Estimating and Managing Carbohydrate Digestibility

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

Introduction

After determining feed intake, we need to determine the potentially digestible nutrients within a ration. Determining the potentially digestible nutrients allows us to estimate the energy and protein being supplied to the cow. The added caveat in the ruminant being the digestibility of the carbohydrates are directly related to the protein that will be supplied to the cow as digestible carbohydrates are critical for microbial protein production. Thus, managing and estimating digestibility of rations fed to cows is critical for continuing to meet their nutrient demands at all stages of lactation.

When assembling rations, the chosen ingredients or nutrients can have palpable effects on digestibility. Feeding increasing starch and reducing NDF digestibility is a common example that we will discuss. Another example is that some specific fatty acids are more digestible than others, which can dramatically affect the energy supplied to the cow – but that is for our next article in this series.  If all the proper interactions and effects are considered, we can supply the necessary nutrients to the cow, whether that be more or less total digestible nutrients.

The Carbohydrate Story

The primary carbohydrates we are concerned about when feeding cattle are NDF, starch, and sugar. Sugar is generally considered completely degraded so we’ll focus on the factors to consider for digestibility of starch and NDF. Both NDF and starch have tremendous variability in digestibility and can be affected by feed source, maturity, dietary concentrations and intake, and other nutrients.

NDF

1. Fiber source, forage maturity, and forage variety.

The source of NDF is critical for digestibility. For example, the 48-hour NDF digestibility of cottonseed is 12.8%, while the NDF digestibility of corn gluten feed is 74% (Figure 1; NASEM, 2021). On the forage side of the equation, the fiber source is equally important; mature cool season grass NDF digestibility is 55%, while a mature rye hay is 45%.

Figure 1. The 48-hour NDF digestibility (NDFD) of common byproducts that are fed to lactating dairy cattle (NASEM, 2021; DDGS = dried distillers grains).

Within the forage NDF sources, maturity and forage variety are critical. As maturity advances, the plant increases concentration of NDF and lignin which results in reduced digestibility. According to NASEM, NDF digestibility declines by 12% units as cool season grass progresses from mid to full maturity. The same holds for legume crops like alfalfa.

An additional consideration for forages is variety selection. The genetics of the plant dictate digestibility. The most obvious example is conventional corn silage compared to brown-midrib (BMR) varieties; the NDF digestibility of BMR varieties can be 10 to 15% units greater than conventional varieties when evaluated in vitro, which often results in greater feed intake and milk yields when fed to the cows (Miller et al., 2021). Low-lignin alfalfa varieties also have greater NDF digestibility when evaluated in vitro (Sulc et al., 2021), but they have less effects when fed to the cow (Buse et al., 2024).

2. Dietary starch, sugar, and fatty acids.

Not only do inherent characteristics to the NDF matter, but other ingredients can negatively influence NDF digestibility. The most consistent is that feeding additional starch reduces NDF digestibility. Currently, it’s estimated that for each 1% unit increase dietary starch above 26% (considered the ‘base diet’ in the model) we expect a 0.6% unit reduction in NDF digestibility (de Souza et al., 2018). There have also been instances where increasing dietary sugar and removing starch has increased NDF digestibility (Broderick et al., 2008); the response could be due to reducing starch or a direct effect of adding sugar which remains to be completely understood.

Fatty acids can help or hinder NDF digestibility depending on the type of fatty acid fed. Oils derived from seeds like corn or soybeans can reduce NDF digestibility (Weld and Armentano, 2017). More recent research though has demonstrated that the addition of palmitic acid may increase the NDF digestibility when feeding dairy cattle (dos Santos Neto et al., 2021). When considering fatty acids and fat-containing supplements, the effects on NDF digestibility must be considered.

Starch

Starch is a critically important and economical source of digestible energy. It is also important for microbial protein production. Thus, extracting the maximum value from starch fed to cows is critical. Fortunately, we can increase starch digestibility from grains in a few ways. 

1. Particle size and processing

The simplest avenue for increasing starch digestibility is to reduce the particle size through grinding or rolling corn. Dry matter and starch digestibilities increase as particle size is reduced from 4,000 to <1,500 μm (Ferraretto et al., 2013). As I’ve discussed in a previous Buckeye Dairy News, “The particle size of corn grain should certainly be less than 1,500 μm with some data suggesting that 800 μm was optimal for milk protein production. Additional reductions in particle size may be beneficial, but there are not concrete peer-reviewed data to suggest that.” When reducing particle size, both hammer mills and roller mills will reduce particle size effectively, although roller mills likely result in a more consistent particle size than hammer mills.

Corn can also be steam flaked which involves the addition of steam and pressure to help break down the corn kernel. Steam flaking corn will increase ruminal starch degradability compared to ground dry corn but is less rumen degradable than high moisture corn (Galyean et al., 1976).

2. Ensiling time

We can also ensile grain as “high moisture” grain to increase the starch digestibility. Ensiling corn results in about 90% ruminal starch degradability, which is greater than other starch processing methods (Galyean et al., 1976). A meta-analysis also suggested that feeding ensiled corn grain results in greater total tract digestibility than dry corn (Ferraretto et al., 2013); the difference in starch digestibility between ensiled grain and dry grain likely shrinks as particle size of dry grains are reduced.

When feeding ensiled grains, like high moisture corn, time is a vital consideration. As ensiling time increases, the starch degradability also increases (Ferraretto et al., 2015). This means that high moisture corn will feed differently as it ages in the silo. The starch digestibility will be less immediately after harvest than 1 month, 6 months, or 12 months later. As high moisture corn ensiling time increases, total dietary starch concentration may need to be reduced to maintain rumen health and function.

Conclusions

Digestibility is a critical component to consider for lactating dairy cow rations. Poorly digestible ingredients and nutrients provide far less usable nutrients to the cow. If the goal is to maximize nutrient intake and use, we must pay attention to digestibility. Thus, we must also pay attention to the factors that affect digestibility, and as we’ve discussed, they’re numerous.  The bottom-line is that when you’re feeding dairy cattle, get the details right. It’s not just about the nutrient, it’s about the form, type, and processing that nutrient underwent.  Perhaps it’s time to give your rations another look - are you optimizing digestibility of the nutrients that you’ve invested in?