By Sandy Johnson, extension beef specialist
We just finished preg checking some replacement heifers from a project conducted earlier this spring. The heifers were all artificially inseminated (AI) at a single fixed-time. Natural service sires were turned in 10 days after AI and removed 20 days later. This short breeding season was used for a number of reasons but most importantly so that there would be no late calving replacement females. The staging of this ultrasound examination was such that we had pregnancies at either 64 days or 54 to 34 days. Differentiating these stages of pregnancy with ultrasound was relatively easy and the information obtained was very powerful. The open heifers were sorted off and hauled to the feedlot that day.
Because of the delay in turning out natural service sires and the timing of the scanning, our accuracy of predicting if heifers were bred to AI or natural service is very high. Having a known breeding date makes predicting a service sire or calving date much easier and a forced gap between AI and cleanup natural service even more so. This is a typical approach used for research. The early pregnancy diagnosis is needed to clearly differentiate date of conception. In one large study where natural service sires were turned in 10 days after fixed-time AI and AI pregnancy determined 30 days after AI, there were a few natural serviced sired calves that were born at the same time as long gestation length AI-sired calves.
So how accurate is ultrasound at staging pregnancies? A recent study where cows were scanned between 32 and 110 days of gestation, found they could predict calving date within ± 15 days, 95% of the time. In a much larger New Zealand study of seasonally bred dairy herds where veterinarians had access to AI dates during examination, 90% of cows calved within 10 days of their predicted calving date. When fetuses were over 13 weeks of age, the probability of calving greater than 10 days later than predicted increased. Accuracy of predictions was 72% for fetal ages ≥ 15 weeks. Variability of fetal measurements increase as gestation progresses so it is not surprising that accuracy of prediction declined with fetal age in the New Zealand data.
I have promoted examining your calving distributions many times in this series as it reflects the job of the manager in matching nutrient resources with cow demand in a timely fashion; it is data you should always summarize. We can get a preview of the next calving distribution if pregnancy diagnosis is done early enough to stage pregnancies. This data can inform management choices as cows continue through gestation. For example, if there are a large number of late calvers, late gestation/early calving rations might be altered or these cows might be targeted for marketing in a good bred cow sale. If young cows are late-bred, specific steps could be taken with their younger counter parts that might avoid the same problem. When drought or other issues impact pasture availability, this information can be used if hard culling decisions must be made.
Herds with a history of a tight calving distribution that argue they don’t need to check or at least not preg check early, miss the opportunity to have time to respond if for some reason there are more open or late pregnancies than expected. This would be comparable to ignoring accounts that are set to pay automatically, because it is all set up and should be working. Then something is changed and the consequences are expensive when you are slow to recognize the problem. Unfortunately, I speak from experience on this matter.
Pregnancy diagnosis can be more valuable if done early enough so that pregnancies can be accurately staged. Your veterinarian can do a much better job in doing so if that occurs within 5 months of the start of the breeding season. Staging can still occur after that point but the data collected might not be as clear. There is a caveat to early pregnancy diagnosis in that embryo and fetal losses are higher early in pregnancy than later, but some fetal loss still occurs in mid to late pregnancy.
When you are making plans with your veterinarian, discuss this timing and the potential benefits. A new app that can be used chute side, called Pregnancy Analytics, allows you to enter data chute side and look at the projected calving distribution when you are finished. Information is power and expected calving dates and projected herd calving distribution is information that more producers should put to work.
And in case you are interested, our fixed-time AI pregnancy rate was 57% and 80% of heifers were pregnant in the 30 day season.
