Dry Matter Intake and The New NRC DMI
Dry matter intake (DMI) is fundamentally important in dairy cattle nutrition because it determines the amount of nutrients available to the animal in a moisture corrected format (NRC 2001). Maximizing DMI provides more nutrients for rumen microbes growth and more nutrients to the cow for milk synthesis, body condition and reproduction. The link between DMI and milk production and thus farm financial efficiency, is closely associated. Therefore, accurate estimation of DMI is important for diet formulation to promote nutrient utilization efficiency.
Many factors affect DMI, such as the animal’s physical limitation (gut fill), metabolic control, interactions of diet and physiological state of the animal. Since 1944, Nutrient Requirements of Dairy Cattle, published by National Research Council (NRC), has been used as the guide for dairy nutritionists in academia and industry professionals to develop and implement nutritional and feeding programs. In the last version (NRC 2001), DMI estimation equation for lactating Holstein cows is:
DMI (kg/d) = 0.372* FCM (kg/d)+ 0.0968 * BW (kg) 0.75)*[1-e (-0.192 * (WOL + 3.67))]
Where FCM = 4% fat-corrected milk (kg/d), BW= body weight (kg), and WOL = week of lactation.
The 2001 equation shows that DMI is predicted based on animal factors (FCM, BW, WOL), since these parameters are easily measured or known. However, this model over-predicted at high DMI (such as early lactation or high producing cows) and under-predicted at low DMI (Allen et al., 2019). In comparison, the new NRC 2021 version which is set to be published by the end of 2021, according to Drs. Mary Beth Hall and Mike Allen’s talk at the 40th Discover Conference, equations for lactating cows take into considerations both the animal factors mentioned above and dietary characteristics such as gut fill effects. The 2021 NRC DMI for lactating Holstein cows when using animal factors is:
DMI (kg/d) = [(3.7 + Parity*5.7 + 0.305 * MilkE (Mcal/d) + 0.022* BW (kg) + (-0.689 + Parity *-1.87) * BCS] × [1 − (0.212 + Parity × 0.136) * e(−0.053 × DIM)]
Where Parity = the proportion of multiparous cows (1 if multiparous and 0 if primiparous), MilkE = milk energy, BCS = body condition score, DIM = days in milk.
The 2021 NRC for DMI prediction (based on 134 treatment means from 34 studies published from 1990 through 2015) when using diet filling effects is:
DMI (kg/d) = 12 – 0.107* fNDF + 8.17 * ADF/NDF + 0.0253* fNDFD – 0.328 (ADF/NDF – 0.602)*(fNDFD-48.3) + 0.225* MY + 0.0039* (fNDFD – 48.3)*(MY – 33.1)
Where fNDF= forage NDF content (%), ADF = acid detergent fiber content (%), NDF = neutral detergent fiber content (%), fNDFD = forage NDF digestibility (%), MY = milk yield (kg/d)
Based on the filling factors equation and the dataset used, DMI decreases linearly as diet fNDF content increases, increases linearly as diet ADF/NDF, and milk yield increases. The limitation for this equation is that the database lacks information on diets with high NDF and fNDF because such diets are not offered to high producing cows. As a compromise, milk yield (MY) is included in the equation. In addition, the equation based on filling factors is not recommended for cows under 60 DIM, therefore the animal factors equation is recommended under that circumstance.
Based on Dr. Allen’s presentation, it is recommended that the equation based on animal factors can serve as a prediction for voluntary DMI for lactating cows and used for ration formulation, and the equation based on filling effects can be used to evaluate DMI as affected by diet forage fiber characteristics.
Fei Sun, PhD, PAS
Dairy Product Technical Manager, Origination LLC.
NRC 2001. Nutrient Requirements of Dairy Cattle. 7th rev. ed. Natl. Acad. Sci., Washington, DC.
S. Allen, D. O. Sousa, and M. J. VandeHaar. 2019. Equation to predict feed intake response by lactating cows to factors related to the filling effect of rations. J Dairy Sci. 102(9):7961-7969.