Effect of age, breed of dam, and breed of sire on pregnancy rates.

Abstract

Proc. Aust. Soc. Anim. Prod. (1974) 10; 33 EFFECT OF AGE, BREED OF DAM, AND BREED OF SIRE ON PREGNANCY RATES T.H. RUDDER*, G.W. SEIFERT** and J.W. LAPWORTH*** Summary Data from a commercial herd in Central Queensland showed that Selection age of cow had a significant effect on pregnancy rates. Lactation stress appears appears effective in improving pregnancy rates. to be important in 2-yr-old lactating cows. However, compar isons may Breed-or-sire effect was significant. have been confounded by select ion and environmen tal effects. observed. There was no breed of dam effect within the range of genotypes Prior selection may have been partly responsible. I. INTRODUCTION Research work at the National Cattle Breeding Station, 'Belmont', has shown that inter-se mating of Brahman x British cattle results in a decline in fertility which is not apparent in Africander x British interse mated cattle (Seifert and Kennedy 1972). Barr (1971) reported a These trends are decline in fertility as Brahman component increased. in general agreement with American work (Cartwright et al. 1964; -m Schilling and England 1968; Kidder et al. 1964; Reynolds 1972). -m The effect of age of cow on fertility is usually reflected in This is lower pregnancy rates in 2-yr-old and 3-yr-old lactating cows. generally attributed to susceptibility to nutritional stress (Barr 1971). However, the effect on fertility of selection by the removal of subfertile cows has usually not been considered. The purpose of this paper is to report initial results concerning the effect of breed and age of pregnancy rates. II. MATERIALS AND METHODS Data from two mating seasons were collected from a trial using This property is approximately a commercial beef herd on 'Mt Eugene'. Long term 50 km south of Rockhampton and 72 km southwest of Gladstone. rainfall in the area averages 674 + 135 mm annually, with 486 2123 mm being received during November to April and 188 2 70 mm during May to October. The land system used for the breeding herd is Torsdale (Speck et Timber treatment has resulted in a grassy woodland with - al- 1968). Eucalyptus crebra, E. tessallaris and E. polycarpa in a pasture of This type of Heteropogon sp., Bothriochloa sp. and Dicanthium sp. pasture is stocked at 4.0 to 4.5 ha per breeding unit, depending on seasonal conditions. Three groups of mixed-age breeders varying in Brahman component were mated to 4 Belmont Red bulls, 4 Droughtmaster bulls and 2 selected Mating intensity was approximately 25 and 30 COWS Santa Gertrudis bulls. The three groups of per bull in 1971/72 and 1972/73 respectively. * ** *** Beef Cattle Husbandry Branch, Q.D.P.I., Rockhampton, Qld, 4700. Division of Animal Genetics, C.S.I.R.O.I Rockhampton, Qld, 47009 Beef Cattle Husbandry Branch, Q.D.P.I., Biloela, Qld, 4715. 33 breeders contained similar proportions of each age group and each genowhere the superscripts indicate the breed component (Brahman and Hereford) and the first letter the cow's sire. The Belmont Red and Droughtmaster bulls were 2- and 3-yr-old at the commencement of the first and second mating season, respectively. The Santa Gertrudis bulls were 3 years older, in addition they had been selected for fertility by means of test matings. All bulls used had satisfactory semen quality according to the method of Watson (1964) whi ch u ses collection of semen by elec tro-ej aculation, and evaluation on the bas is of colour, density, mot ility and 1 ive-dead ratio Pregnancy was determined by rectal palpation 8 weeks after the termination of mating. Estimated foetal age was recorded on the basis of 1 = non-pregnant, 2 = 16 weeks or less, 3 = greater than 16 weeks (early conception). Pregnancy rates were analysed by least squares method (Harvey 1960) using the following model:- Model 2 The data were also analysed with the maiden heifers included and the partial regressions involving calf weight and age excluded. III. RESULTS AND DISCUSSION Table 1 gives the analysis of variance for both models, while Table 2 gives the least squares constants for the significant effects. TABLE 1 Analyses of variance for preqnancy rates Breed of dam effects were not significant and this was probably because higher Brahman component and inter-se mated breeders were not present. But, the effect of prior selection through removal of nonpregnant cows each year may be pertinent. Selection policy should be considered when comparing different breeders for fertility. Breed-of-sire effects were highly'significant in Model and the constants showed the same trend in Model 2. The Santa Gertrudis were selected on fertility and may not have been representative of the breed. 35 Age of dam constants showed a similar trend in both Models. The decline in pregnancy rates from l- to 2-yr-old indicates that lacThis is consistent with tation stress can be important in 2-yr-old cows. published data (Temple 1966). The increase in pregnancy rate with age in a herd where culling non-pregnant females is rigorously adhered to suggests that culling non-pregnant cows is successful in improving This is consistent with reports which suggest that pregnancy rates. the repeatibility of reproduction is high enough to recommend culling cows that fail to conceive (Temple 1966). Simple correlations between cow weight and pregnancy rate were similar in both models (rI = ,242, rII = -258, P * -01). In Model 2 the partial regression on cow weight (b = 0.0006 + 0.003) was significant, while in Model 1 it was not significant (b = 0.0003 2 0.0004). The reason for this may have been that the other regression fitted in Model 1 (calf age and calf weight) were positively correlated to cow weight (r = 0.508 and r = 0.027) and removed a large part of thevariation. The partial regression on calf age (b = 0.0029 + 0.0011) was highly significant and is purely a reflection of the time after calving cows become pregnant. Cows which calved early in the season became pregnant earlier than those which calved later, which is largely a reflection of post-partum anoestrus as well as seasonal conditions. IV. ACKNOWLEDGEMENTS The authors wish to thank Mr P. Maynard, 'Mt Eugene', Jambin, for his co-operation which allowed this work to be done, and Messrs Bean, Beasley and Short for their assistance in collating the data. Thanks are also due to Mr Hodge who undertook the pregnancy diagnosis, and to colleagues for their help, advice and criticism in compiling this paper. V. REFERENCES BARR, N.C.E. (1971). 'Reproduction in Queensland Beef Cattle'. (Queensland Department of Primary Industries: Brisbane). CARTWRIGHT, T-C., ELLIS, G-F., KRUSE, W.E. and CROUCH, E.K. (1964). 'Hybrid Vigor in Brahman-Hereford Crosses'. Texas A. & M., Tech. Monograph 1. May, 1964. HARVEY, W-R. (1960). U.S.D.A., A.R.S. Bul. 20-8. KIDDER, R-W., KOGER, M., MEADE, J.H. and CROCKETT, J.R. (1964). 'Systems of Crossbreeding for Beef Production in Florida'. Uni. of Florida Bulletin 673, May, 1964. REYNOLDS, W.L. (1972). 'Improving Reproduction Efficiency in Beef Cattle'. Proc. of 21st and 22nd Beef Cattle Short Course. Texas A. & M., 1972, p-208. SCHILLING, P.E., and ENGLAND, N.C. (1968). J. Anim. Sci. 27: 1363. SEIFERT, G.W., and KENNEDY, J.F. (1972). Proc. Aust. Soc.%im. Prod. 9: 143. 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