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Occurring in all breeds of cattle, the DMR is the most common midpiece defect (Figures 9.16 and 9.17). The classic appearance is that of a “J” hook involving the very distal end of the midpiece causing the rest of the tail to be whipped around 180°. Cytoplasmic droplet material is usually trapped in the loop but not always; in the absence of droplet material, examiners should look for the classic “J” hook. Other manifestations include a less‐severe 90° bend in the midpiece and second bends involving the principal piece that cause the midpiece and principal piece to loop in a circle. The DMR defect occurs in the epididymis associated with a hormonal chain reaction which ultimately causes a decline in testosterone levels. Sperm are stored in the cauda epididymis prior to ejaculation and epididymal sperm possess a distal cytoplasmic droplet which gets trapped in the reflex and is unable to be shed. Both heat and stress may cause a decline in testosterone and the appearance of DMRs. The DMR defect is most often associated with stress insults to spermatogenesis, appearing within just a few days of a stress occurrence. Common stressful events include severe weather, injuries, and illness of at least three or four days duration. With more severe or long‐standing events, other defects will appear in succession following the DMR according to where they occur during spermatogenesis. In cold, northern regions DMRs are frequently recorded in semen samples collected during the late winter and early spring, suggesting that photoperiod and/or nutrition may be a cause. This defect is seen in all breeds; however, some animals within a breed appear to be more affected than others by stressful events experienced by the group. When a bull fails to meet the minimum morphology standards for a satisfactory classification due at least in part to a high proportion of DMRs it is appropriate to reevaluate the bull in as little as two weeks. Not only do DMRs appear fast following an insult, but also if all goes well the numbers of affected sperm will decline rapidly.

Figure 9.16 Distal midpiece reflexes with and without droplet material and detached heads on an eosin‐nigrosin stained smear.

Figure 9.17 Three images all showing distal midpiece reflexes.