Crossbred ewe lambs of eight genetic types (Dorset X 3/4 Dorset, DD; Dorset X 3/4 Finn, DF; Finn X 3/4 Dorset, FD; Finn X 3/4 Finn, FF; Romanov X 3/4 Dorset, RD; Romanov X 3/4 Finn, RF; Romanov X Western, RW; and Western X Western, WW) were used to determine age and weight at conception, conception rate, ovulation rate, litter size and prenatal mortality. Each cross was represented in five different lamb crops born in winter, spring, summer and fall. High conception rates (greater than 88%) were obtained in all crops of ewe lambs, even in those born in June and exposed to rams the same year. Only Western ewe lambs born in June and exposed to rams in year of birth achieved low conception (59%). Ewe lambs born in October and missing the breeding season in the year of birth were more than 100 d older than ewe lambs born earlier in the calendar year and bred the same year. Prenatal mortalities varied widely (8 to 18%) among lamb crops, with management suspected to be a factor. Additive genetic effects in ovulation rate and litter size of the Romanov were similar to that of Finn. Romanov crosses (RF, RD, RW) conceived at the youngest age (226, 227, 231 d), demonstrating that Romanov is an early-maturing breed.

Crossbred ewe lambs of eight genetic types (Dorset X �90 Dorset, DD; Dorset  �90 Finn, DF; Finn  �90 Dorset, FD; Finn X sA Finn, FF; Romanov  �90 Dorset, RD; Romanov  �90 Finn, RF; Romanov  Western, RW; and Western  Western, WW) were used to determine age and weight at conception, conception rate, ovulation rate, litter size and prenatal mortality. Ninety-eight percent of Iambs were pregnant. Age at conception in days, ovulation rate (number of corpora lutea) and litter size for the eight groups were as follows: DD, 236  5, 1.5  .1, 1.3  .2; DF, 218  4, 1.5 +-.1, 1.4 234177 1.8  228  4, 2.6  .l, 2.3 209

A group of crossbred Dorset or Finnish Landrace ewes maintained under a synthetic light regimen (Light Treatment, 4 mo long days - 16 h light, 4 mo short days - 8 h light) and exposed to rams every 8 mo (January 1, September 1, May 1) was compared for lamb production over a period of 4.5 yr with two other comparable groups under natural daylight conditions, one exposed to rams once a year in the fall (Control I) and the other every 8 mo (Control II). Conception rates across breed type of ewe were 92% for Control I, 66% for Control II and 83% for Light Treatment ewes. Conception rates for May breedings only were 16% in Control II and 88% in Light Treatment ewes. Prolificacies of crossbred Finn ewes were higher (P less than .05) than those in crossbred Dorset ewes of Control I (258 vs 193%), Control II (187 vs 165%) and Light Treatment (238 vs 163%). The annual production at lambing from the crossbred Finn and Dorset ewes in Control I was 251 and 206 lambs/100 ewes exposed to rams, respectively. Corresponding productions were 231 and 178 for Control II and 296 and 211 for Light Treatment ewes. Mortality of lambs from the crossbred Finn ewes (27%) was higher (P less than .01) than that from the crossbred Dorset ewes (12%). Annual attrition of the crossbred Dorset ewes in Control I, Control II and Light Treatment groups was 5.2, 5.4 and 5.4%, respectively. Corresponding percentages of the crossbred Finn ewes were 7.0, 8.8 and 11.2.(ABSTRACT TRUNCATED AT 250 WORDS)

The relationship between daily sperm productions (DSP) and daily sperm output (DSO) was determined in eleven stallions. The DSO was determined by collecting single ejaculates, with an artificial vagina, at 24-hr intervals. The stallions were killed 24 hr after the last collection and the DSP was determined by quantitative testicular histology. The mean DSP was 8-0 X 10(9) (S.E. +/- 0-4 X 10(9), and the mean DSO was 7-0 X 10(9) (S.E. +/- 0-4 X 10(9)). It was estimated that 87% of the spermatozoa produced by the testes were harvested. The correlation between DSP and DSO was 0-80 (P less than 0-01), and between DSP and testis weight was 0-77 (P less than 0-01).

The cycle of the seminiferous epithelium was divided into eight stages on the basis of meiotic divisions, shape of the spermatid nuclei and location of the spermatids with elongated nuclei. Duration of this cycle was 12-2 days (S.E. +/- 0-1) as determined by [3H]thymidine injections and autoradiography. The life-span of primary spermatocytes was 19-0 days, secondary spermatocytes 0-7 days, spermatids with round nuclei 8-7 days, and spermatids with elongated nuclei 10-1 days. Labelled spermatozoa entered the caput epididymidis 35 days, and appeared in the ejaculate 39-9 days, after the isotope injection. The mean minimal time required for spermatozoa to pass through the excurrent ducts was 4-9 days. However, the average transit time was estimated to be between 8 and 11 days. Variation in the duration of the cycle between species is discussed.

