Effects of prepartum lipid supplementation on FSH superstimulation and transferable embryo recovery in multiparous beef cows

Animal Reproduction Science 85 (2005) 61–70

Effects of prepartum lipid supplementation on FSH superstimulation and transferable embryo recovery in multiparous beef cows夽 J.F. Bader a , F.N. Kojima a , M.E. Wehrman b , B.R. Lindsey c , M.S. Kerley a , D.J. Patterson a,∗ a

S132 Animal Science Research Center, University of Missouri, Columbia, MO 65211, USA b Rocky Mountain Reproduction Services., Manhattan, MT 59714, USA c Minitube of America, Verona, WI 53593, USA

Received 30 December 2003; received in revised form 12 April 2004; accepted 12 April 2004

Abstract The objective of this experiment was to determine the effect of prepartum lipid supplementation on the number and quality of embryos recovered following ovarian super-ovulation in postpartum suckled beef cows. Mature cows (n = 40) were assigned to one of two treatments (lipid versus. no lipid) and supplemented for approximately 40 days prior to calving. Supplements provided to cows were isocaloric and isonitrogenous. The treatment group was fed 1.6 kg hd−1 per day of whole soybeans (WSB; 19.8% ether extract, and 41.8% crude protein) and the control group received a supplement consisting of 1.8 kg hd−1 day of a soybean meal and soy–hull combination (SBS; 2.15% EE and 36.81% CP). Cows were synchronized using a GnRH [Cystorelin® 100 ␮g im]–GnRH–PGF2␣ [Lutalyse® 25 mg im] protocol. Cows were administered two injections of GnRH seven days apart and PG seven days after the second GnRH injection. Twenty-eight cows (WSB, n = 15; SBS, n = 13) responded to estrus synchronization and were superstimulated. Super-ovulation was initiated on day 8–10 of the synchronized cycle by twice-daily injections of pFSH (Pluset® ) over four days in decreasing doses using a total of 608.4 IU per cow. Prostaglandin F2␣ was administered 96 and 108 h after super-stimulation was initiated with FSH. Days postpartum (WSB = 59 days; SBS = 57 days) at initiation of FSH treatments were similar (P > 0.10) for both treatments. Cows were monitored for estrus activity by the HeatWatch® Estrus Detection System. Twenty-seven cows (WSB, n = 15; SBS, n = 12) exhibited estrus after FSH and inseminated at 0, 12, and 24 h after the onset of estrus with 1, 2, and 1 units of semen, respectively. Embryos were recovered and evaluated 7–8 days later. Only cows that responded to FSH and that were inseminated were used for statistical analysis. Data were analyzed using the General Linear Models Procedure of SAS. Body condition scores did not ∗ Corresponding author. Tel.: +1 573 882 7519; fax: +1 573 882 4798. E-mail address: [email protected] (D.J. Patterson). 夽 Contribution from the Missouri Agriculture Experiment Station

0378-4320/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.anireprosci.2004.04.033

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differ (P > 0.10) between treatments when cows were evaluated at the initiation of the experiment, two weeks prior to calving, and at initiation of superovulation with FSH. Estrous cyclicity prior to the initiation of estrus synchronization did not differ (P > 0.10) between treatments. There was no difference (P > 0.10) between treatments in recovery of total embryos (WSB, 14.7 ± 3.5; SBS, 17.5 ± 3.0), transferable embryos (WSB, 10.3 ± 2.5; SBS, 13.6 ± 2.6), degenerate embryos (WSB, 3.3 ± 1.1; SBS, 1.6 ± 1.7) or unfertilized ova (WSB, 1.1 ± 0.5; SBS, 2.3 ± 1.2). Cows that were supplemented with whole soybeans prior to parturition failed to produce an increased total number of ova or transferable embryos following super-ovulation. © 2004 Elsevier B.V. All rights reserved. Keywords: Beef cows; Lipid supplementation; Super-ovulation; Embryo

