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Research reveals slim benefits of high-fat diets for calves

Xavier Suarez-Mena and Mark Hill for Progressive Dairy Published on 06 November 2020

Fat is an energy-dense nutrient that is a major component in calf milk replacers (MRs) and dry feeds. Over the last 15-plus years, there have been an array of studies looking at MR formulas with different crude-protein-to-fat ratios.

There has also been a fair amount of research addressing crude protein (CP), amino acids, fat, fatty acids and energy requirements of calves that should influence how MRs are formulated today. Here, we will discuss the research published in expert-reviewed journals relating to fat and how it impacts calf growth and health.

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It’s important to understand the impacts of fat in MRs because as fat content of the MR increases, digestibility of many important nutrients in the diet decreases. In research with single-stomached animals, production of amylase – the starch-digesting enzyme – is reduced as fat intake increases. In research with calves, intake of dry feeds stimulates the pancreas to produce amylase, which is virtually nonexistent in newborn calves. Therefore, if high-fat MRs reduce or delay intake of dry feeds, and digestion is reduced with less enzyme production, growth would be limited near the time of weaning and post-weaning.

With all farm animal species, there are optimal ratios of CP (or amino acids) to energy (or fat) in the diet to support productivity. Several research trials conducted in both the winter and summer months have addressed these ratios in calves. The optimal ratio for bodyweight gain and structural growth is between 51 to 55 grams of CP per Mcal of metabolizable energy. Easier nutrients to remember to meet this ratio for an MR are 24% to 26% CP and 16% to 18% fat (on an as-fed or feed tag basis).

This optimal nutrient density varies with the amount of an MR fed daily and the amino acid and fatty acid fortification of the diet. A typical 20% to 22% CP, 20% fat MR has too little CP relative to energy or fat to optimize calf growth. However, this MR formulation, when fed at approximately 1 pound of solids daily, will support as much growth as higher-protein formulas, albeit less growth than higher-protein formulas fed at 1.5 pounds of solids. We will mention what this low protein-to-fat ratio could mean to calf performance and carcass composition in the next paragraph.

In the winter months, fat supplements are sometimes added to calf MRs. Some companies manufacture MRs that are higher than 20% fat for winter feeding. Research has reported some increase in bodyweight gain with high-fat diets in the first three to four weeks of a calf’s life. However, when calf growth was followed to 8 weeks old, there has been no benefit to bodyweight gain, largely because the calves eat less dry feed. Additionally, high-fat diets promote carcass fattening, so more of the bodyweight gain was fat.

Not only was there more fattening when feeding more fat, but also there is less frame growth and less protein deposition in the carcass. This all falls in line with an optimal CP-to-energy (or fat) ratio, since added fat lowers the ratio of the diet away from the optimal ratio. When there is an excess of fat, the growth shifts to fat versus frame.

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Recently, we investigated the topic of MR feeding rate and fat concentration in MRs in three trials with Jersey calves. In those trials, MRs were fed between 1 and 1.5 pounds per calf daily and measurements were made to 4 months old. We found that when using 24% protein MRs, feeding more than 17% fat in the MR did not increase calf bodyweight gain, and it reduced frame growth. The data in Jersey and Holstein calves are similar.

Looking beyond MRs, there is little research showing benefits to adding small amounts of fat in dry feeds, and most studies show no benefit. Dry feed diets with added fat tend to be consumed at lesser amounts than diets with low fat. This is not desired, as dry feed intake drives development of the rumen and other digestive organs and growth of the young calf.

In the data that we shared, there are no reports of better immunity or health when calves are fed different amounts of fat or a specific ratio of CP to energy or fat. However, there are benefits to immunity or health from supplementing specific fatty acids, but that is a topic for another day. For now, be sure to keep in mind that too much fat for calves can result in less-than-optimal rates of gain. Work with your nutritionist or feed mill to get the right amount of fat to support calf growth and meet your goals.  end mark

Xavier Suarez-Mena, Ph.D., is a calf and heifer specialist with Provimi. Mark Hill, Ph.D., is a ruminant nutritionist with Provimi.

References omitted but are available upon request. Click here to email an editor.

Take-home messages

  • Too much fat in milk replacer can result in less-than-optimal rates of gain.

  • Studies showed a benefit for protein-to-fat ratios in milk replacer between 1.4 and 1.5. This is achieved using a 24% to 26% crude protein (CP) and 16% to 18% fat formula fed at approximately 1.6% of birth bodyweight. (This is approximately 1.5 pounds of solids per calf daily to typical Holstein calves and approximately 1 pound of solids per calf daily to typical Jersey calves.)

  • Greater than 18% fat in a formula can lead to reduced solid feed intake and reduced performance around weaning.

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