Beef Cows Nutrition and the Effect in the Fetal Development – a Meta-Analysis

To quantify the effects of the total digestible nutrients (TDN) and crude protein (CP) in the diet of pregnant beef cows on fetal weight and birth weight of calves, a systematic review and meta-analysis were performed. We searched on two electronic databases (Scopus and ISI Web of Knowledge) from 1945 to October 2016, as well as we checked references of relevant review papers. Data were extracted using pre-defined protocols. Inclusion criteria were complete studies using pregnant beef cows receiving different diets that analysed fetal weight and/or birth weight of calves. Random effect meta-analyses were conducted for each indicator separately with the mean of control and treated group. The analysis was conducted with the TDN and CP levels in relation to cows’ requirements (NRC, 1996). The results were presented with the pooled mean difference (MD) and I² (percentage of total variation between studies that is due to heterogeneity rather than chance). For fetal weight at 4 months (FW4), we included four studies reporting six trials involving 170 animals; for fetal weight at 8 months (FW8), four studies, four trials, and 156 animals; and for birth weight of calves (BW), 48 studies, 125 trials in 9,053 animals. The heterogeneity between studies was high for FW4 (I² = 94.4%), FW8 (I² = 91.08%), and for BW (I² = 96.9%). When control cows received 20-40% less TDN than treated group, the FW4 reduced 2.27 kg (P < 0.001; I² = 65.06), since both groups received 19-25% more TDN than their requirements. The cows from control group tended to show (P = 0.057) a lower MD (1.57 kg; I² = 91.08%) when consumed 30% of CP above their requirements. The fetus from control group were 2.23 kg lighter (P < 0.001; I² = 0%) when their cows consumed 20% TDN above their requirements. The BW was lower (MD = -0.761; I² = 96.4%; P < 0.001) only for cows in the third period of pregnancy. The calves from both groups borned 0.560 kg and 0.761 kg lighter when cows consumed CP (P = 0.0015; I² = 97.1%) and TDN (P < 0.001; I² = 96.4%) more than their requirements, respectively. Our study demonstrates that FW4, FW8, and BW reduced when CP was 19-25% and TDN was 30% above of cows requirements as evidenced in the NRC (1996).