The appropriate sizing of heifer replacement facilities is dependent on many herd-specific factors including size of the adult cow herd, the annual turnover rate and calving interval of the herd, the ratio of female to male calves and use of sexed semen, mortality rates at different stages of development, the age at which male calves are removed from the herd, and the age at which the heifers deliver their first calf. Because of these variables, the total number of heifers on a dairy can easily range from 70 to 130% of the number of adult cows. For an unknown herd size or prospective dairy, it is reasonable to assume that the number of replacement heifers will equal 90% of the total number of lactating adult and dry cows.
Sizing for certain groups of youngstock is most critical for the nursing calves because of the specific housing requirements for this group. The size of the nursing calf facilities is determined by:
- Age at weaning
- Variation in calves born per week and seasonality of the herd
- Facility downtime for cleaning
The 2007 NAHMS survey reports an average weaning age of 8.2 weeks, but the average age of weaning between dairies range from 5 to 12 or more weeks. In herds with relatively stable calving patterns that start weaning at week 8, the range in nursing calf numbers is usually +/-10% of average.
The following example calculations are based upon a hypothetical 500-cow dairy with year-round calving where there might be an expected 4.8 heifers to be born alive per week. The pens are also left unused for a minimum of one week for cleaning, drying, and reassembly. With a weaning age of 8 weeks and 1 week of downtime, housing space requirements should be planned to accommodate 9 weeks of the average number of newborn calves, which yields about 43 nursing calves (4.8 calvings per week x 9 weeks = 43.2 calves). To avoid overcrowding during calving surges, it would be appropriate to plan for an additional 10%, or 48 nursing calves.
While calf barns provide shelter from inclement weather for the caretaker, it can be difficult to break disease cycles and contain outbreaks within a single, permanent structure. Multiple, smaller barns have been used to create physical and time separation between calf occupancies. These barns are stocked all-in over a short period of time, and calves are moved as a group all-out at the same time. The barn is then cleaned and allowed to rest and dry for one week. Some risk management is involved with this method of housing nursing calves as the higher cost of “resting stalls” must be weighed against reducing disease risk and maintaining the ability to contain outbreaks and break disease cycles.
Pre-weaned calves should be completely separated from post-weaned and older calves to reduce their exposure to disease. If pre- and post-weaned calves are housed under one roof, the barn can be partitioned off by a solid wall or clear panels for disease control.
Clear panels separate the nursery from the growing heifer portion of the barn and allow more natural light into the barn
Group Sizes for Post-Weaned Calves and Heifers
Transition calves, from the post-weaning period to 6 months of age, are best housed in small groups of 10 calves or less with a weight range of less than 100 lbs (45.4 kg), or an age range of no more than 2 months. This grouping method will allow calves to learn social behavior while limiting competition at the feedbunk at a particularly sensitive time in nutritional transition.
Close attention needs to be paid to transition calves’ water and feed consumption as they learn to drink from a new water system and eat at a feedbunk, potentially with headlocks or a slatted feedline fence. Ensure waterers are low enough for small calves to easily find and drink from.
For the growing heifer groups, it is desirable to minimize the weight variation within a pen. Minimizing size variation within the pen becomes more critical when freestalls are used because inappropriately sized freestalls can injure heifers that may be oversized for the stalls. A general guideline for freestall pens is to limit the weight range to about 200 lbs (90.7 kg) within the group. Freestalls are an appropriate housing option for heifers greater than 400 lbs (181.4 kg). When heifers are growing at a rate of 1.75 lbs (0.8 kg) per day, the age range would be about 115 days or just less than 4 months. Tighter groupings would be preferred.