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Modeling body condition score at calving by past body condition and forage allowance in grazing beef cow on rangelands
Body condition score at calving affects the length of postpartum anoestrus, probability of pregnancy and the response of temporary suckling restriction and flushing on reproductive performance. Body condition score could be manipulated by forage allowance (FA). Therefore, modeling the effect of FA on BCS could provide valuable information for beef cow managers. The objective was to study the relationships between FA and BCS at middle gestation (BCSm) and calving (BCSc) of primiparous beef cow grazing located in native pastures. We used data from an experiment that evaluated the effect of 2 levels of FA on productivity of primiparous beef cows grazing rangelands. The experiment took place in Facultad de Agronomía, Uruguay. Eighty cows were assigned to a completely randomized experiment of 2 FA in spatial replication on 2 blocks during 2 years. The annual FA averaged 2.5 and 4 kg DM/kg BW for low (L) and high (H) FA, respectively. Cow BW and forage mass were measured monthly and used to adjust FA using the “put and take” method. The experiment started in autumn −150 d postpartum (dpp) and finished 190 dpp. The BCS at -150 dpp (early gestation [BCSe]) was recorded. The BCS was recorded by a visual scale (1-9 points). We used a seasonal value FA before BCS was recorded (FAm and FAc), calving date in Julian days (CD) and the previous BCS and FA to explain BCSm and BCSc. Models were obtained by multiple regressions and variables were selected by Stepwise. An increase in BCSe and FAm improved BCSm (BCSm = 0.16 + (0.66*BCSe) + (0.43*FAm) + (-0.07*(FAm-2.89)2); r2 = 0.63; P <0.01; Mean = 5; RMSE = 0.43) and BCSc was increased as a result of the increase in BCSe and BCSm (BCSc = 0.28+ (0.18*BCSe) + (0.62*BCSm) + (0.008*CD; r2 =0.57; P <0.01; Mean = 4.4; RMSE =0.2). Models have a good explanation capacity and highlight the effect of prepartum BCS and FA on BCSc. Forage allowance indirectly affected BCSc by its positive effect on BCSm. These models could be employed by beef herd managers to control or predict the BCSc and reproductive performance.
Keywords: Body condition score, forage allowance, modeling