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As milk prices remain below break-even, dairy managers continue to fine-tune feeding programs. At the Four-State Dairy Nutrition Conference in June, Mike Allen of Michigan State University discussed an intriguing approach using the HOT concept (hepatic oxidation theory).
There are several factors
to consider when building rations using metabolic aspects of lactating cows:
Fill factor (amount and type of fiber) reduces feed intake in early lactation. Rumen fermentation and feedback (e.g., levels of propionic acid production in the rumen) reduces feed intake in late lactation.
Corn silage has a lower rumen fill factor and produces higher levels of propionic acid. Legume-grass forages have higher fill factor and produce higher levels of acetic acid. High-moisture corn and steam-flaked corn produce more propionic acid (source of blood glucose). Byproduct feeds such as corn gluten feeds and beet pulp produce more acetic acids (less blood glucose).
Early-lactation cows respond to elevated levels of BST, while insulin response to higher glucose levels is low. In late lactation, insulin drives extra energy/glucose to stored body fat.
Body condition factors. Early-lactation cows with high peak milk yields are driven by highly fermentable feeds. These diets in late lactation increase body condition scores as insulin drives extra energy into body fat.
High-producing cows require more glucose (from highly fermentable rations leading to propionate production in the rumen converted to glucose in the liver and starch digestion in the lower tract)
to synthesize lactose. These cows respond to rations with more fermentable carbohydrates (from digestible fiber, starch and sugars) and less bulky ingredients (legume-grass forages), which can limit dry matter intake.
High-yielding cows have the ability to eat more dry matter unless feed intake is limited by gut fill (high-fiber rations with lower digestible fiber). Highly fermentable rations can depress dry matter intake, increase body condition score and lead to milk test depression for lower-producing cows as insulin drives glucose to be stored as body weight and reduces milk yield.
Glucose demand for late-lactation cows declines as milk lactose synthesis is lower (less milk produced). Because of insulin sensitivity and lower bovine somatotropin hormone secretion, late-lactation cows drive circulating glucose to body condition gain.
The fill factor in a ration is determined by the level of fiber and digestibility of the forage. Research at Michigan State demonstrates that a one-unit increase in forage neutral detergent fiber (NDF) digestibility will increase a cow's milk yield by 0.55 lb. of 3.5% fat-corrected milk within a forage type.
Michigan researchers also report that grass-based forages have greater fill factors compared to high-quality
legume and corn silage forages due to slower passage rates with grass. Based on these studies, grass and legume-grass mixtures could be used by lower-producing cows when gut fill is less important and could be metabolically favorable to slow body weight gain.
Table 2 outlines the feed strategies for grouping cows to coordinate their metabolism, hormonal changes, available forage, concentrate choices and body condition changes. The Michigan State concept may result in lower feed costs while optimizing body condition scores and improving health in future lactations.
More on HOT rations from the Four-State Dairy Nutrition and Management Conference
(starts on page 8)