Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/10/2022
Publication Date: 3/25/2022
Citation: Hall, M., Zanton, G.I. 2022. Substitution of cane molasses for corn grain at two levels of degradable protein. I. Lactating cow performance, nutrition model predictions, and potential basis for butterfat and intake responses. Journal of Dairy Science. 105(5):3939-3953. https://doi.org/10.3168/jds.2021-21241.
Interpretive Summary: If we can gain a better understanding of how carbohydrates and protein in dairy cow diets interact to affect cow performance, we will be better able to feed them to appropriately meet their nutritional needs. In a study with high producing dairy cattle, increasing substitution of molasses, a sugar source, for corn grain, a starch source, in diets with more or less rumen degradable protein (RDP) resulted in decreased feed intake and milk yield but no change across diets in feed efficiency or milk fat yield; milk fat% increased linearly. The effect of molasses on intake and milk fat in this study may be explained by the rapid ruminal production of the organic acids that can be used to produce milk fat, and that which can reduce intake, but more research is needed to determine this. Two nutritional models tested for their ability to predict the cow responses showed a general underprediction of production with more RDP, and overprediction with less RDP based on predicted metabolizable protein supply to the cow. This indicates a need for improvement in the models’ predictive accuracy to efficiently feed cows to support lactation performance. Both improvement in our understanding of what drives the production of milk components and improved accuracy of prediction of our nutritional models will give us tools to enhance animal production, welfare, and efficiency.
Technical Abstract: There is little data available on our ability to predict the combined effects on the performance of high producing dairy cows of modifying diets with feeds rich in sugars or starch and ruminally degradable protein (RDP). The objective of this study was to compare responses of 59 lactating Holstein cows to molasses (Mol) versus dry corn grain (CG) at three levels of Mol and two levels of RDP (+RDP or -RDP) in a randomized complete block design with a 3 x 2 factorial arrangement of treatments. Cows were individually fed a common diet during a two week covariate period followed by eight weeks on experimental diets. Diets were formulated to be isonitrogenous and provide similar amounts of starch (ST) + water-soluble carbohydrates (WSC). Experimental diets contained on a dry matter (DM) basis 35% corn silage, 20% alfalfa silage and 16.6% crude protein; the 0, 5.25, and 10.5% Mol diets respectively contained, 19.0, 14.5, and 10.0% CG, 28, 25, and 22% ST, and 5.5, 8.5, and 11.5% WSC. At 10 weeks on study, cows averaged 45.5 kg 3.5% fat- and 3.2% protein- corrected milk (FPCM). The DM intake (DMI), and yields of milk, milk protein, and FPCM, and milk N/intake N declined linearly with increasing Mol. Differences among diets were not detected for milk fat yield and FPCM/DMI. No RDP or interaction effects were detected for these measures. That milk production efficiency did not differ across diets suggests that DMI was a primary driver of performance. The similar FPCM/DMI and maintenance of milk fat yield would not have been predicted based on Mol and CG composition but may relate to differences in fermentation rates and products. It is hypothesized that more rapid evolution of volatile fatty acids post-ingestion with Mol as compared to CG may have provided masses of acetate and butyrate in excess of basal needs that were shunted to milk fat production, and propionate that depressed intake. The 2001 Dairy National Research Council model and the Cornell Net Carbohydrate and Protein System 6.55 in NDS Professional (2021) estimates of metabolizable protein-allowable energy corrected milk (ECM) underestimated actual ECM for +RDP diets by 4.7 and 2.5 kg, respectively, and came close or overestimated for -RDP diets by 0.6 and 5.3 kg, respectively. Prediction discrepancies suggest issues with valuation of protein based on degradability. Improved understanding of factors mediating these results would likely enhance our ability to predict animal responses.