Ruminant Nutrition: Dairy: Forages and Heifers

Dietary forage NDF (fNDF) offers the potential of combining fiber and its particle size influences in a common index. For this reason, it may be more precise than dietary NDF or proportion of concentrate to predict dairy cow responses. To evaluate this hypothesis a database was compiled from 116 published experiments (n = 289 treatments) where dietary NDF or concentrate varied. Forages were long or coarsely chopped. Dietary NDF averaged 34.4 ± 8.2% of DM while fNDF was 26.2 ± 10.2% (from 11.0 to 59.6). Meta analyses with GLM allowed to focuse on within experiment regressions. Chewing index (CI; 38.2 ± 11.8 min chewing/kg DM Intake) was linearly related to fNDF (CI = 19.8 + 0.70 fNDF, n = 195, nexp = 80, rmse = 2.9). Mastication time also increased with fNDF, but obtained a plateau of 865 min/d at fNDF around 52%. Dry matter intake (20.4 ± 3.5 kg/d) was related negatively and curvilinearly to fNDF ( = 22.6 0.003 fNDF2, rmse = 1.1), and milk yield (28.3 ± 7.1 kg/d) presented a similar shape ( = 22.6 0.005 fNDF2, rmse = 1.4). Opposite responses were observed for milk percentages of fat ( = 2.5 + 0.07 fNDF 0.00008 fNDF2, rmse = 0.17) and protein ( = 3.3 + 0.001 fNDF 0.00001 fNDF2, rmse = 0.08). Ruminal pH was positively related to fNDF ( = 5.62 + 0.023 fNDF 0.00014 fNDF2, n = 145, nexp = 56, rmse = 0.08). Acetate/propionate ratio increased with fNDF ( = 1.05 + 0.10 fNDF 0.001 fNDF2, n = 163, nexp = 66, rmse = 0.24). Values of pH = 6.1 and A/P = 3.0 corresponded to fNDF = 24%. An increase in fNDF significantly increased the rumen liquid load (RLL = 73 ± 11.1 kg, = 53.6 + 1.14 fNDF 0.014 fNDF2, n = 76, nexp = 32, rmse = 5.0) and rumen liquid outflow rate. In contrast, fNDF decreased the organic matter digestibility ( = 74.1 0.139 fNDF, n = 186, nexp = 73, rmse = 2.3) and the energy balance became negative when fNDF > 31.5%. In conclusion, most predictions were curvilinear and fairly accurate. As a measure of fill effect, the minimization of fNDF enhances the milk performance, however when fNDF is < 20–25% DM the risk of acidosis increases.