Substitution of molasses for corn grain at two levels of degradable protein. II. Effects on ruminal fermentation, digestion, and nitrogen metabolism

Authors: Zanton, Geoffrey; Hall, Mary Beth

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2021
Publication Date: 3/25/2022
Citation: Zanton, G.I., Hall, M. 2022. Substitution of molasses for corn grain at two levels of degradable protein. II. Effects on ruminal fermentation, digestion, and nitrogen metabolism. Journal of Dairy Science. 105(5):3954-3968. https://doi.org/10.3168/jds.2021-21240.
DOI: https://doi.org/10.3168/jds.2021-21240

Interpretive Summary: Dairy cows use dietary protein and carbohydrates to make milk components and finding the optimal balance among different dietary sources could lead to improved feeding practices. Finely-ground, dried corn was exchanged for molasses and rumen degradable protein (RDP) was exchanged for rumen undegradable protein feed sources to evaluate ruminal fermentation, digestion and nitrogen metabolism. Concentration of some end products of rumen fermentation related to milk fat synthesis changed with carbohydrate source whereas the effects of protein source was more limited. Increased ruminal lactate when increasing molasses with high RDP should be explored further to optimize microbial efficiency and rumen health.

Technical Abstract: Our objective was to evaluate cow N metabolism and ruminal measures with diets containing three different levels of molasses or finely-ground, dry corn grain with two levels of ruminally degradable protein (RDP). Twelve lactating, ruminally cannulated Holstein cows (parity 2.25±0.62; 185±56 days in milk; 41.3±6.3 kg milk initially) were housed in tie stalls with ad libitum access to feed and water throughout the duration of the split-plot, replicated 3 × 3 Latin square designed experiment where each period lasted 28 days. Six experimental diets were formulated according to a 2 × 3 factorial arrangement of treatments where two levels of RDP (solvent soybean meal [SBM] as the primary supplemental protein source or expellers SBM replacing a portion of solvent SBM: + RDP and - RDP, respectively) were fed throughout the experiment as the whole plot and three levels of molasses (0, 5, or 10% of dry matter replacing finely-ground, dry corn grain) were fed in sequences in the replicated Latin squares balanced to account for the effects of previous treatment. Data were analyzed as a split-plot, Latin square where contrasts of interest were the main effects of protein degradability, linear and quadratic effects of carbohydrate type, and the linear and quadratic interaction between carbohydrate type and protein degradation where statistical significance was declared when P < 0.05. Dry matter intake did not differ by diet, although ash concentration increased and potentially digestible neutral detergent fiber corrected for ash (pdNDFOM) declined with increasing dietary molasses resulting in ash increasing linearly and intake of pdNDFOM declining with increasing molasses. Ruminal pH, organic acid concentration, and ammonia concentration were not affected by diet and followed a similar temporal distribution across the sampling day. Ruminal acetate molar percent decreased linearly and butyrate molar percent increased linearly with increasing levels of molasses. Ruminal lactate concentration was minimized at the intermediate levels of molasses. Ruminal free amino acid concentration was greater for diets formulated with higher RDP and was increased at 10.5% molasses whereas branched chain volatile fatty acids declined linearly with increasing molasses. Rumen content mass was greater for diets formulated as - RDP due to greater quantities of rumen liquid. As a result, ruminal acetate pool size was greater with - RDP and butyrate pool size was greater in cows fed increasing concentrations of molasses. Total tract apparent dry matter and organic matter digestibilities based on spot sampling were not affected by diet, however ash digestibility increased linearly with increasing levels of molasses. Calculated urine output was greater for cows fed diets with increasing levels of molasses and for cows fed + RDP. Grams of N distributed to excretion pools were not different across diets, although as molasses increased a lower proportion of N intake was excreted in urine. Overall, including different levels of molasses and RDP individually affected ruminal and excretion responses, however these factors did not interact.