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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
Department of Animal and Range Sciences, Montana State University, Bozeman 59717
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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Key Words: Biostimulation • Bovine • Bulls • Pheromone • Postpartum Interval
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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; Custer et al., 1990; Fernandez et al., 1993). The biological stimulus that must emanate from bulls to alter reproductive activity of postpartum suckled cows is unknown. Most investigators assume or speculate that bulls produce a primer pheromone that acts via an olfactory pathway to evoke this response (for review, see Rekwot et al., 2001). Barauh and Kanchev (1993) reported that bull urine sprayed into the nasal passages of dairy cows 7 d after calving increased systemic LH and FSH concentrations within 70 min of exposure. Likewise, Fernandez et al. (1996) found that postpartum cows exposed intermittently to bulls increased mean LH and LH pulse frequency at each 2-h exposure; however, cows did not resume ovarian cycling activity any earlier than control cows in either case.
There are no reports in the literature that have focused on the exact form of the sensory information produced by bulls that causes early resumption of ovarian cycling activity in postpartum anestrous cows. The nature of this sensory information does not seem to be related to the body configuration of bulls because androgenized cows can elicit the same effect as bulls in decreasing postpartum anestrus (Burns and Spitzer, 1992).
Our objective was to evaluate the influence of excretory products of mature bulls on the resumption of postpartum luteal function in primiparous, restricted-suckled beef cows. The hypotheses were that postpartum interval to resumption of luteal function would be decreased and that proportions of cows that resumed luteal function by the end of the experiment would increase when primiparous cows were exposed to excretory products of bulls.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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Sixty-two spring-calving, 2-yr-old Angus x Hereford crossbred primiparous cows, and five 3-yr-old epididymectomized Angus bulls were used in this experiment conducted at the Bozeman Livestock Teaching and Research Center, Montana State University. All experimental procedures and animal handling and care protocols used in this experiment were approved by the Montana State University Institutional Animal Care and Use Committee.
The calving season began January 28 and ended March 3. The experimental period extended from March 10 to May 18 (approximately 70 d). Cows and calves were maintained as a group, in a single pasture, from calving until d 35 ± 2 d (mean ± SE) after calving (d 0 of the experiment).
Cows were stratified by calving date, calf BW, sex of calf, dystocia score, cow BW, and BCS, and then assigned randomly to one of four treatments in a completely randomized design: 1) exposed continuously to the physical presence of a bull (BE; n = 15); 2) exposed to excretory products of bulls (EPB; n = 16); 3) exposed to excretory products of cows (EPC; n = 15); and 4) not exposed to a bull or excretory products of bulls or cows (NE; n = 16). Body weights and BCS (Richards et al., 1986) were obtained from each cow 2 wk after calving and on the last day of the experiment. At calving, each cow was assigned a dystocia score of 0, 1, 2, 3, or 5 (calving was unassisted, required some manual assistance, continuous manual assistance, mechanical assistance, or surgical intervention, respectively).
Cows exposed to bulls or excretory products of bulls had no contact with bulls throughout pregnancy and after calving until they were placed into treatments approximately 35 d (d 0) after calving. Cows exposed to excretory products of bulls, cows not exposed to bulls or exposed to excretory products of bulls or cows, and cows exposed to their own excretory products had no contact with bulls throughout pregnancy and the experiment.
Lots Used for Exposure Type
Two lots were used for this experiment, designated north and south by their geographic location. Each lot contained four pens (41 x 18 m) that were identical in east-west configuration, bunk space, aspect, slope, and connection to open-shed shelters. Lots were approximately 0.35 km apart, and the arrangement was such that the prevailing wind blew from the south to the north. Animals housed in one lot were not able to see or smell animals housed in the other lot; however, it may have been possible that sounds made by animals in one lot could be heard by animals in the other lot. These lots and pen arrangement within lots have proven to be effective in previous experiments involving bull–cow interactions (Fernandez et al., 1993, 1996). Pens within the north lot had not held bulls for approximately 9 mo, whereas pens in the south lot had not contained bulls for more than 3 yr. Pens within each lot were isolated from each other by draping and securing tarpaulins over the entire length of the 3-m-high fences that separated each pen.