1. Introduction Meeting the nutrient requirements of beef cattle is critical in assuring optimal reproductive performance. Lipid supplementation enhanced reproductive function in beef cows independent of dietary energy intake (Wehrman et al., 1991). Supplemental fat may partially alleviate negative energy balance during the early postpartum period in dairy cows, although in many cases the positive influence of supplemental fat on reproductive traits occur independently of the cow’s energy status (Staples et al., 1998). Changes in ovarian function and metabolism can occur through intake of dietary fat (Hightshoe et al., 1991; Williams, 1989). Thomas et al. (1997) reported that consumption of polyunsaturated fatty acids stimulated a greater number of medium sized follicles in cattle compared with intake of saturated and highly polyunsaturated fatty acids. Characteristics of the intra-follicular environment to which the pre-ovulatory oocyte is exposed may be a major factor influencing the variability in embryo recovery and viability (Espey, 1981). The possibility that lipid supplementation would influence the number and quality of embryos was first evaluated in beef cows that were supplemented with lipids during the postpartum period (Ryan et al., 1992; Thomas and Williams, 1996). The hypothesis that prepartum lipid supplementation could influence the number or quality of embryos recovered from donor females following super-ovulation originates from previous research in our laboratory. Graham et al. (2001) reported a 23% improvement in first service conception rate among postpartum suckled beef cows that were fed 1.6 kg of whole soybeans for a 40-day-period that preceded parturition. This improvement in first service conception rate occurred despite similarities between whole soybean supplemented and control groups in prebreeding estrous cyclicity rates, body condition score (at calving or breeding), estrous response during the synchronized period, and final pregnancy rate. Based on the previous study from our laboratory we hypothesized that prepartum lipid supplementation may potentiate increased numbers of transferable embryos among superovulated donor females. The objective of this study was to compare recovery rates that included the total number of ova and transferable embryos obtained following super-ovulation of mature, suckled beef cows that were supplemented with a whole soybean or a soybean meal and soybean hull supplement during the prepartum period.

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2. Materials and methods 2.1. Experimental supplements Mature suckled beef cows (n = 40) were randomly assigned to receive either a supplemental treatment consisting of whole soybeans (WSB) or a soybean meal and soybean hull supplement (SBS; control supplement) Supplements were offered for approximately 40 days prior to parturition. Cows were acclimated to bunk feeding for two days with the supplementation of soyhulls before treatments began. Supplements were formulated to be isoenergetic and isonitrogenous. The experimental group received 1.6 kg hd−1 per day of whole soybeans (19.8% ether extract, 41.8% crude protein) and the control group received 1.8 kg hd−1 per day of soybean meal and soybean hull supplement (2.15% EE and 36.81% CP). All cows selected for this experiment conceived on the same day and were bred artificially to the same sire during the previous breeding season. Calf birth weights were recorded at parturition to compare treatments and their effects on calf birth weight. Cows were removed from supplemental treatments at calving and both groups were combined to ensure similar intake of forage during the postpartum period. Body weight and body condition score (BCS) were recorded before cows were allocated to treatments. Weight and BCS were recorded again 10 days prior to calving and at the initiation of FSH treatment. 2.2. Supplement analysis Supplement samples (Table 1) were dried in a 55 ◦ C forced air oven and ground (Willey Mill; Thomas Scientific, Swedensboro, NJ) to pass through a 2 mm screen. Nitrogen analysis of feed was completed using thermoconductivity (LECO Corporation, St. Joseph, MI) with crude protein being calculated by multiplying nitrogen by 6.25. Dry matter and ether extract were determined by AOAC methods (1984). For total fatty acid determination of supplements (Table 2), ether extract from supplements was evaporated under nitrogen at 55 ◦ C to dryness (N-EVAP Analytical Evaporation; Organomation Assoc., Inc., Berlin, MA) after filtering through granular anhydrous sodium sulfate. A small aliquot of remaining oil was transferred to a clean 15 ml screw top glass culture tube and mixed with 0.5 ml Table 1 Chemical composition of whole soybeans (WSB) and the control supplement (SBS) fed to cowsa Supplement WSB SBS a

Dry matter (%)

Organic matter (%)

Ash (%)

Ether extract (%)

Crude protein (%)

90.3 90.8

93.6 93.8

6.4 6.2

19.8 2.2

41.8 35.9

Determined by AOAC approved procedures.

