Research Accomplishment Reports 2009

Metabolic Relationships in Supply of Nutrients for Lactating Cows

K.R. McLeod
Department of Animal and Food Sciences

 

Non-Technical Summary

Over 55% of the calcium, 17% of the protein, and 15% of the energy in the US diet are supplied by dairy products; thus, the US consumer is a major stakeholder for the project. Consumers want dairy products that are safe and inexpensive, but increasingly they also want an environmentally friendly dairy industry that promotes animal well-being. Natural resources are used efficiently when milk production per unit feed and per cow is high.

To efficiently produce milk, a cow must have a well-developed mammary gland and be able to supply the gland with the nutrients it needs. Nutrition in the first year of life affects mammary gland development, and nutrition around the time of calving and throughout lactation has a major effect on the health, productivity, and efficiency of cows. Feeding for optimal nutrient intake requires not only the provision of the necessary nutrients for milk production but also consideration to the effects of diet on mammary capacity and on appetite, health, and metabolic regulation of the cow.

Because feed costs account for half of all costs on a dairy farm, nutrition also significantly impacts farm expenses. This project considers all of these factors for optimal feeding. For example, if we could maintain current milk production while feeding diets with 4 percentage units less total protein, we would decrease N losses to the environment in the US by 470,000 metric tons per year and save US dairy farmers $1 billion per year in feed costs.

This type of progress only can be made if we take an integrated approach, combining quantiative measures of metabolites and genes involved in metabolic pathways with the use of mechanistic bio-mathematical models that accurately describe metabolism and production of cows. Refinement of the current nutrition programs will allow more exact formulation of diets for lactating dairy cattle and have immediate impact. Feed costs and environmental impacts from excess dietary nutrients will be reduced for dairy farmers. Consumers will benefit from both reduced environmental impacts of food production and lower prices due to increased supply and lower input costs. Consumers will also benefit from possible alteration of the fatty acid content and composition of milk fat, resulting in a healthier diet. A more strict scientific understanding of the dairy cow will allow scientists to further refine and leverage research efforts to improve efficiency.

2009 Project Description

Results have been published in a thesis and have been accepted in manuscript form to the Canadian Journal of Animal Science.

2009 Impact

A study was conducted to examine the influence of ractopamine (RAC) on whole-body and splanchnic energy balance. Six growing Holstein steers surgically fitted with an arterial and portal, hepatic, and mesenteric venous indwelling catheters were used in a repeated measures study. Treatments were a basal diet of alfalfa cubes fed at approximately 1.5x maintenance energy requirements (d 1-21) and basal plus RAC (430mg/hd/d; d 22-42). On d 14 of each period splanchnic and portal-drained viscera (PDV) energy balance was determined as the product of arterio-venous O2 difference and blood flow. Blood flow was determined using down-stream dilution of p-aminohippuric acid. Whole-body energy balance was determined on d 15-21 of each period, which included 7 d total excreta collection and 3 d of respiratory gas exchange measurements.

Body weight (BW) and DM intake were greater for steers receiving RAC compared with those receiving the control diet, however, no difference was observed in either BW or DMI when expressed on a metabolic BW basis (MBW). Similarly, as a function of MBW, whole-body heat production and retained N and energy were unaffected by RAC. In contrast, RAC decreased energy use by splanchnic tissues, largely due to a reduction in energy use by the PDV.

These data indicate that although whole-body energy use is not affected by RAC, energy use by splanchnic tissues is decreased, thereby increasing energy use by peripheral tissues.