Dry Matter Intake by Cattle
Animal productivity is highly related to ration quality and dry matter intake (DMI). On high forage diets, animal performance is directly related to DMI. Understanding and managing the factors that influence DMI is key to the old saying, “The eye of the master finishes the cattle.”
The factors that determine DMI fall into three classes: animal characteristics that drive DMI, ration characteristics that limit DMI, and environmental factors that impact DMI (Table 1).
Table 1. Factors that drive and influence dry matter intake (DMI) in cattle.
|Drive DMI||Limit DMI||Environmental factors|
|Bodyweight||Ration NDF||Air temperature|
|Milk production||Ration NE||Plant toxins|
|Body condition||Forage mass/allowance||Water requirement and intake|
Drivers of DMI Demand
Body weight (BWt) is the animal characteristic that has a primary influence on DMI (Figure 1A). Large animals have a greater DMI than small animals, when all else is equal. This is why we often define DMI as a percentage of BWt. Milk production has a major impact on DMI by lactating animals. Cows producing more milk have a greater DMI drive than cows producing less milk (Figure 1B). In lactating animals, fat corrected milk production is the second most important factor driving DMI, after BWt. Milk production and DMI are in a dynamic equilibrium. If ration quality does not provide adequate nutrition to maintain a given level of milk production, production will go down, and the DMI drive will go down. That is why when feeding dairy animals in early lactation, the practice is to lead feed the animals, meaning that the manager ensures that ration quality does not limit nutrient intake and milk production in early lactation. Once animals have passed peak milk production and are rebred, the animal’s nutrient requirements go down and ration quality can follow it down since the ration only needs to maintain milk production and restore body condition.
Figure 1. A) Body weight (BWt) and B) 4% fat corrected milk (FCM) production influence
absolute dry matter intake (DMI_lb) and DMI expressed as a percent of BWt (DMI_pct_BWt),
Body Condition Score
Body condition score (BCS), or how fat an animal is, affects DMI. Thin animals eat more than fat animals. The effect of BCS on DMI has been viewed as a function of metabolic body weight, BCS in dairy animals, and weight in growing bulls (Figure 2). The simplest description is a straight line for dairy cattle and beef bulls. All DMI versus BCS models give approximately the same relative DMI response when BCS is equal to or greater than four. The fat on the back of an animal serves as insulation during cold weather and provides reserve energy when energy intake is low. That is why “a fat cow is half wintered.”
Air temperature affects DMI. The DMI by cattle, relative to their DMI when daily air temperature averages 70 F, decreases greatly when daily average air temperature increases above 70 F (Figure 3). Relative DMI increases gradually as average air temperature decreases to below freezing. Cattle take about 30 days to adjust to changes in temperature. This can be observed in the spring when the weather has been cold, then turns warm over a day or two. Cows under this scenario can show heat stress even though the temperature is only in the 70s. Likewise, in the fall, if the weather has been warm and then a cold front comes through taking the temperature down, cattle can experience greater cold stress than expected since they are not adapted to the cold. In winter, cold stress is also affected by hair depth, if the hair is wet or muddy, and by wind speed.
Figure 2. Relative dry matter intake (relDMI) by dairy cattle and yearling bulls as a function of beef cattle body condition score (BCS).
Figure 3. Dry matter intake (DMI) of cattle, relative to their DMI when air temperature averages 70 F, decreases greatly as air temperature increases above 70 F, and increases gradually as air temperature decreases to below freezing.
Implants in growing and finishing beef cattle increase DMI by 4% to 16%. The NRC uses an average DMI increase due to implants of 6% to 8% when modelling beef cattle DMI and growth (NRC Beef 2000, p.92).
Limits to DMI
In high forage diets, forage neutral detergent fiber (NDF) has a major impact on DMI and ration digestibility. As ration NDF increases, DMI of lactating dairy cattle and beef cattle decreases (Figure 4A). Steers have a similar DMI response to ration NDF as do lactating cattle. As previously mentioned, there is an equilibrium between ration NDF, milk production and DMI. For lactating cattle, as ration NDF increases, the level of milk production that the animal will be able to maintain decreases (Figure 3B). In a similar manner, average daily gain in growing animals decreases as ration NDF increases.