Daily sperm production (DSP) was estimated by quantitative histological analysis in two groups of stallions. The DSP was 7.8 billion (109) for stallions in Group I (sacrificed in June 1971) which was not significantly different from the 8.1 billion for stallions in Group II (sacrificed in July 1972). However, sperm production per gram of testicular parenchyma averaged 19.3 × 106 for stallions in Group I compared to 22.3 × 106 for stallions in Group II(P< .05). Daily sperm output (DSO) accounted for 98% and 81% of the DSP of stallions in Groups I and II, respectively. The correlation between DSO and DSP was 0.80 (P< .01), while the correlation between DSP and testicular weight was 0.77 (P<.01).

Two experiments were conducted to determine the effects of regular semen collections at 96- and 72-hr. intervals, starting when the boars were 198 and 206 days old, respectively, on subsequent semen characteristics and development of the reproductive organs. In the first experiment, 44 boars averaging 198 days (SE ± 1) of age and weighing 105 kg (SE ± 1) were assigned to five groups differing in semen quality. The different groups were placed on a regular 96-hr, semen collection schedule at 198, 235 and 271 days of age, and the experiment terminated when the boars were 328 days old. Semen characteristics of boars ejaculated since they were 198 days old were similar to semen characteristics of boars sexually rested during the first part of the experiment. Furthermore, semen quality improved more rapidly with time in boars that had initially low quality semen than in boars that had initially high quality semen. Thus, boars that had low numbers of total and motile sperm per ejaculate at 6.5 months of age, did not necessarily have low numbers of total and motile sperm per ejaculate when they reached 10 months of age. In the second experiment, 48 boars averaging 206 days (SE ± 1) of age and weighing 106 kg (SE ± 1) were assigned to three groups so that the groups were similar as to age, body weight and mean number of motile sperm in two ejaculates. The groups were placed on a regular 72-hr. semen collection schedule at 206, 245 and 281 days of age, and the experiment terminated when the boars were 315 days old. Semen characteristics of boars sexually rested until they reached 245 and 281 days of age were similar to semen characteristics of boars ejaculated since they were 206 days old. Age effects on semen characteristics were most pronounced when boars aged from 6.5 to 8.5 months. The development of the testes and accessory sex glands was not significantly influenced by regular 96- or 72-hr, ejaculation starting when the boars were 6.5 to 6.8 months of age.

Summary. The cycle of the seminiferous epithelium of the stallion was divided into eight stages, using as criteria the presence of meiotic divisions, shape of the spermatid nuclei and location of spermatids with elongated nuclei in the tubule. The mean frequencies of stages 1 to 8 were 16·9, 14·9, 3·2, 15·8, 7·4, 13·5, 12·6 and 15·7%, respectively. The duration of one cycle of the seminiferous epithelium was 12·2 days (S.E.±0·1) as determined by injecting a single dose of 700 μCi of [3H]thymidine into each spermatic artery of six stallions and removing testes at different intervals after the isotope injection. The life-span of primary spermatocytes was 19·0 days, secondary spermatocytes 0·7 days, spermatids with round nuclei 8·7 days, and spermatids with elongated nuclei 10·1 days. Radioactive spermatozoa were observed in the caput epididymidis 35 days after [3H]thymidine injection. The volumetric percentages of testicular components were: spermatogonial nuclei, 0·6; primary spermatocyte nuclei, 4·2; secondary spermatocyte nuclei, 0·1; round spermatid nuclei, 2·1; elongated spermatid nuclei, 1·0; Sertoli cell nuclei, 1·6; tubular cytoplasm, 45·7; lumina, 3·4; basement membranes, 2·6; and intertubular spaces, 38·7%. The seminiferous tubules made up 61·3% of the testicular volume. The diameters of the seminiferous tubules varied significantly among stallions, but not among stages. The average length of the seminiferous tubules per testis was 2419 m (range 1667 to 3726 m).

