Effects of ruminal degradability of ensiled whole crop maize varieties on feed intake and milk production of dairy cows.

The feed value of whole crop maize silage (WCMS) depends on nutrient composition, ruminal degradability and whole tract digestibility. However, as the ruminal degradation rate is involved in physical regulation of feed intake, ruminal degradability of WCMS may also affect feed intake and milk production of dairy cows. Thus, the aim of this study was to examine relationships between nutrient composition, ruminal degradability, and whole tract digestibility of WCMS and feed intake and milk production of dairy cows. Nine varieties were tested in 3 consecutive years. Nutrient composition analyses included proximate analysis and determination of cell wall constituents. Whole tract digestibility was determined in vivo using wethers and ruminal degradability was examined in situ using four rumen-fistulated steers. Feed intake and milk production were measured using nine cows per variety. Cows were fed a ration consisting of 75.0% WCMS, 8.5% hay and 16.5% soya bean meal (dry matter basis) ad libitum. Variety did not influence nutrient composition, except for the concentration of ADF (ADFom), ADL and utilisable CP (uCP). In contrast, variety had a significant effect (P<0.05) on ruminal degradability of NDF (aNDFom) and on whole tract digestibility of organic matter (OM) and non-fibre carbohydrates. Dry matter intake (DMI) of WCMS tended to be affected by variety (0.05<P<0.10) whereas no effect on energy-corrected milk production (ECM) was observed. The year of harvest had an influence on net energy for lactation and uCP concentration, ruminal degradability and whole tract digestibility of nutrients, as well as on DMI and ECM of dairy cows. Whole tract aNDFom digestibility of WCMS was positively correlated with aNDFom concentration (R 2=0.48) and whole tract OM digestibility (R 2=0.58). Furthermore, ruminal OM degradability was positively correlated with DMI (R 2=0.57) and ECM (R 2=0.49) of dairy cows. The results indicate that ruminal degradability and whole tract digestibility have a greater impact on the feed value of WCMS than nutrient composition and should be focused on to optimise feed intake and milk production of dairy cows. Thus, maize breeders should include ruminal degradability and whole tract digestibility parameters in variety testing programs to increase the informative value of variety descriptions for farmers.