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Dry Matter Intake – for Sep. 6, 2010

I’d like to start this first column by acknowledging the late Dr. David Price. While I never had the opportunity to meet Dr. Price, I read his column with interest and appreciated his practical approach to cattle feeding. I hope to carry on his tradition.

The topic I would like to start with is the concept of dry matter intake. The ability to predict feed intake is critical for the cow-calf operator and the cattle feeder. Without it, you can’t accurately assess if you have enough forage and grain on hand, or develop feeding programs that consistently target performance objectives.

While many factors influence feed intake, two of the most important are the level and quality of forage and grain that make up the ration. When forage makes up the majority of the ration, intake is primarily regulated by its physical characteristics. Poor-quality forage limits intake by reducing digestibility and slowing the passage rate from the rumen. With high-grain diets it is not the bulky nature of the diet that influences feed intake but rather the amount of energy consumed.

Cattle typically eat to meet their needs for maintenance and growth and stop eating once these needs are met. Beyond that, factors such as health, stress, the environment and bunk management influence whether cattle meet and maintain their maximum intake.

Feed consumption is usually expressed on an “as fed” or “dry matter” basis with the difference being the moisture content of the feed. Grain and hay diets typically contain 85 to 88 per cent dry matter (DM). The addition of silage reduces dry matter content; straight silage contains only 30 to 35 per cent dry matter.

Why does it matter how we express feed intake? It’s needed for accuracy. If you are feeding

30 pounds of alfalfa brome hay at 13 per cent moisture, that’s really 26 pounds of dry matter. That may not seem like a large difference, but consider what it means if you are feeding

30 pounds of silage at 65 per cent moisture. In this case you are feeding only 11 pounds of dry matter, and your animals are likely crawling the feed bunk looking for something to eat.

Nutritionists express an animal’s requirement for specific nutrients as absolute amounts per day (ie. pounds of TDN or protein, Mcal of net energy) or as a proportion of the diet dry matter (i. e. TDN per cent, CP per cent, Mcal/kg NE). In either case, you need to know how much dry matter an animal is expected to consume in order to put the right combination of feeds together to meet its nutrient needs.

In so doing, when we compare feeds or feeding

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programs, we compare “apples to apples.”

As many of you no doubt know, there are practical rules of thumb we can use to determine how much cattle should be eating. For example, a 1,350-pound wintering beef cow typically can be expected to eat two per cent of her body weight on a dry matter basis, or 27 pounds of DM, per day. However, as discussed above, forage quality has a great influence on how much she will actually eat. Straw or slough hay will be consumed at much lower rates, as low as 1.25 to 1.5 per cent of body weight, or 17 to 20 pounds per head per day. Intake of high-quality forages such as alfalfa or alfalfa grass hay can approach 2.5 per cent of body weight, 34 pounds per day, or better. When feeding the poor-quality forage, you must provide supplemental feed to meet her nutritional needs while with the alfalfa hay you may actually have to restrict intake to prevent her from becoming over-conditioned.

As with the breeding herd, dry matter intake (DMI) in feeder cattle is commonly expressed in one of two ways, pounds of feed dry matter per day, or dry matter intake as a percentage of body weight. It is important to understand both terms as they are used in day-to-day feeding programs. Expressed as a percent of body weight, DMI typically starts low (1.5 to 2.0 per cent) particularly for newly weaned calves, reaches a maximum (2.6 to 2.8 per cent), and then plateaus at 2.4 to 2.5 per cent of body weight. Much of our feeding management is aimed at maintaining this plateau as long as possible. However as time on feed increases and cattle gain weight, it is natural to see dry matter intake decline where cattle close to market weight are eating 1.8 to 2.0 per cent of their body weight or less.

Dry matter intake of new arrivals, particularly recently weaned calves can be unusually low due to the stress of weaning and mixing at the feedlot. In contrast, when expressed as pounds of dry matter per head per day, dry matter intake of feeder cattle typically increases with time on feed and eventually plateaus at 23 to 25 pounds depending on finishing weight. Good feeding management dictates that this increase is slow and steady, without marked fluctuations.

The take home message is that in order to accurately feed cattle for today’s market or to develop a wintering program for your cow herd, you need to have a handle on what cattle are expected to eat and what they are actually consuming.

Understanding dry matter intake and the factors that influence it is a good first step. To gain that first step you need one other thing, an accurate scale.

———

JohnMcKinnon isabeefcattle nutritionistat theUniversityof Saskatchewan

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John McKinnon

John McKinnon is a Professor Emeritus at the University of Saskatchewan and a consulting nutritionist who can be reached at [email protected].

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