Table 2 Fatty acid composition (%) of whole soybeans (WSB) and the control supplement (SBS)a Supplement 16.0 (palmitic) 18.0 (stearic) 18.1 (oleic) 18.2 (linoleic) 18.3 (linolenic) Total 18:2 and 18:3 WSB SBS a

11.0 15.8

4.5 5.1

25.0 17.7

Determined by acid methylation of ether extracted lipids.

51.9 47.9

6.8 9.8

58.7 57.7

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benzene containing trinonadecanoin (triglyceride 19:0; 2 mg/ml) as an internal standard. Benzene was used as an aid in solublizing dietary triglycerides. Methyl esters of fatty acids were formed by adding 4 ml of 4% sulfuric acid in anhydrous methanol to the culture tubes containing samples and heating capped at 90 ◦ C for 60 min. After heating, reactions were stopped by adding 3.0 ml double-distilled H2 O. Methyl esters were extracted by adding 8.0 ml of chloroform, vortexing and centrifuging at 900 × g to aid in phase separation. The chloroform layer was transferred through a sodium sulfate filled glass Pasteur pipet into 15 ml glass screw-top conical bottom tubes and evaporated to dryness under nitrogen. Fatty acid methyl esters were resuspended in heptane and determined using a gas chromatograph equipped with a flame ionization detector and integrator (Varion Model 3400; Varian Associates, Walnut Creek, CA). The column used was a fused silica capillary column with helium as the carrier gas (30 m × 0.25 ␮m film thickness; Supelco SP 2380; Bellefonte, PA). The initial oven temperature was 150 ◦ C, held for 7 min and increased at a rate of 2 ◦ C per min until reaching a final temperature of 220 ◦ C. The injector and detector temperatures were set at 250 and 240 ◦ C, respectively. A mixture of either plant or animal derived fatty acid methyl esters were used as external standards for peak identification (PUFA I and PUFA II; Supelco; Bellefonte, PA). Reported chemical analysis is the average nutrient composition of batch and ingredient samples. All chemicals were purchased through either Fisher Scientific (Pittsburgh, PA) or Sigma (St. Louis, MO). 2.3. Estrus synchronization and superstimulation Cows were synchronized using a GnRH (Cystorelin® , Merial, Athens, GA: 100 ␮g im)–GnRH–PGF2␣ (PG: Lutalyse® Sterile Suspension, Pfizer Animal Health, New York, NY: 25 mg im) protocol. Cows were administered two injections of GnRH 7 days apart and PG 7 days after the second GnRH injection (DeJarnette et al., 2001; Kojima et al., 2000). Estrus activity was monitored from 36 to 144 h after PG to determine response to estrus synchronization (n = 28: WSB, n = 15; SBS, n = 13). Cows were observed three times daily for estrus activity and K-mar® heatmount detectors (Kamar, Inc. Steamboat Springs, CO) were used as an aid in estrous detection. Ovarian superstimulation was initiated on days 8–10 of the synchronized estrous cycle by twice daily treatments of pFSH (Pluset® , Calier, Barcelona, Spain) over 4 days in decreasing doses (4, 4, 3, 3, 1.5, 1.5, 1 and 1 ml) using a total of 608.4 IU per cow. Prostaglandin F2␣ was administered on AM and PM (12 h apart) of the last day of FSH treatment. Cows were monitored for estrus continuously after FSH by the HeatWatch® Estrus Detection System (DDx Inc., Denver, CO). Estrus was defined by HeatWatch when cows exhibited three or more mounts, greater than or equal to 2 s in duration over a 4 h period. Cows detected in standing estrus (n = 28: WSB, n = 15; SBS, n = 13) were artificially inseminated at 0, 12 and 24 h with 1, 2 and 1 units of semen, respectively. Inseminations were performed by one experienced AI technician. One AI sire was used for all cows and the semen used was collected from the same ejaculate. 2.4. Embryo recovery Seven to eight days after insemination, embryos were non-surgically recovered using ViGroTM Complete Flush Solution (AB Technology, Pullman, WA). Medium recovered

