Primig Effect of Melegestrol Acetate (MGA) ad Moesi o Puberty ad Subsequet Reproductive Performace i Crossbred Beef Heifers J. L. Roberts, R. P. Del Vecchio, G. T. Getry, Jr., D. Saders, P. E. Humes ad R. A. Godke Fidigs Heifers receivig MGA oly ad those receivig moesi ad MGA had more advaced reproductive tract scores at the start of the breedig seaso compared with those of the cotrol heifers. Heifers receivig moesi oly ad those receivig moesi ad MGA were bred earlier i the breedig seaso compared with cotrol heifers. Pregacy rates i crossbred beef heifers were icreased whe aimals received moesi compared with cotrol heifers. Itroductio Studies have show that beef heifers are more profitable throughout their lifetime calvig as 2-year-olds, producig more total calves i a lifetime tha those that calve first as 3-year-old heifers. Most beef producers that attempt to breed heifers at 16 to17 moths for calvig as 2-year-olds have experieced lower tha expected pregacy rates. Research has show that heifers mated at the secod or third estrus followig the oset of puberty have higher pregacy rates tha those heifers mated o the pubertal estrus. This study was coducted to determie if melegestrol acetate (MGA ) could be used to ehace the oset of cyclicity, so that heifers will have completed more estrous cycles sooer tha ormal at the same age. The ioophore, sodium moesi (Rumesi ), was also evaluated to determie if ay additive effects would be associated whe fed i combiatio with MGA to ehace pregacy rates i these heifers. Rumesi, a cattle growth promotat, has bee show to improve the aimal s efficiecy to covert feedstuffs by icreasig the rate of gai, especially i heifers fed high-roughage diets. The overall aim of this study was to ehace the oset of puberty i beef heifers, so they could be subsequetly mated at a earlier age. More specifically, the focus of this 2- year study was to evaluate the effects of repeated progestoge primig with MGA combied with Rumesi to ehace the oset of puberty i beef heifers, followed by icreased pregacy rates i these heifers mated at 15 to 17 moths-of-age. Puberty was defied i this study as whe a female first exhibits a fertile estrus ad settles producig a pregacy or cotiues to cycle if ot mated. Experimetal Approach I Year I, 45 crossbred beef heifers (¼ Hereford, ¼ Brahma, ½ Agus) were weaed i September stratified by weight ad radomly allotted to oe of four treatmet groups as follows: cotrol ( = 11), MGA ( = 11), Rumesi ( = 11), ad Rumesi plus MGA ( = 12). All heifers were supplemeted with a basal diet of 90% cracked cor ad 10% soybea meal (12% crude protei) at 5 lbs/head/day. Each heifer treatmet group was maitaied i separate 2.5 acre pastures. The cotrol group received the basal ratio oly. The MGA group was fed the basal diet plus 0.5 mg/head/day MGA for 14 days. The first MGA primig bega whe the average age of the heifers reached 10 to 11 moths, the secod treatmet bega whe the average age of the heifers was 12 to 13 moths-of-age ad the third whe the average age of the heifers 55
was 14 to 15 moths-of-age. The Rumesi group was fed the basal diet plus Rumesi at a iitial rate of 100 mg/head/ day for 5 days ad the icreased to 200 mg/head/day for the remaider of the feedig period. The Rumesi plus MGA group was fed a diet that was a combiatio of both the MGA ad the Rumesi treatmets. Startig with the iitial MGA treatmet, estrual activity was moitored by Estrus Alert patches checked oce daily at feedig. Body weight ad body coditio scores were recorded at 28- to 30- day itervals. The ovaries of radomly selected heifers were scaed every other day ad follicle developmet was recorded begiig the day after each MGA withdrawal ad cotiued for 14 days. Two weeks after the 3 rd MGA primig all heifers received two ijectios of prostagladi F 2 alpha (Lutalyse ) 11 days apart to sychroize their estrous cycles. After the secod Lutalyse ijectio, the heifers were heat checked twice daily (morig ad eveig for ~45 miutes) ad artificially isemiated 12 to 18 hours after the oset of stadig estrus. After 5 days, fertile bulls were placed with the heifers for the remaider of the 60-day breedig seaso. Thirty days after the ed of the breedig seaso, all the heifers were palpated ad ultrasoically examied for pregacy. I Year II, a similar group of 40 crossbred beef heifers (¼ Hereford, ¼ Brahma, ½ Agus) were agai weaed i September ad pre-coditioed for 45 days. After pre-coditioig the heifers were stratified by weight ad radomly allotted ( = 10 heifers/group) to the same treatmet groups used i Year I (cotrol, MGA, Rumesi, ad Rumesi plus MGA groups). Heifers were maitaied i 10 acre pastures. Body weight ad body coditio scores were collected at 28- to 30-day itervals. groups were fed the same diet as i Year I. The first MGA treatmet bega whe the average age of the heifers reached 10 to 11 moths, the secod whe the average age was 11 to 12 moths-ofage ad the third whe the average age was 12 to 13 moths-of-age. Rumesi was added to the diet ad fed cotiuously from December 1 st to April 23 rd. Rumesi was agai fed at a iitial rate of 100 mg/head/day for 5 days ad the icreased to 200 mg/head/day for the remaider of the feedig period. Startig with