Why Do Dairy Cows Need Fat? A Guide to Using Fats in Cattle Diets

Introduction

Nutritionally, fats, also known as lipids, are an incredible source of energy. Fat provides many benefits to livestock diets including increasing energy density and improving reproductive performance. Fats contain 2.25 times more energy than carbohydrates, which means that small amounts of fat can add a large amount of energy to a ration (Feinstein, 1983). This energy boost can impact milk production and reproduction. It is especially relevant during early lactation when the cow is experiencing negative energy balance and uses her stored energy (in the form of body fat) to support milk production. But harnessing the benefits of fat relies on providing the right kinds and amount of fats in the diet. This article will define what fat is, how much fat your cows need, and what happens when the cow consumes it, and when fat supplementation may be beneficial.

What is fat?

The one characteristic that defines all fats is that they are not soluble in water, regardless of whether they are in the form of solids or liquids (oils). Fats are composed of smaller units called fatty acids (FA).  The properties of a specific fat are determined by the composition of these fatty acids. Fatty acids are chains of carbon atoms that may contain single or double bonds.  When all bonds are single, the fatty acid is classified as saturated, whereas the presence of one or more double bonds in a fatty acid will classify it as unsaturated. In livestock feed and animal products like milk, fat is typically found in the form of triglycerides which are molecules consisting of three fatty acids attached to a glycerol backbone. Triglycerides are the primary form of lipid stored in animal tissues and the main type of fat in milk (98%) (MacGibbon et al., 2009).  

What happens to the fat in the cow?

When fat arrives at the rumen, normally in the form of triglycerides, the glycerol backbone is separated from the fatty acids in a process named hydrolysis. Glycerol groups are used as energy sources for rumen microbes while the fatty acids will go through different processes depending on their length and saturation level. For example, unsaturated FA can be toxic for some rumen bacteria, especially those that ferment fiber. Due to this phenomenon, unsaturated FA goes through a process called biohydrogenation which converts the FA’s double bonds to all single bonds (Maia et al., 2010). If the process goes to completion, the result is a saturated fatty acid. For that reason, many unsaturated fatty acids entering the rumen will leave in the form of stearic fatty acid, which is saturated. On the other hand, saturated FAs do not undergo major changes after hydrolysis (Jenkins et al., 2008). 

After leaving the rumen, the fatty acids are absorbed in the small intestine. From there they travel through the blood to different parts of the body depending on the needs of the cow. When no extra energy is required or excess energy is fed in the diet, fat is stored as adipose tissue (body fat). The cow can pull on these energy reserves when needed (Bauman et al., 1980).  In early lactation, for instance, the cow is going through a negative energy balance due to the high energy requirements of milk production and will use fat from body storage for extra energy, leading to body condition loss. Excessive body fat storage can contribute to metabolic issues or poorer reproductive performance. 

Together, dietary fatty acids from forages, grains, and byproducts typically amount to roughly 3% of diet dry matter.  In general, it is recommended that lactating dairy diets do not exceed 7% of dietary dry matter as fatty acids, and less than 5% for fresh cow rations to minimize the negative impact of higher amounts of fat on rumen fermentation (Jenkins, 1993; Van Soest, 1994). Adding fat to a diet increases energy density and can support milk production, reproductive performance, and improve body condition. On the other hand, feeding excess fat can reduce microbial activity in the rumen, decrease dry matter intake, cause excessive body condition gain, and lead to metabolic diseases and reproductive issues. 

When does the cow need more fat?

There are some situations when the cow can benefit from an increase in dietary fat content without dropping intake such as: 

• In early lactation to support milk production or mitigate body condition loss. 
• High-producing cows to keep up with the energy needs of their high performance. 
• During heat stress when cows can reduce their dry matter intake and therefore, they need to obtain more energy per unit of feed consumed. 

Types of fat in dairy cow diets

Saturated Fats

In ruminant nutrition, saturated fats are typically found in animal-based sources such as tallow and choice white grease. Saturated fats are typically solid at room temperature. In the rumen, saturated fats remain relatively unchanged, while unsaturated fats are hydrogenated by rumen microorganisms. Some examples of saturated fatty acids are 

• Palmitic acid (C16:0) → Commonly used in fat supplements for dairy cattle.  
• Stearic acid (C18:0) → Commonly found in animal fats and dominant fatty acid leaving the rumen. 
• Myristic acid (C14:0) → Present in smaller amounts in common feeds used in dairy diets. 

Unsaturated Fats

In ruminant nutrition, unsaturated fats are typically found in plant-based sources, including vegetable oils and oilseeds.  Unsaturated fats are typically liquid at room temperature. In the rumen, unsaturated fats can be toxic to some rumen bacteria and will therefore undergo extensive modification compared to saturated fats. The main process of transformation is known as biohydrogenation. Due to those changes, you can find unsaturated fatty acids in animal tissue too but often in lower portions. Some examples of unsaturated fatty acids are 

• Oleic acid (C18:1) → Common in many plant oils. 
• Linoleic acid (C18:2) → Abundant in oilseeds such as soybeans. 
• Linolenic acid (C18:3) → Common in forages and some oilseeds. 

Other ways to classify fat

Protected Fats

Protected fats are also known as bypass fats. These fat supplements are typically coated by a substance that protects them from interactions with rumen microbes. The purpose of protecting fatty acids is to avoid biohydrogenation, allowing the fat to remain unchanged for better absorption in the small intestine.