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from each uterine horn was passed through a 70 ␮m screen filter (Em-Con® , Veterinary Concepts, Spring Valley, WI) to harvest embryos. Embryos were washed and graded in ViGro® Holding Plus Media (AB Technology, Pullman, WA). Grading of embryos was performed by one of the two qualified embryologists. Embryos were assigned a developmental stage and quality grade according to standards set forth by the International Embryo Transfer Society (Savoy, IL). Developmental stage codes were: 3 = early morula; 4 = compact morula; 5 = early blastocyst; and 6 = blastocyst. Embryo quality codes ranged from 1 to 4. Embryos assigned a score of 1 were considered to be of excellent or good quality. The spherical embryo mass was symmetrical with individual blastomeres that were uniform in size, color, and density with at least 85% of the cellular material intact. Embryos assigned a score of 2 were considered to be of fair quality. These embryos contained moderate irregularities in overall shape of the embryonic mass or in size, color, and density of individual cells with at least 50% of the cellular material intact. Embryos that received a quality score of 3 were considered to be of poor quality, and degenerate embryos were assigned a score of 4. 2.5. Blood sampling and radioimmunoassay Blood samples (10 ml) were collected by jugular venipuncture one week before initiation, and at each time injections were administered during the estrus synchronization protocol. Two blood samples obtained 10 days and one day prior to initiation of estrus synchronization were used to confirm estrous cyclicity status of the cows. Cows were considered to be estrous cycling if either one or both samples contained concentrations of progesterone (P4 ) in serum ≥ 1 ng/ml. After centrifugation, serum was harvested and stored at −20 ◦ C. Concentrations of P4 in serum were determined by a single radioimmunoassay (Kirby et al., 1997; Coat-A-Count® , Diagnostic Products, Los Angeles, CA). The intra-assay coefficient of variation was 4.9% and assay sensitivity was 0.05 ng/ml. 2.6. Statistical analysis Age, days postpartum, body condition score, calf birth weight, days on feed, total number of embryos, number of transferable embryos, number of degenerate embryos, and number of unfertilized embryos were analyzed using the General Linear Models Procedure (GLM) of SAS. Embryo quality grades were ranked among cows and between treatments using the Rank Procedure of SAS and analyzed by GLM of SAS. Estrous cyclicity at the initiation of estrus synchronization and subsequent comparisons of luteal tissue presence (determined by P4 concentrations ≥ 1 ng/ml) between treatments were analyzed using Chi-square analysis (StatView® soft-ware package, SAS Institute Inc., 1999).

3. Results The mean number of days postpartum (WSB = 59 days; SBS = 57 days) at the initiation of FSH super-ovulation was similar (P > 0.10) for both groups (Table 3). Body condition scores did not differ (P > 0.10) between treatments at the initiation of the experiment,

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Table 3 The mean age, number of days postpartum (DPP), body condition score (BCS), calf birth weight (BW) and days fed supplement for cows on the whole soybean (WSB) or control (SBS) treatmentsa Treatment (year)b

Age DPP (day)c Initial BCSd Intermediate BCSe Final BCSf Calf BW (kg)g Days on feed (day)

WSB

SBS

6.1 ± 0.8 59 ± 0.9 6.2 ± 0.1 6.5 ± 0.1 5.2 ± 0.1 39.2 ± 1.9 38.1 ± 0.9

6.6 ± 0.8 57 ± 0.9 6.1 ± 0.2 6.6 ± 0.1 5.4 ± 0.1 38.8 ± 3.0 39.3 ± 0.9

Data reported as means ± S.E.M. Determined at the initiation of supplementation. c Determined at the initiation of FSH treatments. d Scored at initiation of prepartum supplementation. e Scored 10 days prior to calving. f Scored at initiation of FSH treatment. g Determined at the time of parturition. a

b

Table 4 The recovery of embryos (total, transferable, and degenerate) and unfertilized ova from cows fed whole soybeans (WSB) or a control (SBS) supplementa Supplement WSB SBS P-value