Exposure Types
BE and EPB treatments. Pens within the north lot were used to maintain cows exposed to the physical presence of bulls (BE) and cows exposed to the excretory products of bulls (EPB), whereas pens within the south lot were used to maintain cows that were not exposed to bulls (NE) and cows exposed to the excretory products of cows (EPC). Cows assigned to treatments were placed into pens within each lot on d 0.
Cows assigned to the BE treatment were placed into the easternmost pen of the north lot containing a single bull. The bull-to-cow ratio for BE cows was 1:15. Cows assigned to the EPB treatment were placed into the westernmost pen of the north lot. This pen was adjacent to a pen that had a specially designed enclosure for exposing cows to the excretory products of bulls. The enclosure was approximately one-third the area of a pen (approximately 245 m2) and was used to alternately house bulls and cows during the course of each day. Bulls (n = 4) were placed into this enclosure at approximately 0800 and removed at 1830 (approximately 10.5 h). Cows in this treatment were then moved into the enclosure overnight between 1830 and 0800 (approximately 13.5 h) each day throughout the experiment. The enclosure was bedded with a thick mat of straw, and cows and bulls had free access to feed and water while in this enclosure. The enclosure was cleaned and rebedded every 4 to 5 d. Bulls were moved from the enclosure and maintained overnight in a pen approximately 0.75 km north of this lot. Cows in the EPB treatment had no visual contact with these bulls. Access to the enclosure was through a steel gate covered with a tarpaulin. The entire enclosure was made of plywood sheeting from the ground to approximately 0.66 m from the top of the shed, which allowed light and air to pass through the enclosure. The entire enclosure was surrounded by plastic wrap (4 mil thick), except for the gate. The distance between pens that contained BE and EPB cows was approximately 54 m.
NE and EPC treatments. Cows assigned to the NE treatment were placed into the easternmost pen of the south lot. These cows were maintained in this pen throughout the experiment. Cows assigned to be exposed to their own excretory products of cows (EPC) were placed into the westernmost pen of this lot, adjacent to a pen that had a specially designed enclosure for exposing cows to their own excretory products. The enclosure was identical to that of the enclosure used for cows and bulls in the EPB treatment. Cows in the EPC treatment were placed into this enclosure at 1830 and removed from it at 0800. The enclosure was left empty between 0800 and 1830, so that these cows were exposed daily to their own excretory products for approximately 13.5 h throughout the experiment. All other housing and cleaning aspects of this enclosure were the same as that for the enclosure used for cows in the EPB treatment.
Suckling
The following suckling strategy was employed in this study to equalize the suckling stimuli among treatments because the enclosure used for EPB and EPC cows could not accommodate calves during the period of enclosure. This strategy was a modified version of the model used by Stagg et al. (1998). In their model, cows were allowed to suckle calves once daily accompanied by isolation of cows from calves. They reported that this procedure resulted in a very rapid return to ovarian cycling activity, whereas suckling once daily with calves housed adjacent to dams resulted in a slower return to ovarian cycling activity. Our modification included one additional suckling bout (twice daily) to more closely simulate the effect of full-contact suckling in primiparous cows.
Beginning on d 0, cows in all treatments were restricted to suckling twice daily: once in the morning from 0800 to 0830 and again in the evening from 1800 to 1830. Between suckling events, calves had no physical contact with their dams. This was accomplished by housing and maintaining calves of BE and EPB cows in the south lot adjacent to pens in which EPC and NE cows were maintained, respectively, whereas calves of EPC and NE cows were housed and maintained in the north lot adjacent to pens that housed BE and EPB cows, respectively. Calves were moved to the pens that contained their dams for each suckling bout (morning and evening). Under these conditions, BE and NE cows were in fence-line contact with "alien" calves throughout the experiment, whereas EPB and EPC cows were in fence-line contact with "alien" calves during the daylight period only throughout the experiment.
