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Making Milk From Forage: Principles Are Important

03 February 2015

To optimize milk component production from forages we must understand rumen fiber digestion and passage, says a leading US forage researcher.

Digestion characteristics of neutral detergent fibre (NDF) influence feeding and rumination, rate of particle breakdown, rumen turnover and fill, dry matter intake, and overall rumen and productive efficiency.

This is according to Rick Grant, William H Miner Institute, where much research takes place to build understanding about rumination and feeding of cattle.  

Mr Grant writes that, traditionally, nutritionists have focused primarily on measures of NDF digestibility, but recently the focus has included undigested NDF as well because of the recognition of its importance in setting the extent and influencing the fast and slow rates of rumen fiber fermentation.

Differences Between Forage Type

Grasses, legumes, and grain-containing forages such as corn silage behave differently in the rumen, and we must understand their unique digestion and passage characteristics. Legumes such as alfalfa have more fragile NDF than grasses and their forage particle size decreases more rapidly with rumination.

Across a wide range of forage types we’ve observed a positive relationship between 24-hour NDF digestibility and forage fragility measured as rate of particle reduction during milling. Grasses tend to increase the rumen pool size or amount of large fiber particles compared with legumes thereby retaining more small fiber particles and contributing to a slower passage rate from the rumen (i.e. selective retention). Consequently, grasses contribute to greater rumen fill and mass of physically effective NDF.

Eating and Ruminating

In addition to increasing rumen fill, higher forage diets with slower fermenting forage-fiber require substantially longer to process by the cow (eating and ruminating) which can pose an often-overlooked time budgeting constraint, especially with overstocked feed bunks.

In contrast, diets containing highly fermentable forage-fiber that’s too highly fragile can result in lower chewing time, rumen pH, fat output, and efficiency of solids-corrected milk production (i.e. cows consume more dry matter without a corresponding increase in milk yield).

This lower rumen and productive efficiency can be corrected by addition of forages that elicit greater chewing per unit of NDF such as straws or grass hays.

What About High Yielders?

High-producing cows with their greater intake and appetite will be more quickly limited by rumen fill with average quality grasses versus legumes. The typical NDF digestion curves for legume and grass forages show that legumes such as alfalfa have a 15-20 per cent faster initial rate of NDF digestion versus grasses, but the extent of NDF digestion is 30-40 per cent greater for grasses reflecting 30-40 per cent less lignin.

For average grasses and legumes, the digestion curve lines cross at approximately 24-30 hours of fermentation. Beyond this point, the greater extent of grass NDF digestion will be an advantage.

Recent research indicates that the mean rumen retention time for marked haycrop silage and corn silage NDF particles is approximately 35-45 hours for cows consuming about 60 lb/day of dry matter and producing 100 lb/day of milk.

These data indicate that highly productive cows can effectively utilize grass forage as a source of fermentable NDF. A critical management goal is to shorten the fermentation time needed for the two forage digestion curves to cross. The normal range in 30-hour NDF digestibility for grass silage measured in the U.S. is about 55 - 70 per cent.

We need to manage grass for harvesting at the upper end of this quality range. This quality of grass will fall within the range of rumen retention that we have measured for haycrop and corn silage particles of medium size.

Take Home Goals

Overall, the nutritional goal is to maximize productive response to forage NDF whether you are feeding a higher forage diet or simply making strategic use of smaller amounts of forage in the ration.

Particularly for grasses, we are learning how to optimize their use in the ration for highly productive cows.

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