the iitial Rumesi treatmet, estrual activity was moitored by HeatWatch (DDx. Ic., Dever, CO). The ovaries of all aimals were scaed every 3 days by ultrasoud begiig 3 days after MGA withdrawal ad cotiuig for ~2 weeks. Ovary scas were also performed at the iitiatio of each MGA treatmet ad 7 days after iitiatio to assess status of the ovaries. Reproductive tract scores (1 = small to 6 = large) were assiged to all heifers at the begiig of the experimet ad prior to each MGA treatmet. Two weeks prior to the start of the 60-day breedig seaso all heifers were give two ijectios of Lutalyse 11 days apart for estrus sychroizatio. Followig the secod ijectio, heifers were moitored for estrual activity with the HeatWatch system ad artificially isemiated (AI) 12 to 18 hours after the oset of stadig estrus. After 5 days of AI, fertile bulls were placed with the heifers for the remaider of the 60-day breedig seaso. Both years, bulls were rotated betwee groups at 2-week itervals durig the 60-day breedig seaso to remove ay bull effect. At 30 days after the ed of the breedig seaso, heifers were pregacy checked by ultrasoud/rectal palpatio. Results ad Discussio Average adjusted weaig weight, average daily gai, average breedig weight ad average breedig body coditio score are preseted i Table 1. Reproductive data for 56
Years I ad II for the umber of detected estrus periods per aimal prior to the breedig seaso, umber of aimals respodig to the Lutalyse ijectio ad overall pregacy rates are preseted i Table 2. More heifers were pregat (P<0.05) i the Rumesi group whe compared with the cotrol or the MGAtreated group ad more heifers were pregat (P<0.05) i the Rumesi plus MGA-treated group tha i the cotrol group. At palpatio the fetuses were older (P<0.05) at 30 days after the breedig seaso i the Rumesi-treated ad Rumesi plus MGA-treated groups whe compared with those of the cotrol group but were ot differet from those the MGA-treated group (Table 3). Reproductive tract scores (RTS) were recorded ad aalyzed oly for Year II. The Rumesi plus MGA-treated group had sigificatly (P<0.05) greater RTS scores tha either the Rumesi-treated group or the cotrol group (data ot show). The Rumesi plus MGA-treated group also had sigificatly greater (P<0.05) average chages i RTS throughout the treatmet period tha the Rumesi-treated ad cotrol groups. Follicle size scores, luteal tissue developmet ad ovary size scores were also aalyzed (data ot show). The Rumesi plus MGA-treated group developed more (P<0.05) corpora lutea prior to the breedig seaso tha all other treatmet groups. Durig Year I, the Rumesi-treated group had sigificatly more (P<0.05) large follicles ( 10 mm i diameter) per aimal per examiatio tha all other groups. The MGA-treated ad Rumesi plus MGA-treated groups also had more (P<0.05) large follicles per aimal per examiatio tha the cotrol group. I Year II, the Rumesi plus MGA ad MGA-treated groups had more (P<0.05) medium follicles (5 to 9 mm) per aimal per examiatio tha the Rumesi-treated ad cotrol groups. Also, the Rumesi-treated ad Rumesi plus MGA-treated groups had more large follicles ( 10 mm) per aimal per examiatio tha the cotrol ad MGA-treated groups. Also, the MGA-treated group also had more (P<0.05) large follicles per aimal per examiatio tha the females i the cotrol group. I coclusio, this study provides evidece for the positive effect of sodium moesi o ehacig puberty, icreased cyclicity prior to the expected breedig seaso, icreased overall pregacy rates ad icreased umber of aimals pregat early i the breedig seaso whe usig artificial isemiatio i cojuctio with atural cover. However, a additive effect of melegestrol acetate, if ay, is yet to be idetified. 57
Table 1. Combied aimal performace data for Years I ad II 1 Adj. Weaig Wt 2 Daily Gai Breedig Wt Breedig BCS 3 Cotrol 21 406 ± 7.84 1.11± 0.07 651± 13.87 5.5± 0.14 MGA 21 421 ± 7.84 1.10± 0.07 673± 13.87 5.5± 0.14 Rumesi 21 420 ± 7.84 1.28± 0.07 697± 13.87 5.8± 0.14 MGA + Rumesi 22 425 ± 7.68 1.17± 0.07 681± 13.62 5.7± 0.13 1 Mea ± stadard error. 2 205-day weight = {[(Actual weight-birth wt)/age i days]*205}+ birth weight. 3 Body coditio score (1 = thi to 9 = obese). Table 2. Combied reproductive data for peripubertal heifers for Years I ad II Number of Estrus Periods/Female* Respose to Lutalyse Overall Pregacy Rate Cotrol 21 1.10 2/21 (10%) MGA 21 0.92 12/21 (57%) Rumesi 21 0.51 1/21 (5%) MGA + Rumesi 22 0.91 9/22 (41%) 13/21 a (61%) 15/21 a,c (71%) 20/21 b (95%) 20/22 b,c (91%) *Estrual behavior was moitored by visual observatios ad by the use of Estrus Alert patches i Year I ad estrual behavior was moitored by visual observatios ad the HeatWatch System i Year II. a,b,c Differet superscripts are sigificatly differet (P<0.05) for treatmets withi colums. 58
Table 3. Combied fetal age data for yearlig heifers for Years 1 ad 2 Pregat Fetal Age (days)* Cotrol 13 46.9 ± 5.3 a MGA 15 59.4 ± 5.3 a,b Rumesi 20 72.8 ± 5.3 b MGA + Rumesi 20 79.5 ± 5.1 b *Fetal age ± stadard error determied by ultrasoography usig crow-rump ad head diameter measuremets. Days = pregacy determiatio 30 days after breedig seaso. a,b,c Differet superscripts are sigificatly differet (P<0.05) for treatmets withi colums. 59