To achieve maximal fertility, rams should be physically examined for reproductive fitness to detect any abnormalities that may limit mating. (Also see Breeding Soundness Examination of the Male in Sheep.) The scrotum and its contents and the penis and prepuce must be carefully examined. The size and symmetry of both testes and epididymides should be assessed, and both testes should be firmly palpated for consistency and resilience. Any palpable lesions, particularly of the epididymides, should be considered potentially contagious (eg, Brucella ovis and Actinobacillus seminis). Appropriate tests should be performed to establish a flock diagnosis to initiate a test and eradicate program for infected rams. Semen can be collected and evaluated to screen potential sires, particularly in single-sire mating systems. All screening procedures should be done 6–8 wk before mating to allow management changes of the ram team or purchase of replacements for defective rams.
Supplementary feeding of the rams can be started 6 wk prior to joining. High-protein grains, particularly lupines, increase both testicular size and the number of cells in the germinal layers of the testicle, resulting in increased sperm production. However, dietary protein levels >14% have been associated with infectious balanoposthitis due to Corynebacterium renale (see Ovine Posthitis and Vulvitis, also called pizzle rot).
Mating activity can be monitored by using a breeding harness on the rams and changing the crayon color every 14–17 days. When fewer than expected ewes are marked, poor ram libido, a low ram:ewe ratio, or anestrus is suggested. When ewes are serially marked with different colors, conception failure or early embryonic death is suspected.
The ram:ewe ratio varies with breed, maturity of ram, and whether synchronization or induction of estrus is being practiced. Ratios of 1:40 are common in farm flocks, but excellent fertility can be achieved with a lower ratio if rams from prolific breeds are used (eg, Finnish Landrace). For ram effect, the ratio should be 1:20; for estrus synchronization, 1:15 (in season); and for estrus induction (out of season), 1:5 to 1:7.
Length of ram exposure during the ovulatory season should be limited to 2–3 cycles so as to tighten the lambing period to optimize lambing management and lamb survival. Excellent fertility can be achieved with a breeding exposure of 35 to 42 days (2–2½ cycles). Poor fertility indicates an issue with the breeding management. Flock dispersion should be avoided at mating, but normal handling should not affect mating. Because younger ewes have a shorter, less intense estrous period, they are better mated separately from older ewes with experienced, though not necessarily older, rams.
Collection of Semen
The artificial vagina is used most commonly for collection of ram semen. It is prepared for collection by the introduction of warm water (100–130°F [40–55°C]) and air between the outer casing and soft inner sleeve, lubrication with petrolatum in the end where intromission of the penis occurs, and attachment of a graduated collecting glass at the opposite end. Rams quickly learn to mount a restrained ewe, and intromission and ejaculation are extremely rapid.
The second method of semen collection is by electroejaculation, for which the ram may be restrained on its side. The lubricated bipolar electrode is inserted into the rectum. The withdrawn penis is held with a piece of gauze to facilitate insertion of the glans into a 10- to 15-mm diameter graduated collecting tube. Ejaculation usually occurs after a few short electrical stimulations; “stripping” of the urethra may be helpful when expulsion of semen seems incomplete. Electroejaculation is less reliable than the artificial vagina; specimens vary in quality and can be contaminated with urine.
The volume of semen collected with the artificial vagina is 0.5–1.8 mL, and the concentration of the spermatozoa is 2.5–6.0 × 109/mL. Semen obtained by electroejaculation generally is of larger volume but lower concentration.
Evaluation of Semen
Immediately after collection, the semen is assessed for contamination, volume, concentration of spermatozoa, and sperm motility (wave motion and sperm progression).
Extension of Semen
Semen can be processed by extending or diluting, packaging, and storing. Semen may be extended 5-fold, depending on the initial concentration, the processing and storage method, and whether the semen will be used fresh, chilled, or frozen-thawed. Most semen extenders or diluents are based on Tris, egg yolk, and additional cryoprotectants such as glycerol. Commercial extender concentrates contain cryoprotectants and require the addition of egg yolk and double-distilled water. These extenders can be used for either fresh or frozen semen.
Extenders for fresh and chilled semen include whole, skimmed, or reconstituted cow's milk that has been heated to 92–95°C for 8–10 min in a water bath to inactivate toxic factors, egg yolk/glucose/citrate (15% egg yolk, 0.8% glucose [anhydrous], 2.8% sodium citrate dihydrate in glass-distilled water). The addition of Tris or glycerol improves the sperm survival of frozen-thawed semen. The reconstitution of frozen-thawed semen with fresh seminal plasma improves its fertilizing ability when used for intracervical insemination but not for intrauterine insemination. The number of motile spermatozoa and the volume of an insemination dose for the ewe depends on the site of insemination and the method of processing. For vaginal insemination, 0.3–0.5 mL with 300 million motile spermatozoa is used; for cervical insemination, 0.05–0.2 mL is used with 100, 150, and 180 million spermatozoa of fresh, liquid-stored, and frozen-thawed semen, respectively. Intrauterine insemination by laparoscopy requires 0.08–0.25 mL (with a total of 20 million motile spermatozoa) into each uterine horn.
Storage of Semen
Ram semen may be stored for up to 24 hr by cooling the extended semen to 35–41°F (2–5°C) over 90–120 min and by holding at this temperature. Fertility decreases rapidly and is low by 48 hr.
Freezing and storage of ram semen in 0.25–0.3 mL, 3-dose pellets or in 0.25 mL, single-dose synthetic straws at liquid nitrogen temperature (-320°F [-196°C]) is successful in maintaining sperm viability, but there may be a range in post-thaw motility and fertility between rams or processing batches. Use of frozen-thawed semen may result in lambing rates of 50% with cervical insemination and 50–80% with intrauterine insemination.
Freeze-thawing reduces the numbers of motile sperm. Chilling results in membrane changes that reduce the longevity of sperm. The membrane changes are similar to capacitation and acrosome reactions, and affected sperm are thus ready to fertilize oocytes. Fresh seminal plasma mitigates the effects of some of the capacitation changes.
Last full review/revision July 2011 by Paula I. Menzies, DVM, MPVM, DECS-RHM