Total number of ovab

Number of transferable embryosc

Number of degenerate embryos

14.7 ± 3.0 17.5 ± 3.5 0.55

10.3 ± 2.5 13.6 ± 2.6 0.38

3.3 ± 1.1 1.6 ± 0.5 0.20

Number of unfertilized ova 1.1 ± 0.5 2.3 ± 1.2 0.31

Data reported as means ± S.E.M. Number includes all embryos Grades 1–4 (freezable, transferable, degenerate embryos and unfertilized ova). c Number includes all embryos Grades 1–3 (freezable and transferable embryos). a

b

two weeks prior to calving, and at initiation of FSH super-ovulation (Table 3). Estrous cyclicity prior to the initiation of estrus synchronization did not differ (P > 0.10) between treatments [determined by P4 concentrations; (WSB, 1/15; SBS, 1/13)]. Calf birth weights and days of supplemental feed did not differ (P = 0.80 and 0.34, respectively) between treatments (Table 3). There was no difference (P > 0.10) between treatments in number of total embryos (Table 4; WSB, 15.0 ± 3.0; SBS, 18.0 ± 3.5), number of transferable embryos (WSB, 10.0 ± 2.5; SBS, 14.0 ± 2.6), degenerate embryos (WSB, 3.0 ± 1.1; SBS, 2.0 ± 0.5) or unfertilized ova (WSB, 1.0 ± 0.5; SBS, 2.0 ± 1.2). Ranked embryo quality grades did not differ (P > 0.10) between treatments (Table 5).

4. Discussion Based on the previous study in our laboratory, we hypothesized that prepartum lipid supplementation would influence the number or quality of embryos recovered from superovulated donor females. Graham et al. (2001) reported a 23% improvement in first service

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Table 5 Ranked embryo quality grades from cows fed whole soybeans (WSB) or a control (SBS) supplementa Quality grade

1 2 3 4 Unfertilized ova Transferable embryos Total ova a

Treatment WSB

SBS

13.8 ± 7.8 12.0 ± 7.0 15.5 ± 7.0 15.1 ± 8.5 13.5 ± 6.6 12.4 ± 8.1 13.1 ± 8.4

14.3 ± 8.4 16.5 ± 8.5 12.1 ± 7.8 12.7 ± 6.5 14.6 ± 7.9 16.0 ± 7.5 15.2 ± 7.5

P-value 0.87 0.15 0.24 0.43 0.69 0.25 0.50

Data reported as means ± S.E.M.

conception rate among postpartum suckled beef cows that were fed 1.6 kg of WSB for the 40-day-period that preceded parturition. This improvement occurred despite similarities between WSB supplemented and control groups in prebreeding estrous cyclicity rate, BCS at calving or breeding, estrous response during the synchronized period, and pregnancy rate at the end of the breeding season. The basis for this hypothesis was also made from reports that a period of approximately 40 days is required for bovine follicles to grow from the primordial through the antral stage (Lussier et al., 1987). Furthermore, it is well documented that environmental changes influence the quality of follicles over extended periods of time (Howell et al., 1994; Wilson et al., 1998). Fat supplementation to beef cows during late gestation alleviated the negative impacts of prepartum nutritional inadequacy on reproduction; including postpartum return to estrus, conception, and maintenance of pregnancy (Hess et al., 2002). Collectively, these reports support the concept that the effect(s) of prepartum supplementation manifest their expression on reproductive endpoints that are measured after parturitition. The importance of prepartum nutrition on subsequent postpartum reproduction is well established (Randel, 1990; Short et al., 1990; Dunn and Moss, 1992). Cows that received a high energy ration prior to parturition returned to estrus sooner following calving than cows that received supplementation after parturition (Wiltbank et al., 1962). Furthermore, the response of cows to the level of energy provided during the postpartum period was more highly influenced by the energy intake of the cows during the prepartum period. These reports support more recent studies that point to the difficulty in effectively compensating for and/or reversing the negative effects associated with inadequate nutrition prior to parturition through nutritional inputs that are made during the postpartum period (Lalman et al., 2000). Dietary lipid intake for the first 3 weeks postpartum was associated with an increase in serum and follicular fluid lipoprotein–cholesterol, androstenedione biosynthesis, and folliculogenesis (Wehrman et al., 1991). Changes that occurred as a result of lipid intake were thought to be associated with enhanced thecal or granulosal cell development prior to ovulation, or an increase in the pool of follicles from which a competent preovulatory follicle is selected. It is important at this point, to consider reports in the literature that involved prepartum supplementation of fat and the associated effects that were seen at the time of calving or that occurred subsequently following parturition. Bellows et al. (2001) reported an increase