Nutrition
Cows and calves had free access to good-quality chopped mixed-grass alfalfa hay and any pasture grasses that were available before they were moved into their respective treatment pens. After cows were moved into treatment pens, they were given free access to the same hay, 0.25 kg of cracked barley per animal daily, water, and a mineralized-salt supplement until the end of the experiment. The TDN of the diet on a DM basis exceeded the NRC requirement for lactating beef cows with a mature BW of 545 kg by approximately 18% (NRC, 1996). Bulls were fed the same diet as cows.
Blood Sampling for Progesterone Analysis
Blood samples (7 mL per sample) were obtained from each cow in each treatment by jugular venipuncture starting on d –1 (d 0 = 35 d after calving and start of treatment), and every 3 d thereafter until the end of the experiment. Serum was harvested and stored at –20°C until assayed for progesterone. Progesterone was assayed using solid-phase RIA kits (Diagnostic Products Corp., Los Angeles, CA) validated in our laboratory for bovine serum (Custer et al., 1990). Intra- and interassay CV for a serum pool that contained 0.45 ng/mL were 9.4 and 13.3%, respectively; for a pool that contained 3.5 ng/ mL, CV were 6.8 and 9.6%, respectively. Progesterone concentration patterns were used to assess the resumption of ovarian cycling activity. An increase in baseline progesterone concentrations in three consecutive samples that exceeded 1.0 ng/mL during the experimental period was used as the criterion for resumption of ovarian cycling activity. The graphic representation of the pattern of progesterone concentrations used to validate this criterion is given in Fernandez et al. (1993) for primiparous suckled beef cows.
Statistical Analyses
Calving date, calf BW, calf sex ratio, dystocia score, cow BW and BCS, and change in cow BW and BCS over the experimental period were analyzed by separate ANOVA for a completely randomized design using PROC GLM of SAS (SAS Inst., Inc., Cary, NC). The model included treatment only as the independent variable. Treatment means were evaluated with the PDIFF option of SAS. Calving date (40.4 ± 2 d of yr; mean ± SD), calf BW (33.1 ± 5.3 kg), calf sex ratio (male-to-female calves; 0.46 ± 0.1), dystocia score (1.5 ± 0.2), cow BW (499 ± 33 kg) and BCS (4.3 ± 0.26) at the start of the experiment, and change in cow BW (20.4 ± 16.5 kg) and BCS (0.27 ± 0.16) during the experiment did not differ among treatments, indicating that the stratification process was successful.
Postpartum interval to resumption of luteal function for cows that had not exhibited an increase in progesterone by the end of the experiment was calculated by assuming that resumption of luteal function occurred on the last day of the experiment and then subtracting d 0 or calving date from the last day of the experiment. Data for postpartum interval to resumption of luteal activity from d 0 were analyzed by a one-way ANOVA for a completely randomized design using the GLM procedure of SAS. Treatment was the independent variable included in the model. Treatment means were compared using the PDIFF option of SAS.
Proportions of cows among treatments that exhibited resumption of luteal function by the end of the experiment were analyzed by a contingency 
2 test using PROC FREQ of SAS. Cumulative percent distributions for proportions of cows that resumed luteal function were constructed for 10-d intervals after d 0. Proportions of cows among treatments that resumed luteal function for each 10-d interval were analyzed by contingency 2 analyses using PROC FREQ of SAS. Nonlinear regression analyses were performed on the cumulative distribution of percentages of cows among treatments that resumed luteal function at 10-d intervals from d 0 to the end of the experiment using the curve-fitting software of SigmaPlot (Systat Software, Inc., Point Richmond, CA).
