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Submitted to: American Society of Animal Science Proceedings
Publication Type: Proceedings Publication Acceptance Date: 4/29/2014 Publication Date: 10/2/2014 Citation: Gunter, S.A. 2014. Predicting the forage intake by lactating cows. Western Section of American Society of Animal Science Proceedings. 65:158-161. Interpretive Summary: The current intake model that is used throughout the world to estimate the dry matter intake by cows is based on a single equation related to metabolic size and net energy density of the diet. However, research has indicated that the observed forage intake by grazing cows can be influenced by animal demands, largely determined by body weight, physiological state, genetics, or any combination of the three. The lactating cow has the highest nutrient requirements of all the physiological states she can experience. A broad-based database was developed from published experiments where forage intake was reported for lactating cows. New equations were developed using stepwise regression modeling. It was found that cow body weight only explains a small portion of the variation noted in forage intake; regression modeling showed that forage crude protein, calf weaning weight, calf average daily gain, and milk production significantly affected the cow’s forage intake. Forage intake models that included cow milk production explained the most variation. However, milk production is difficult to measure in field application. Hence, a model was constructed using calf weaning weight as a segregate for milk production. The model using calf weaning weight accounted for less variation than the milk production model, but performed nearly as well. Hence, the new intake models to predict the forage intake by lactating beef cows will need to contain other independent variables than just cow body weight; models probably will need to contain independent variables that quantify metabolic demands that are altered by the cow's physiological state. Technical Abstract: The current National Research Council (NRC) model to estimate the dry matter intake by cows is based on a single equation related to metabolic size and net energy density of the diet. However, research has indicated that the observed dry matter production by grazing cows can be influenced by animal demands, largely determined by body weight (BW), physiological state, genetics, or any combination of the 3. The lactating cow has the highest nutrient requirements of all the physiological states she can experience. A broad-based database was developed from published experiments where forage intake was reported for lactating cows. New equations were developed using stepwise regression modeling. It was found that cow BW only explains a small portion in the variation noted in forage intake; regression modeling showed that herbage crude protein, calf weaning weight, calf average daily gain, and milk production significantly (P < 0.01) affected forage intake. Forage intake models that included cow milk production explained the most variation (r squared =0.55). However, milk production is difficult to measure in field application, hence, models were constructed using calf weaning weight as a segregate for milk production. The model using calf weaning weight accounted for less variation (r squared = 0.30) than the milk production model, but only had a 4% downward bias. Hence, the new NRC models to predict the forage intake by lactating beef cows will need to contain other independent variables than just cow BW; models probably will need to contain independent variables that quantify metabolic demands that are altered by the cow's physiological state. |
