared to a 1,250 pound cow consuming the same average quality forage, she would consume 26 lb/da of the forage on a dry matter basis. Both females will require about 2.2 to 2.3 pounds of crude protein daily the last month prior to calving. The diet for the 1st-calver needs to be 9.5% to 10.0% crude protein and the mature cow needs a diet that is about 8.8% crude protein to meet their respective protein requirements. The point of this discussion is that with young females, the rumen you are working with is smaller compared to a mature cow. Although the pounds of a nutrient needed may not be much different, the diet quality needed to meet this requirement is much different. This is a primary reason that beginning at least three weeks before calving that 1st-calvers need to be managed and fed separate from the mature cows.
Research conducted at the University of Nebraska by Tim Loy reported in the 2004 Nebraska Beef Report indicates that as the first-calf-heifer that is within three weeks of calving experiences a 17% decrease in daily feed intake. This data further illustrates the need to separate first-calf-heifers from mature cows beginning at least three weeks before the start of the calving season. This data also suggests that the nutrient density of the diet has to be high because intake is restricted. Intake is re-established to more “normal” levels by about one week post-calving. The reduction in feed intake is not yet understood. The most logical explanation would be that the fetus has increased in size and takes space that the rumen would normally be occupying. Another explanation could be that hormones being produced late in gestation impact apatite.
Because we don’t normally weigh individual cows in a normal production setting, feeding to a specific body condition is the best route to monitor your feeding program. Feed 1st-calf-heifers to calve in BCS 5.5 to 6.0 to at least give her a chance to be a productive part of your cow herd.