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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), but it did not differ between BE and EPB cows. Postpartum interval to resumption of luteal function for EPC cows did not differ from that for NE cows; however, interval to resumption of luteal function for EPC cows was longer (P = 0.06) than that for BE cows and was longer (P < 0.05) than that of EPB cows (Figure 1).
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) indicated that proportions of cows resuming luteal function by d 30 did not differ among BE, NE, and EPC cows, but the proportion of EPB cows that resumed luteal function was greater (P < 0.05) than that for NE cows by this day. Conversely, proportions of cows that resumed luteal function by d 30 did not differ among BE, EPB, and EPC cows. More (P < 0.05) BE and EPB cows resumed luteal function by d 40 than either NE or EPC cows. Proportions of cows that resumed luteal function by d 50 did not differ (P = 0.26) between NE and EPC cows; however, the proportions of BE and EPB cows that resumed luteal function by this day were greater (P < 0.05) than those of NE and EPC cows. Proportions of BE, EPB, and EPC cows that resumed luteal function did not differ (P = 0.52) but were greater (P < 0.05) than that for NE cows by d 60 and 70. Proportions of BE and EPB cows that resumed luteal function did not differ (P = 0.47) by d 70, whereas the proportion of EPB cows that resumed luteal function was greater (P = 0.06) than that of EPC cows. Moreover, the proportion of BE cows that resumed luteal function did not differ (P = 0.62) from that of EPC cows (Table 1).
Data for cumulative distribution of percentages of cows among treatments that resumed luteal function at 10-d interval from d 0 to the end of the experiment were fitted to a sigmoidal-shaped curve with three parameters: y = a/(1 + {e–[(x – x0)/b]}), where y = estimated %; a = maximum %; e = 2.718; x = days; x0 = days at 50% of maximum %; and b = slope, %/d. The results of the curve fitting procedure are depicted in Figure 2. Coefficients of determination were 0.993, 0.994, 0.983, and 0.994 for curves generated from data of BE, EPB, EPC, and NE cows, respectively. Estimates of a, x0, and b were 88.6 ± 2.3% (± SE), 38.8 ± 0.9 d, and 6.3 ± 0.7%/d for BE cows; 102.4 ± 3.3%, 36.0 ± 1.1 d, and 9.0 ± 0.9%/d for EPB cows; 85.7 ± 7.1%, 51.0 ± 1.6 d, and 4.9 ± 1.4%/d for EPC cows; and 57.4 ± 4.4%, 59.6 ± 1.3 d, and 6.0 ± 0.8%/d for NE cows. All estimates of these coefficients differed from 0 (P < 0.05). Visual examination of these patterns and estimates of the parameters used to generate these curves indicated that the distributions for BE and EPB cows differed from those of NE and EPC cows. Furthermore, the distribution for EPC cows seemed to be different from that of NE, BE, and EPB cows.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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; Fernandez et al., 1993, 1996; Fike et al., 1996). More important is the question of whether resumption of luteal function was altered by exposing cows to excretory products of bulls. In answer to this question, we found that postpartum interval to resumption of luteal function from d 0 (d 35 after calving) was significantly decreased, and the proportion of cows that resumed luteal function by the end of the experiment was greater for cows exposed to excretory products of bulls (EPB cows) for 12 to 14 h daily than for cows not exposed to bulls (NE cows). Although this is an important result, there also is the question of whether the responses shown by cows exposed to the excretory products of bulls mimic those of cows exposed to a bull. Examination of the postpartum intervals from d 0 to the resumption of luteal function; cumulative percentages of cows that initiated luteal function at 10-d intervals beginning on d 0; fitted patterns of the cumulative distribution of percentages; and proportions of cows that resumed luteal function by the end of the experiment indicated that the pattern of response for these variables did not differ between cows exposed to a bull (BE cows) and cows exposed only to the excretory products of bulls (EPB cows). The interpretation of these observations is that exposing postpartum anestrous cows to excretory products of bulls had the same biostimulatory effect as exposing cows to the continuous presence of a bull.
