Decisions as to how and when to check if cows are pregnant are not as easy as they used to be. Rectal palpation for pregnancy has long been the method of choice and is typically performed at 45-60 days of pregnancy (although some can diagnose pregnancy as early as 35 days). Now, ultrasonic diagnosis is an alternative possibility. The use of ultrasound allows pregnancy checks to be made as early as 28 days after insemination. In addition, one can use ultrasound for a limited time in pregnancy (60-90 days of gestation) to diagnose sex of the calf. Use of milk progesterone assays can push  pregnancy diagnosis to as early as 21 days after insemination.

There are advantages and disadvantages of performing pregnancy diagnosis early in pregnancy.  The main reason to do so is to identify nonpregnant cows so that they can get another opportunity to be rebred. One strategy is to start OvSynch (i.e., give an injection of GnRH) 7 days before pregnancy diagnosis.   Cows diagnosed non-pregnant then receive an injection of prostaglandin on the day of pregnancy diagnosis, GnRH two days later and insemination.

Since ultrasound has become more common, it has been realized that many cows pregnant at day 28 of pregnancy subsequently lose their pregnancies (about 15%). Therefore, you cannot consider a pregnancy check at day 28 a definitive determination of the cow's pregnancy status - confirmation of pregnancy at day 45 or so still needs to be done.  Whether it is worthwhile to do an earlier pregnancy check probably depends on the cost of the procedure as well as the fertility of the herd.  If conception rates are typically low, it may be cost-effective to perform pregnancy checks at day 28 so that open cows get another chance to conceive as soon as possible.  

Websites that are "How to.". Manuals

Reproductive Applications of Ultrasound in Cattle - Here Bill Beal from Virginia Tech lays out the principles of uses of ultrasound for pregnancy diagnosis and more and tells you what to look for.

The Use of Milk Progesterone Assays for Reproductive Management - R.C. Rhodes explains the theory and practice of using milk progesterone assays to diagnose pregnancy.  In practice, milk progesterone is probably a more difficult technique to manage than ultrasound at day 28. 

Bovine Rectal Palpation - you will still need to practice, practice, practice to get good but this introductory procedure should help you orient yourself.

Websites Discussing Issues Related to Pregnancy Diagnosis

A Comparison of Methods for Early Pregnancy Diagnosis - a good review of the methods available from Brent Broaddus and colleagues at the University of Florida