A meta-analysis and meta-regression of the effect of substitution of steam flaked corn instead of finely ground corn on dry matter intake, nutrients digestibility, lactation performance, and rumen fermentation in dairy cows

The objective of this meta-analysis was to investigate the effects of substitution of steam flaked corn (SFC) instead of finely ground corn (FGC) on dry matter (DM) intake, nutrients digestibility, milk yield (MY), milk composition, and rumen fermentation of dairy cows. A literature search was conducted to identify papers published from 1990 to 2020. Effect size for all outcomes was reported as standardized means difference (SMD) with 95 % confidence intervals. Heterogeneity was determined using Q test and I² statistic, while meta-regression was used to examine factors influencing heterogeneity. The meta-analysis results indicated no effect of SFC inclusion on DM intake (SMD = -0.214; 95 % CI = -0.549 to 0.121; P = 0.210). Starch digestibility (SMD = 0.987; 95 % CI = 0.668–1.307; P < 0.001) increased and DM digestibility (SMD = 0.345; 95 % CI = -0.027 to 0.718; P = 0.069) tended to increase as SFC replaced FGC. In this regard, DM digestibility decreased with increasing DM and starch intake, and increased with increasing forage level and days in milk. Milk yield tended to increase (SMD = 0.195; 95 % CI = -0.021 to 0.411; P = 0.077) and milk protein yield and content increased as SFC replaced FGC, but milk fat yield and content decreased. Inclusion of SFC instead of FGC significantly decreased MUN (SMD = -0.332; 95 % CI = -0.659 to -0.005; P = 0.047). Replacing SFC instead of FGC did not influence the ruminal pH (SMD = -0.021; 95 % CI = -0.278 to 0.236; P = 0.871) and total VFA concentration (SMD = -0.092; 95 % CI = -0.359 to 0.174; P = 0.497), however, decreased acetate (SMD = -0.579; 95 % CI = -0.862 to -0.296; P < 0.001), isovalerate (SMD = -0.388; 95 % CI = -0.694 to -0.082; P = 0.013), and isobutyrate (SMD = -0.607; 95 % CI = -0.947 to -0.266; P < 0.001) proportion. Substitution of SFC for FGC decreased the rumen NH₃ concentration (SMD = -1.005; 95 % CI = -1.341 to -0.668; P < 0.001). Egger’s test showed publication bias was detected for rumen acetate, propionate, butyrate, valerate, acetate to propionate ratio, and NH₃ (P < 0.1). We conclude that SFC has the potential to impact on lactation performance, digestibility, and rumen fermentation of dairy cows without any negative effect on DM intake and rumen environment, but its effects for some variables depend upon DM intake and forage inclusion rate in diets.