Another unique finding in the present study was that postpartum interval from d 0 to resumption of luteal function for EPC cows was intermediate between the intervals for NE and BE cows; however, the proportions of EPC cows that resumed luteal function by 60 d and by the end of the experiment were greater than those for NE cows. In fact, they were similar to those observed in BE cows. Cows exposed to excretory products of cows (EPC cows) were isolated from the bull or excretory products of bulls, but they were enclosed for 12 to 14 h daily and exposed to their own excretory products during this time. This treatment was included to act as a control for social and environmental factors that were associated with the housing of cows within the special enclosures. It is interesting to note that this treatment decreased the postpartum interval to resumption of luteal activity relative to that for cows not exposed to a bull. An explanation for this result is not obvious. Perhaps this observation is a manifestation of a female-to-female interaction mediated by a primer pheromone excreted in the urine, feces, or other bodily fluids of cows, similar to that of the excretory products of bulls. This idea is supported by the results of several studies. Burns and Spitzer (1992) reported that intact, androgenized cows decreased the interval to resumption of luteal function to the same extent as bull exposure. This result indicated that exteroceptive cues for the biostimulatory effect of bulls are androgen-dependent and can be expressed by intact adult cows. Thereafter, Wright et al. (1994) found that the proportion of postpartum anestrous cows isolated from other cows and exposed to an estrual cow or cervical mucus discharge of estrual cows, which resumed luteal function, was greater than that of cows not exposed to an estrual cow or exposed to distilled water. These studies provide evidence for a possible female pheromonal action that may influence early onset of ovulatory cycles in postpartum anestrous cows. That this may be the case in the present study is illustrated by the result that as soon as 33% of cows exposed to their own excretory products initiated luteal function, resumption of luteal function in the remaining anovulatory cows increased by more than 50% within 21 d. Thus, it would seem that crowding cows in an enclosure may have exposed anestrous/anovulatory cows to a pheromone(s) present in the cervical mucus or excretory products of estrual cows and accelerated resumption of luteal function in a high proportion of these cows.
For decades, researchers in this area have speculated that the biostimulatory effect of bulls on reproductive activity of postpartum anestrous cows is mediated by pheromones released by bulls (for reviews, see Izard, 1983; Rekwot et al., 2001); however, there is no direct scientific evidence for this idea in the literature. The findings of the present study are the first report of compelling data, under controlled conditions, that support the concept that the biostimulatory effect of bulls to accelerate resumption of ovarian cycling activity in postpartum cows is mediated by biochemical signalers (pheromones) presented in urine, feces, and other bodily fluids of males. Our study was not designed to specifically determine what particular component of excretory products of bulls contains the pheromone(s) that might mediate this effect. Nonetheless, most pheromones, but not all, that involve the male-to-female interactions that affect ovarian activity seem to be transmitted in the urine of males in many wild, domestic, and laboratory species (for review, see Vandenbergh, 1983). Further study is necessary to determine the actual source and biochemical nature and mechanism of transference of this pheromone(s).
In conclusion, the biostimulatory role of bulls to decrease the postpartum anestrous interval to resumption of luteal function in primiparous suckled beef cows seems to be mediated via a pheromonal mechanism involving the excretory products of bulls. The result of this pheromonal interaction is to temporally accelerate the reproductive neuroendocrine-endocrine cascade that culminates in the resumption of ovulation and luteal function in postpartum anovulatory cows.
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Footnotes |
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3 Current address: The Ohio State Univ., 185 Hamilton, 1645 Neil Ave., Columbus 43210.
2 Correspondence—phone: 406-994-5574; fax: 406-994-5589; e-mail: jgb{at}montana.edu.
Received for publication February 23, 2005. Accepted for publication June 1, 2005.
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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