There is a rule of thumb that cattle can only eat 1.2% of their body weight in NDF. This rule of thumb originated for high producing dairy cows and was needed to provide a minimum of 35% NDF (19% ADF) in the ration to prevent milk-fat depression. This minimal level of NDF is needed to maintain rumen health. Cattle that are not producing high levels of milk can consume much greater level of NDF than can high producing dairy cows. This is the value of beef cattle consuming relatively low quality, high NDF forage and converting it into steaks and burgers.
Figure 4. A) Ration neutral detergent fiber (rNDF) content affects dry matter intake as a percentage of body weight (DMI_pct_BWt) and B) milk production as pounds of 4% fat corrected milk (FCM_lb) for angus cross beef cattle (Ang-X), lactating Holstein cows (Hol), and Holstein steers (Hol steer).
NDF Affects Forage Digestibility & DMI
Animal productivity is powered by solar energy fixed in forage organic matter. This energy becomes available as the forage is digested in the rumen and intestines and then metabolized into the animal’s body for maintenance, body growth and fattening, milk production or fetal growth. A classic measure of forage organic matter digestibility is total digestible nutrients (TDN). As NDF increases, the TDN content of the forage decreases (Figure 5). The spread of TDN values around the NDF regression line is a function of the digestibility of the NDF fiber. Livestock nutritionists use one of several energy systems. These energy systems are TDN, digestible energy (DE), metabolizable energy (ME), net energy lactation (NEL), net energy maintenance (NEm), and net energy gain (NEg) (Figure 6). The TDN in feeds is directly related to DE, with 1.0 pounds of TDN providing 2.0 megacalories of DE. For beef cattle, 1.0 megacalorie DE provides 0.82 megacalorie of ME. The measures of NEL and NEm are almost identical (Figure 6). As energy is used in the body, part of the energy is lost as heat and part is used to metabolize energy into the NEg in the meat and fat in the growing body or NEL in milk produced by a lactating animal.
The NDF in a forage or ration influences rate of passage and digestibilty of the ration. When NDF drops below 55% dry matter, rate of passage increases resulting in decreased digestibility of the forage NDF. When forage NDF is above 55%, forage NDF digestibility decreases and the TDN value of the dry matter decreases.
When cattle are feed grain in feedlot rations, NDF decreases as grain is increased in the ration. The inclusion of high energy grains in the ration results in high net energy intake which reduces DMI (Figure 7, Beef feedlot A). This differs from lactating cows and growing yearlings feed high forage diets (Figure 7, Dairy, Beef cows & steers).
Figure 5. As forage neutral detergent fiber (NDF) increases the total digestible nutrient (TDM) content of the forage decreases. Variation about the average linear regression line is due to the digestibility of the NDF and the rate of passage from the rumen.
Figure 6. Total digestible nutrients (TDN) as a percent of dry matter (DM) in a forage or ration is directly related to the energy available to the animal measured as digestible energy (DE), metabolizable energy (ME), net energy lactation (NEL), net energy maintenance (NEm), and net energy gain (Neg).
Figure 7. Ration neutral detergent fiber (Ration NDF%) content affects dry matter intake (DMI %BWt) in lactating dairy cattle (Dairy A and Dairy RF), lactating beef cows and growing steers (Beef cows & steers), and feedlot fed beef cattle (Beef feedlot A) (adapted from Arelovich et al. and Rayburn and Fox).
Pasture Forage Mass and Forage Allowance Impact DMI
The availability of pasture and how much forage is allocated to each animal on the pasture affects DMI (Figure 8A) and animal daily gain (Figure 8B). When forage mass drops below 1,000 pounds of forage dry matter per acre forage mass limits DMI. A pasture ruler height of 4 inches is about 1,000 pounds of forage dry matter per acre. Pasture forage quality determines animal performance across all levels of forage mass (Figure 8B). When pasture FM is greater than 1,000 pounds of forage dry matter per acre, DMI is a function of forage quality and selective grazing by the livestock (Figure 8A). Selective grazing is the ability of livestock to select and eat forage of a higher quality than the average forage in the pasture. Selective gazing can increase forage quality by up to 20%. When FM is greater than 1,000 pounds of forage dry matter per acre, forage quality is the main factor determining animal DMI and productive performance (Figure 8B). When FM is less than 1,000 pounds of forage dry matter per acre, forage availability limits animal performance.
