of Dairy Cattle
Many dairy producers believe pregnancy rate is a major problem for their herds. To realistically evaluate reproductive performance, one needs to know what is normal as well as what is possible. First, review the average reproductive performance for Midwestern dairy herds in 1996:
Calving interval: 14 months.
Heat detection rate: 40%.
Conception Rate: 50%.
Voluntary waiting period: 60 days.
Cull rate for reproductive failure: >10%.
Compare this with a set of goals which have been achieved on many well-managed dairies:
Calving interval: 12.5 to 13 months.
Heat detection rate: 70%.
Conception Rate: 60%.
Voluntary waiting period: 45 days.
Cull rate for reproductive failure: <10%.
Pregnancy loss rate (abortion and reabsorption): <5%.
Pregnancy rate is the percentage of cows conceiving during a 21-day period. Heat detection rate x conception rate = pregnancy rate. For the average Midwestern herd, this is 40% x 50% = 20%. This means that once breeding is started, only 20% of the cows will conceive during the first 21 days. During the next 21 days, 20% of the remaining 80% (16%) will conceive.
Using this information, compare two herds, the first with average reproductive performance and another that meets the previously stated goals (see Table 1). To compare the bottom line, figure a loss of $700 per animal culled (the difference between the cost of a springer and the price of a cull cow), add $2.50 x 45 days for lost production due to extra days in milk. For a 100-cow herd, this equals $21,750 in net profit.
Five factors determine calving interval:
Heat detection rate.
Voluntary waiting period.
Cull rate for reproductive failure.
Pregnancy loss rate (abortion and reabsorption).
Following is a discussion of these factors, listed in order of importance.
Heat Detection Rate
There are two reasons why cows are not observed in heat. Cows are not cycling or they are missed because of inadequate heat detection methods. Well over 90% of dairy cattle are cycling by 45 days post-partum. The exception would be cows, and especially heifers, with feet and leg problems or those with excessive weight loss because of post-partum health problems, such as retained placenta or displaced abomasum. These animals may not begin to cycle until they achieve a positive energy balance, which could be several months. This is one important aspect of how nutrition impacts reproductive performance. Many of the above problems are caused by improperly balanced rations during the dry cow and fresh cow periods. In addition, a severe negative energy balance during the post-partum period will delay the onset of estrus.
If heat detection is a problem (less than 70% heat detection rate on DHIA or can be calculated as 21 ÷ breeding interval x 100%), missing cows in heat can usually be corrected by moving cows to an open area twice daily and observing for 20 minutes per turn-out. Cows should be observed when first turned out. The surface of the lot should provide adequate traction to allow mounting, and bunks should be free of fresh feed. Heat detection is probably the most important step in achieving the goals stated previously. In the example cited in Table 1, a producer could make over $90 per hour for the time spent detecting heat, well worthwhile.
Heat detection is by far the major factor contributing to successful reproductive performance, which is the reason some producers have moved to "Target Breeding Programs." These programs use hormone injections along with careful recordkeeping to bring cows into heat for "scheduled" breeding. These programs can work well, and will usually enable a producer to meet the goals stated previously. These programs should also lower cost compared to traditional herd health palpation programs. For more information on these programs, consult your local veterinarian.
Many producers feel their cows do not settle, implying a conception rate problem. With an average 50% conception rate, 50% of the cows will need to be bred two times, 25% will need to be bred three times, 12.5% will need to be bred four times and so on. This is normal and does not imply a problem.
Several factors can lower conception rate, such as excess summer heat. In extreme conditions, such as those experienced in Florida or Arizona during the summer, conception rates may temporarily fall to 20% or less. Semen handling techniques can also cause conception problems, such as an inaccurate thermometer in the thaw jar which results in over-heating of semen. Contact your A.I. representative if you have concerns about semen handling procedures.
Another factor influencing conception rate is uterine health. Retained placenta or metritis can cause reduced conception rates. Nutrition can have an influence on these problems. If the incidence of retained placenta and/or metritis exceeds 10%, check nutrient levels in the dry cow diet. Also, lack of cleanliness at calving time can contribute to metritis.
Recent research shows cows that lose excess body condition during the first 60 days after calving have reduced conception rates. According to one study, cows losing more than one full body condition score will have first service conception rates in the 20% range. Loss of body condition can be nutritionally related. Cows with low body condition scores often have metabolic disorders or are fed low-energy diets that contribute to weight loss and subsequent poor conception rates.
Recently, attention has focused on blood urea nitrogen (BUN) levels. Current understanding is levels in excess of 20 mg/100 ml can cause a decrease in conception rates. To control BUN levels, rumen degradable protein and ruminal available carbohydrates must be balanced.
Voluntary Waiting Period
The voluntary waiting period (the amount of time between calving and when a producer decides the cow is ready for breeding) is simply a decision the producer makes. Forty-five days is the recommended waiting period. Longer periods will result in an extended calving interval. Cows subject to waiting periods less than 45 days usually have reduced conception rates.
Cull Rate for Reproductive Failure
Cull rate for reproductive failure is determined by the previously discussed three factors. It is important to understand, however, that calving interval can be artificially lowered by excessive culling.
Pregnancy Loss Rate
Pregnancy loss from abortion and reabsorption is largely beyond a producer's control and typically runs about 5% to 6%. This is probably normal and does not indicate a problem. Hot weather may temporarily increase reabsorption rate. Maintaining a good vaccination program for IBR, BVD, and leptospirosis is also recommended. IBR and BVD are the leading infectious causes of abortion and can be prevented.
Reproductive performance is mainly determined by heat detection rate,
conception rate, and voluntary waiting period. Reproductive performance
data can be obtained from DHIA or other record systems to determine if
there is a problem, cause of the problem, and herd comparison to the
"average" and progress toward attaining set goals.
Nutritionally, producers should focus on minimizing the degree and
duration of negative energy balance, avoiding periparturient metabolic
problems, and balancing diets according to established guidelines.