Supplying Energy to Dairy Cows – When to Add Starch or Fat?

Dr. Kirby Krogstad, Assistant Professor, Department of Animal Sciences, The Ohio State University

Introduction

Throughout 2025, I have focused on various aspects of carbohydrate nutrition. I am going to continue that theme again to complete this year of Buckeye Dairy News. Carbohydrates are the largest portion of a lactating cow ration and starch comprises about half of the total carbohydrate fraction. Generally, we feed starch because it provides degradable nutrients in the rumen and postruminally. Each kg of starch digested equates to 4.2 Mcal of energy for the cow to partition amongst her various needs (NASEM, 2021). Fortunately, starch is not the only energy source we can feed to cows. Both protein and fat provide energy; fatty acids have 9.4 Mcal/kg of energy while protein has 5.65 Mcal/kg (NASEM, 2021). For this article, we’ll focus on the comparison of starch to fatty acids as an energy source for cows.

The choice of how much energy to provide each cow and which form to provide is an important strategic choice. We need to consider a variety of factors like stage of lactation of the cows being fed, level of forage NDF in the ration, the degradability of the starch sources in the ration, the type of fatty acids to which you have access, the marginal cost of additional starch or fat, and the marginal benefit of including additional starch or fat when making this choice of how to meet the cow’s energy needs.

The goal of this article is to provide some food for thought about when additional starch or fat may be useful for lactating dairy cattle. When done right, providing proper amounts of starch and fat can maximize milk component production, animal health, and profitability.

The following are 3 considerations when determining whether additional starch or additional fat is the right choice for meeting a cow’s energy needs:

1.  Do you need more milk fat or milk protein?

The choice of energy source has a direct effect on both milk fat and milk protein production from lactating dairy cattle. During 2024, milk fat price averaged at $3.29/lb while protein averaged $1.90/lb and producers were incentivized to increase milk fat production. The quickest way to increase milk fat production under market conditions like this is by providing fatty acids. One of the most potent fatty acids for increasing milk fat is palmitic acid; supplementing approximately 400 g/d of palmitic acid supplement increased milk fat yield by 110 g/d (Shepardson and Harvatine, 2021, Bales et al., 2024).

On the other hand, adding starch to increase energy may reduce milk fat yield in some cases (dos Santos Neto et al., 2025), but it also may increase milk protein yield (Morris et al., 2020, Krogstad and Bradford, 2023). Starch increases milk protein yield because it may increase microbial protein production and alter mammary metabolism such that protein synthesis increases.

If you’re short on milk fat and the economics make sense, additional fatty acids may help. If you’re short on protein, some additional starch may be part of the solution from an energy perspective.

2.  Managing body condition.

Applied nutrition and energetics can be thought of as an exercise in body condition management. If cows are getting too thin, they may need additional energy or an energy source that is more likely to be partitioned to body tissue than to milk. Or, perhaps, cows are getting too fat, and the goal is to limit additional body condition gain.

If your goal is to increase body condition gain or bodyweight gain, additional starch generally increases bodyweight gain in lactating dairy cattle. Cows fed a 21% starch with 2.7 % animal fat gained bodyweight while cows fed 15% starch diet with 3.5% animal fat lost bodyweight in early lactation (Piccioli-Cappelli et al., 2014). Similar results have been observed in mid and late lactation cows, too; in diets with similar energy concentration, starch, in place of fatty acids and fiber, increased bodyweight gain (Boerman et al., 2015, Morris et al., 2020, dos Santos Neto et al., 2025). Of course, this strategy can be overdone as additional starch may reduce milk fat production (van Knegsel et al., 2007, Boerman et al., 2015).

Supplementing palmitic acid, depending on the stage of lactation, also influences body weight change. Supplementing 300 g/d of palmitic acid during early lactation has resulted in greater bodyweight loss (de Souza and Lock, 2019) while supplementing palmitic acid to mid-lactation cows did not alter bodyweight gain – both palmitic acid fed cows and control cows gained bodyweight (de Souza and Lock, 2018).

3.  What does the basal diet include?

When deciding to make ration changes to increase energy, it is critical to factor in what the diet already includes. For example, if you’re considering adding starch to the ration you must ensure that you have adequate forage NDF. In two experiments we’ve conducted at OSU we’ve demonstrated this concept. When forage NDF is low (~12.5% of diet DM), increasing starch reduces milk fat production from 1.9 kg/d to 1.6 kg/d. When forage NDF was adequate (~19% of diet DM), increasing starch did not affect milk fat production. Another consideration is the concentration of unsaturated fats in the diet; including additional starch with increasing concentrations of unsaturated fatty acids may increase the risk of milk fat depression (Ramirez Ramirez et al., 2015)..

If you’re considering adding fat to the ration you MUST monitor DMI. Increasing calcium salts of fatty acids can reduce DMI by 1 kg for each percentage unit of FA increased above 3% of the diet in the ration (Weld and Armentano, 2017). Thus, although adding fat to a diet increases energy density of the ration, it may reduce DMI which mitigates the potential increases in energy intake.

Conclusion

As with any nutritional decision, whether to add starch or fat to meet the energy needs of the cow depends on the context, goals, and economics of the farm and the time. When milk fat is economically valuable, like it was for much of 2024, feeding additional fat from sources like palmitic acid may be the best option for increasing energy in the diet. When fat is less valuable as it is currently ($2.19/lb) then you may want to re-evaluate the supplementation of fatty acids. If protein is more valuable, like it was during 2022, then additional starch may be more economical. The other considerations include body condition management and what the basal diet includes. Balancing rations that maximize profitability will always be three-dimensional chess where there is almost never 1 correct answer.