<strong>Arkansas</strong> Animal Science Department Report 2001 mental feed can be reduced. Calves were weaned in October at an average <strong>of</strong> 204 d <strong>of</strong> age. After weaning, calves were penned for 2 wk and fed BG hay and 2.2 lb/d <strong>of</strong> commercial mixed feed. Subsequently, calves were moved back to their preweaning forage environment (i.e. BG or E+). Calves were managed postweaning for moderate gains consistent with a common backgrounding program by supplementing with cottonseed meal, corn, and (or) a commercial mixed feed. Tall fescue or BG hay was provided free choice for calves on E+ or BG, respectively. Supplemental feed (same as above) was provided from weaning to late October and was continued to April <strong>of</strong> the following spring. Amounts <strong>of</strong> supplemental feed were adjusted based on visual estimates <strong>of</strong> forage availability and ambient temperatures Growth parameters <strong>of</strong> A and k were estimated on 177 Angus (AA), Brahman (BB), Angus x Brahman (AB) and Brahman X Angus (BA) heifers born from 1988 to 1991 using the three-parameter growth curve model as described by Brody (1945). Data were collected every 28 d until approximately 18 mo and then at prebreeding, postcalving, and weaning <strong>of</strong> calf. In a preliminary analysis, year was not a significant source <strong>of</strong> variation, therefore data were pooled over year and analyzed by the general linear model (GLM) procedure <strong>of</strong> SAS (SAS Inst. Inc., Cary, NC). Included in the models for A and k were the independent variables <strong>of</strong> breedtype, forage and breed-type x forage interaction. Sire <strong>of</strong> calf was not included in the model due to the fact that sires were rotated among breeding pastures in both forage treatments to prevent confounding <strong>of</strong> sire and forage effects. Results and Discussion There was a significant (P < 0.01) breed-type x forage interaction for mature weight in this study. Presented in Table 1 are the least squares means and standard errors for estimated mature weight by breed-type and forage environment. There was no difference (P > 0.05) in mature weight <strong>of</strong> straightbred Angus cows on either forage with means <strong>of</strong> 1,298 and 1,344 lb, respectively, for BG and E+ forages. These estimated values for Angus cows are higher than those reported by Stewart and Martin (1983) and Brown et al. (1972) who reported mean A values <strong>of</strong> 1,067 and 970 lb, respectively. Angus x Brahman cows grazing E+ were heavier (P < 0.05) at 1,283 lb than were their counterparts grazing BG at 1201 lb. It is not entirely clear why these Angus x Brahman cows which grazed BG had smaller A values than the mean <strong>of</strong> their parental breed-types, however, this could be due to an interaction between the direct breed effects <strong>of</strong> the Angus cattle and the maternal breed effects on the Brahman cows grazing this BG forage. There were no differences (P > 0.05) in mean A values for the Brahman x Angus cows on either forage. The Brahman cows seemed to have a difficult time coping with the negative effects <strong>of</strong> the E+ forage as they had a mean A value <strong>of</strong> only 1,120 lb which is smaller (P < 0.05) than all other breed-type x forage combinations with the exception <strong>of</strong> the Angus x Brahman crosses on BG at 1,201 lb. Determining the ideal weight for maximum animal production is an important question that needs an answer. Stewart and Martin (1983) reported that in Angus cows, their optimum estimated mature weight in order to achieve maximum maternal performance was 1,045 lb. This weight was considerably lower than our estimated mature weights <strong>of</strong> 1,298 to 1,344 lb, however, Kapps et al. (1999) reported similar values for mature weight in Angus cows <strong>of</strong> 1,320 lb. Shown in Table 2 are the least squares means and standard errors for k by breed-type. There was no breed-type x forage interaction for k in this study. Angus x Brahman crosses had the earliest (P < 0.05) rate <strong>of</strong> maturing <strong>of</strong> all breedtypes maturing at a rate <strong>of</strong> 0.053. There were no differences (P > 0.05) between the k values between straightbred breedtypes with Angus at 0.039 and Brahman at 0.042. These values <strong>of</strong> k are considerably lower than values reported in the literature. There was however a difference between the two reciprocal crosses with Brahman x Angus cows maturing at a slower (P < 0.05) rate than did the Angus x Brahman (0.049 vs 0.053). This increase in rate <strong>of</strong> maturing over the purebred cattle can be expected due to the effect that heterosis has on this trait. Nelson et al. (1982) reported a percentage heterosis increase in maturing rate <strong>of</strong> 3.5 % in Brahman x Angus cross cattle thus supporting our results. In a growth curve study by Tawah and Franke (1985), they used 574 straightbred and crossbred cows and reported results stating that generation one crossbred cows had a 0.034 greater k value than did the straightbred cattle. Implications These results suggest that the growth parameters <strong>of</strong> A and k differ by breed-type. These differences are <strong>of</strong> importance to the biological and economical efficiency <strong>of</strong> beef production and need to be carefully considered when attempting to correctly match breed-type to available production resources. Further research is needed in this area, particularly in the field <strong>of</strong> crossbreeding to help producers deal with different biological types <strong>of</strong> animals in a wide variety <strong>of</strong> production environments, including those with limited resources. Literature Cited Brody, S. 1945. Bioenergetics and Growth. Reinhold Publishing, New York. Brown, J.E, et al., 1972. J. Anim. Sci. 34:525-537. Kapps, M., et al., 1999. J. Anim. Sci. 77: 569-574. Nelson, T.C. et al., 1982. J. Anim. Sci. 55:280-292. Stewart, T.S and T.G. Martin. 1983. J. Anim. Prod. 37: 179-182. Tawah, L.C. and D.E. Franke. 1985. J. Anim. Sci. 61 (Suppl.) 8. 54
AAES Research Series 488 Table 1. Least-squares means and standard errors <strong>of</strong> estimated mature weight (lb) for genotype x forage interaction. Genotype a Forage AA AB BA BB Bermudagrass 1298 + 37 bcdef 1201 + 35 fgh 1373 + 42 b 1316 + 44 bcde Fescue 1344 + 37 bcd 1283 + 40 bcdefgh 1351 + 48 bc 1120 + 46 h a AA = Angus x Angus, AB = Angus x Brahman, BA = Brahman x Angus and BB = Brahman x Brahman. bcdefgh Means with different superscripts differ (P < 0.05). Table 2. Least-squares means and standard errors for rate <strong>of</strong> maturing by genotype. Genotype a Forage AA AB BA BB Rate <strong>of</strong> maturing 0.039 + 0.002 d 0.053 + 0.002 b 0.049 + 0.002 c 0.042 + 0.002 d a AA = Angus x Angus, AB = Angus x Brahman, BA = Brahman x Angus and BB = Brahman x Brahman. bcd Means with different superscripts differ (P < 0.05). 55
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