INTRODUCTION
Efficiency of reproduction is one of the most critical aspects of a profitable herd. Economic losses resulting from delayed reproduction have multiple facets:
- The lifetime milk production of the cows is reduced because peak milk production does not occur as often and the dry periods are extended;
- The number of calves born per year decreases, giving fewer opportunities to cull cows with low milk production, and slowing the possible gain in the genetic value of a herd;
- The direct costs for treatment of reproductive disorders, breeding and veterinary fees are increased.
HEAT DETECTION
In order to maximize productive life, a cow must be bred within 80 to 90 days after calving. This will enable her to produce a new calf every 12.5 to 12.8 months. Longer calving intervals have detrimental effects on lifetime milk production. Whether a producer uses artificial insemination or natural service, heat detection is a critical component of good reproductive management on the farm. In either case, recording of cows in heat and dates of services is necessary to predict future heat or calving dates an to manage the cows accordingly.
What is Heat?
Heat is a period of acceptance for mating (sexual receptivity) that normally occurs in non-pregnant, pubescent heifers and non-pregnant cows. This period of receptivity may last from six to 30 hours and occurs every 21 days on the average. However, the interval between two heats may vary normally from 18 to 24 days.
Signs of Heat
Detection of heat calls for acute observation. Most cows have a pattern of behavior that changes gradually from the beginning to the end of a heat. The best indicator that a cow is in heat is when she stands and allows herself to be mounted by herdmates or a bull (Figure 1). A series of signs that may help to identify cows that need to be observed closely are summarized in Table 1.
Daily Patterns in Signs of Heat
The onset of heat activity follows a distinct pattern, with most activity occurring in the late evening, through the night, and in the early hours of the morning. Research shows that more than 70% of mounting activity takes place between 7:00 at night and 7:00 in the morning (Figure 2). In order to detect more than 90% of the heats in a herd, cows should be observed carefully in the early hours of the morning, the late hours of the evening, and at four- to five-hour intervals during the day
Other factors influencing the expression of heat
Expression and detection of heat may be more or less easy depending on a number of factors.
For example, the type of housing (stanchion barn, free stall, pasture, walking path along a fence, etc.) provides various degrees of ease for the cow to express signs of heat and for the producers to detect cows in heat. In larger herds, more than one cow may come in heat at the same time.
When this occurs, the chance of detecting cows in heat increases dramatically because mounting activities also increase considerably.
For example, two cows in heat at the same time (sexually active group) cause a tripling of mounting activity. In contrast, factors such as high temperature and humidity, wind, rain, snow, confined space, and conditions that may cause slipping, falling or hoof pain tend to inhibit the expression of heat.
Absence of Heat
Heat may not be detected in cows for the following reasons:
The cow is pregnant;
- The cow has calved and the heat cycle has not yet resumed (silent heat);
- The cow is anestrus because of poor nutrition, severe infection of the reproductive tract, or other complications after calving;
- The cow has cystic ovarian disease;
- The producer fails to detect a cow that actually came in heat