Seven semen characteristics were studied in two ejaculates collected at 24-h intervals from each of 41 Yorkshire (body weight 86 ± 6 kg) and 54 Lacombe (body weight 95 ± 8 kg) boars between 21 and 26 wk of age (group 1), and from each of 56 Yorkshire (body weight 94 ± 7 kg) and 47 Lacombe (body weight 107 ± 10 kg) boars between 27 and 31 wk of age (group 2). Means for certain semen criteria of the Yorkshire and Lacombe boars in group 1 were, respectively: semen volume 180 ml, 184 ml; sperm per ml 111 × 106, 104 × 106; total sperm per ejaculate 15.9 × 109, 13.9 × 109; motility 58%, 53%. Means for these criteria of group 2 Yorkshire and Lacombe boars were, respectively: semen volume 194 ml, 199 ml; sperm per ml 202 × 106, 126 × 106; total sperm per ejaculate 29.5 × 109, 17.8 × 109; motility 66%, 55%. Group 1 boars had fewer sperm per ml (P < 0.01), fewer total and motile sperm per ejaculate (P < 0.01), and fewer progressively motile sperm (P < 0.05) than group 2 boars. First ejaculates differed (P < 0.01) from second ejaculates for all criteria measured except percent motile sperm. In general, Yorkshire boars between 21 and 31 wk of age had better quality semen, as measured by sperm per ml and motile sperm per ejaculate, than Lacombe boars. A third ejaculate was collected 24 h after the second from 33 Yorkshires in group 1 and from 17 Yorkshires in group 2. Most semen criteria showed significant changes from first to second and from second to third ejaculates, but correlations among ejaculates for all criteria were significant. Nine 36-wk-old, six 86-wk-old, and seven 139-wk-old Yorkshire boars were ejaculated at 72-h intervals for 63 days, followed by daily ejaculations for 13 days. Mean semen criteria for the 36-, 86-, and 139-wk-old groups on the 72-h collection schedule were, respectively: semen volume 170 ml, 368 ml, 404 ml; sperm per ml 209 × 106, 128 × 106, 146 × 106; total sperm per ejaculate 25.6 × 109, 36.8 × 109, 45.9 × 109; motility 79%, 79%, 79%. On the 24-h collection schedule the means for these criteria were, respectively: semen volume 157 ml, 277 ml, 316 ml; sperm per ml 93 × 106, 39 × 106, 56 × 106; total sperm per ejaculate 9.9 × 109, 8.3 × 109, 12.8 × 109; motility 79%, 80%, 78%. On an ejaculate basis, the three groups showed highly significant decreases in all semen criteria except gel volume and motility when the ejaculation interval was changed from 72 to 24 h. Sperm output per unit time was greater on the 72- than on the 24-h collection schedule.

Four methods of measuring daily sperm production (DSP) were evaluated using seventy-two boars.The DSP was determined; I, by unilaterally castrating boars 0 and 48 hr after depletion of the epididymal sperm reserves and then measuring changes in the reserves (fourteen boars) ; II, by measuring changes in the epididymal sperm reserves of four groups of boars killed 5, 24, 52 and 72 hr following depletion of the reserves (forty-six boars) ; III, by dividing the total number of sperma- tozoa in the right and left epididymis of sexually active boars by the epididymal transit time (twelve boars) ; IV, by quantitative testicular histology (same twelve boars as for Method III).Method I was un- suitable for estimating the DSP of individual boars because of normal variation in sperm numbers between epididymides within boars.Method II revealed that there was too much variation in epididymal sperm reserves among boars to obtain a reliable mean estimate of DSP for the forty-six boars.Method III did not give reliable results for individual boars because epididymal transit time varies among boars.It was concluded that the DSP of individual boars could best be measured by Method IV though Methods III and IV gave similar estimates for the mean DSP of the twelve boars (15\ m=.\ 0\ m=x\ 109 versus 16\m=.\2\m=x\ 109).The sperm output of these boars averaged 15\m=x\5\m=x\109/day when ejaculates were collected at 72-hr intervals, suggesting little or no absorption of sperma- tozoa from the epididymides or excretion of spermatozoa in the urine of sexually active boars.The mean DSP/g of net testis was 24\m=.\5\m=x\106 (S.E.\m=+-\0\m=.\3)as determined by quantitative histology.Only spermatozoa from the cauda epididymidis were ejaculated during depletion.

The structural composition of the testes, the sperm output, and the daily sperm production were determined in seven 18-month-old Shorthorn bulls. The testicular components classified composed the following percentages of the total testis volume: spermatogonium nuclei,.62%; primary spermatocyte nuclei, 4.57%; secondary spermatocyte nuclei,.16%; spermatid nuclei, 3.52%; spermatozoa,.34%; tubular cytoplasm, 57.79%; lumen, 4.67%; basement membrane, 2.60%; intertubular space, 23.63%; and Sertoli cells, 2.04%. The seminiferous tubules made up 76.4% of the testis volume. It was estimated that the testes contained on the average 1630 m (range 960–2240 m) of seminiferous tubules. The tunica albuginea comprised 12.8% of the testis weight.Three procedures are described for measuring daily sperm production from quantitative testicular histology, and the results obtained with Shorthorn bull testes are presented. The limitations of the three procedures are discussed. It was estimated that 18-month-old Shorthorn bulls produced 5.3 × 109 sperm per day. This represented a production of 16.9 million sperm per gram of testis per day. Within a given species sperm production appears to be primarily a function of testis size. The seven bulls averaged 2.7 × 109 sperm per ejaculate when semen samples were collected every other day with an electroejaculator. This represented about 25% of the sperm production by the testes.