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in calf birth weight and final pregnancy rate when cows were fed supplemental fat for the last 65 days of gestation. These investigators suggested that there was a carry-over effect of fat supplementation late in gestation that enhanced subsequent postpartum reproductive performance. Lammoglia et al. (1999) demonstrated that prepartum supplementation with safflower seed high in linoleic acid, improved calf survivability at parturition. The improvement in calf survival was thought to occur as a result of an increase in activity of brown adipose tissue that is essential for thermogenesis in the newborn calf. These results indicate that metabolic pathways may be altered which may affect the ovarian and (or) uterine environments. Despite these reports, birth weights of calves in this study were not increased among cows that received supplementation with fat prior to calving. The mechanism(s) by which prepartum fat supplementation affects postpartum reproduction is not well understood. Mattos et al. (2000) describes several possible mechanisms by which lipid supplementation affects reproduction including: synthesis and inhibition of prostaglandins, LH secretion, corpus luteum function, steroidogenesis, and gene expression. The influence of diet on phospholipid pools of fatty acids may lead to carry-over effects (Staples et al., 1998) that subsequently influence reproduction in cows supplemented with fat during late gestation. Change in endocrine profiles at critical times in the development of follicles during the preantral stage potentially impacts future reproductive success or failure. This experiment was designed to determine whether the quality or quantity of embryos recovered following super-ovulation was improved as a result of prepartum supplementation with whole soybeans. We observed no improvement in embryo quality or quantity between treated and control groups. It is possible that the benefits of prepartum fat supplementation, if they indeed exist were not detected with the methods used to compare treatments, and perhaps alternative methods may have detected differences. Direct measurement of embryo fatty acid profiles or subsequent fertility of frozen/thawed embryos may have expressed such benefits. The presence of a dominant follicle at the time FSH was administered may have inhibited the growth of medium-size follicles and prevented differences between treatments from occurring (Ryan et al., 1992). Administration of FSH was initiated on 8–10 days of the synchronized cycle to avoid this problem. However, depending on the number of follicular waves of individual cows, the suppressing effects of a dominant follicle on the development of subordinate follicles is possible. Therefore, presence of a dominant follicle perhaps masked potential benefit of fat supplementation on follicular recruitment and response to gonadotropins (Ryan et al., 1992). Furthermore, the pool of follicles affected by lipid supplementation may have been lost in the subsequent ovulations that occurred during the presynchronization schedule. The lack of difference between treatments may too, have been affected by the moderate body condition of the cows (Initial BCS; WSB, 6.2; SBS, 6.1) that were used in this experiment. Lipid supplementation to beef cows that were below average in body condition at calving resulted in follicular development among those cows that was comparable to cows with higher BCS (Ryan et al., 1994). The potential increase in follicular development because of the fat supplementation may have been realized among cows in sub-optimal body condition. There is also the possibility that the balance between Omega 3 (n−3) fatty acids (alphalinolenic acid; 18:3n−3) and Omega 6 (n−6) fatty acids (linoleic acid; 18:2n−6) was not