Figure 8. A) When forage mass in a pasture drops below 1,000 pounds dry matter per acre, dry matter intake (DMI) decreases since animals cannot eat as much forage as possible due to the lack of forage intake per bit. When forage mass is above 1,000 pounds of forage dry matter per acre, relative DMI is a function of forage quality in the pasture and animal selective grazing. B) The effect of forage mass (FM) and forage quality (invetro digestible dry matter IVDDM) on DMI is reflected in average daily gain (ADG) of steers on pasture.
Crude Protein to TDN Ratio
The balance of crude protein (CP) to TDN affects bacterial digestion rate of forage in the rumen, which can limit DMI. When the ratio of CP to TDN is below 0.20, the low CP limits bacteria reproduction and growth, which limits forage digestion rate (Figure 9A). When a protein supplement is added to a low CP hay, the digestion rate in the rumen increases and DMI of the hay increases (Figure 9B). This enables us to feed supplemental CP to increase hay intake so that the TDN needed by the animal can come from home grown forage, reducing the need for purchased energy supplements. When feeding a protein supplement, such as soybean meal, distiller’s grain or corn gluten pellets, 1.0 pound of added CP can increase hay and supplement intake by 5 pounds of TDN. An interesting side to this CP to TDN ratio for bacteria is that it is a bacteria-to-its-food-supply relationship, not the animal’s nutritional need. The ratio applies in the rumen, in the soil and in the compost pile when expressed as available nitrogen to actively degradable carbon ratio.
Another aspect of the CP to TDN ratio is for high CP forages when the ratio is significantly greater than 0.20. These pastures and hay crops can be used as the source of supplemental protein for low protein hays or pastures. Also, for pastures having a CP to TDN ratio well above 0.20, the excess CP above the needs of the bacteria and the animal will be excreted as urea. This is an energy cost to the animal, resulting in lower-than-expected average daily gain or milk production. When weaning calves on high CP pasture, supplementing with a high carbohydrate feed, such as corn or barley, provides additional carbon for the rumen bacteria so they can make use of this otherwise excess CP.
When comparing National Research Council book values for beef cattle DMI to the DMI values based on NDF as used here, the differences in DMI are due to the CP to TDN ratio in the recommended rations.
Figure 9. A) Rumen bacteria need adequate crude protein (CP) to reproduce and digest the total digestible nutrients (TDN) in the forage. When the ratio of CP to TDN is below 0.20 CP limits bacterial growth, forage digestion rate and dry matter intake (DMI). B) An example of how adding a protein supplement at four increasing levels to a low CP hay increases hay DMI and total ration DMI.
Plant Toxins Impact DMI
Alkaloids and other toxins in plants lower forage palatability and DMI. Our most common plant toxins are found in tall fescue containing toxic endophytes. These tall fescue toxins induce heat stress in the cattle and reduce forage digestibility both of which reduce forage DMI. In Alabama, as endophyte levels increased from 1% to 34%, and then to 90%, steer average daily gain decreased from 2.1 to 1.76, and then to 1.41 pounds per day, respectively.
Adequate high quality water is needed by livestock to maintain high feed DMI. As average air temperature increases, cattle require more water per pound of dry matter consumed. At average daily temperatures of 60 F to 70 F, cattle need 4 pounds of water for every pound of dry matter consumed. At cooler temperatures, the water requirement decreases slightly to 3 pounds of water per pound of ration dry matter. When average daily temperature increases above 80 F, water requirement increases greatly to over 10 pounds of water needed for each pound of forage dry matter. In pasture containing 20% dry matter and when temperatures are below 70 F, much of the animal’s water requirement is contained within the forage. High quality water is the least expensive nutrient and should be available to livestock as needed.
Figure 10. As average daily air temperature increases above 60 F, water intake per pound of ration dry matter (DM) increases at an increasing rate. Cattle breeds differ slightly in their water requirements per pound of ration DM.
Forage DMI is impacted by a number of factors related to the animal, the forage and the environment. At first, this looks like a complex system. However, by evaluating each factor in turn the manager can determine which one of the factors is limiting production and how to manage their system to optimize animal nutrition, health and productivity.
Author: Ed Rayburn, Retired WVU Extension Specialist – Forage Agronomy