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optimized to effect improvements in embryo quality. Increasing the availability of either n−6 or n−3 fatty acids or altering the ratio of n−6/n−3 fatty acids may alter specific reproductive processes (Thatcher et al., 2004 ). These authors state that embryo-maternal interactions may be influenced in a manner to improve conception rates and subsequent embryo survival. Competence of the oocyte and embryo is related to fatty acid composition; specifically, phospholipid content of the cellular membrane plays a vital role in development during and after fertilization (McEvoy et al., 2000). The cows in this experiment were managed on pasture, which precluded monitoring of daily feed intake. Hence, the actual balance of n−3:n−6 fatty acids, was not determined for cows in this study. Because of the selectivity and individual intake variation among cows on pasture, the possibility exists that total fatty acid intake may have differed among cows and (or) between treatments. The combined effects of fatty acid content and antioxidant potential (Vitamin A and E) also contributes to differences in embryo quality (McEvoy et al., 2000). Embryo quality was improved in super-ovulated beef cows that were injected with Vitamin A on the first day of FSH administration (Shaw et al., 1995). In this experiment, the correct balance of the Omega fatty acids and/or the combination of fatty acids and antioxidant potential contained in various oilseeds perhaps were not achieved because of the differences in environment, or forage quality and availability. The results from this experiment indicate that prepartum supplementation of WSB failed to influence quantity and quality of embryos recovered following super-stimulation in multiparous suckled beef cows. References AOAC. Official methods of analysis, fourteenth ed. Association of Official Analytical Chemists, Washington, D.C., 1984. Bellows, R.A., Grings, E.E., Simms, D.D., Geary, T.W., Bergman, J.W., 2001. Effects of feeding supplemental fat during gestation to first-calf beef heifers. Prof. Anim. Sci. 17, 81–89, issue no. 2. DeJarnette, J.M., Day, M.L., House, R.B., Wallace, R.A., Marshall, C.E., 2001. Effect of GnRH pretreatment on reproductive performance of postpartum suckled beef cows following synchronization of estrus using GnRH and PGF2␣ . J. Anim. Sci. 79, 1675–1682. Dunn, T.G., Moss, G.E., 1992. Effects of nutrient deficiences and excesses on reproductive efficiency of livestock. J. Anim. Sci. 70, 1580–1593. Espey, L.L., 1981. Ovulation as an inflammatory process—a hypothesis. Biol. Reprod. 22, 73. Graham, K.K., Bader, J.F., Zumbrunnen, C.N., Patterson, D.J., Kerley, M.S., 2001. Prepartum supplementation with whole soybeans increases first service conception rate in postpartum suckled beef cows. J. Anim. Sci. 79 (Suppl 2), 340. Hess, B.W., Rule, D.C., Rule, Moss, G.E., 2002. High fat supplements for reproducing beef cows: Have we discovered the magic bullet? Proceedings of the 37th Annual Pacific Northwest Animal Nutrition Conference, Vancouver, BC, Canada, October 8–10, pp. 59–84 Hightshoe, R.B., Cochran, R.C., Corah, L.R., Kiracofe, G.H., Harmon, D.L., Perry, R.C., 1991. Effects of calcium soaps of fatty acids on postpartum reproductive function in beef cows. J. Anim. Sci. 69, 4097–4103. Howell, J.L., Fuquay, J.W., Smith, A.E., 1994. Corpus luteum growth and function in lactating Holstein cows during spring and summer. J. Dairy Sci. 77, 735–739. Kirby, C.J., Smith, M.F., Keisler, D.H., Lucy, M.C., 1997. Follicular function in lactating dairy cows treated with sustained-release bovine somatotropin. J. Dairy Sci. 80, 273–285. Kojima, F.N., Wood, S.L., Smith, M.F., Patterson, D.J., 2000. Does pretreatment with GnRH prior to a GnRH-PGF2␣ (PG) protocol improve synchronization of estrus in beef cattle? J. Anim. Sci. 78 (Suppl 1), 210.

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