Advice on Metabolic Grouping Presented
I had the opportunity to listen to Dr. Mike Allen of Michigan State University present an excellent paper (Allen and Bradford, 2011) on physiological and physical factors controlling dairy cow feed intake at the recent Western Dairy Management Conference in Reno, Nev.
His proceeding paper outlined the same metabolic factors controlling feed intake that I summarized in my February column (Feedstuffs, Feb. 14).
This column will be drawn from Allen's slides and comments and will focus on practical grouping strategies based on the changing metabolic status cows experience from close-up to late lactation.
The goal for close-up cows is reducing fat mobilization and maintaining rumen fill through the transition period. Stress should also be minimized as stress hormones increase fat mobilization, which is already high during this period.
According to Allen, diet formulation should target: (1) no supplemental fat since cows are already experiencing reduced intakes from high non-esterified fatty acid (NEFA) levels, (2) forage sources with a relatively long rumen retention time and (3) moderate inclusion rates of highly fermentable starch to increase insulin levels and potentially reduce fat mobilization.
In defense of lower inclusion levels of highly fermentable starch in close-up diets, reference was made to a study by Dann et al. (1999) that compared cracked corn and steam-flaked corn in prepartum diets. Cows fed the steam-flaked corn had lower plasma NEFA concentrations during the prepartum period, presumably due to a reduction in fat mobilization as a result of increased glucose precursors.
Fresh cow diets
The goal for fresh cows is to: (1) increase plasma glucose and insulin levels, (2) reduce plasma NEFA levels and liver fat content and (3) maintain rumen fill to stimulate rumen buffering and provide distention for reducing the incidence of displaced abomasums.
Diet formulation should target: (1) moderately high forage fiber concentrations with a relatively long rumen retention time and (2) low to moderate starch fermentability to allow for higher starch inclusion levels without causing acidosis or reducing meal size.
The need for propionate to produce glucose (driving lactose and milk yield), stimulate insulin and reduce fat mobilization must be balanced against the tendency for too rapidly available propionate levels to reduce meal size, resulting in an overall reduction of total glucose precursors supplied to the cow.
Fresh cows typically have a large pool of acetyl co-enzyme A (CoA) from partial NEFA oxidation. Under these conditions, propionate is used for glucose production. While this appears advantageous, highly fermentable diets can result in propionate stimulating the oxidation of acetyl CoA quickly within meals, increasing adenosine triphosphate production and signaling satiety to the brain sooner.
More research is clearly needed on how altering starch fermentability plays a role in stimulating fresh cow intakes.
Allen said he frequently observes herds where cows are kept in the fresh cow pen for too long, and as a result of the lower-fermentability/high-rumen fill diets, high-producing cows tend to "stall out."
Michigan State currently has a research interest in the appropriate time to move cows to peak-lactation diets.
Cows peaking in milk have high levels of circulating growth hormone, partition more energy to milk production and respond well to more fermentable diets.
Rumen fill is the primary mechanism limiting feed intake as cows enter this phase of production, so diet formulation should target low rumen fill by: (1) minimizing perennial grasses, which are very filling compared to legumes, (2) maximizing corn silage (an annual grass) due to fiber that digests and passes the rumen relatively quickly, (3) using highly fermentable starch sources to maximize total energy intake and drive microbial protein production and (4) limiting intake of unsaturated fats, which can reduce rumen motility and further contribute to rumen fill intake impediments.
Allen referenced research from Dr. Dale Bauman's laboratory at Cornell University indicating that the trans-10, cis-12 conjugated linoleic acid, which has been shown to down-regulate gene expression for milk fat synthesis, also appears to be a very important repartitioning agent by increasing adipose tissue gene expression for fat synthesis.
The change in the dominant mechanism of intake regulation from hepatic oxidation to rumen fill can occur in as quickly as a week for some cows and can take as long as three weeks for other cows. Practical signs of when to switch to peak-lactation diets are lower plasma NEFA and ketone concentrations, increased gut distention and steadily increasing feed intake. Fat cows generally take longer to make this transition because high NEFA levels from larger fat stores depress intakes and lower tissue insulin sensitivity.
One-group total mixed ration herds that are being managed to prevent fat cows (e.g., higher fiber, lower starch) are likely not attaining their peak milk, which they could attain if they followed a metabolic grouping strategy and implemented these peak-lactation diet recommendations.
Cows in later lactation: (1) are more insulin sensitive, (2) partition more energy to adipose, (3) require less glucose and (4) are more susceptible to milk fat depression. Rumen fill is now less of a concern than at peak lactation, and intake is again being regulated more by metabolic factors.
Late-lactation diets should target more of a maintenance approach to maintain body condition scores of 3.0 or slightly higher by formulating with: (1) forages with a longer rumen retention time, (2) less-fermentable starch sources, (3) more non-forage fiber sources to help dilute starch concentrations and (4) limited unsaturated fat for cows are already prone to milk fat depression.
The Bottom Line
Dairy cow feed intake is primarily influenced by forage neutral detergent fiber concentration, forage digestion characteristics (e.g., fragility, filling effect and digestibility), diet fermentability - particularly starch - and the type/amount of fat.
Metabolic grouping is an approach to varying the diet formulation from close-up to late lactation based on the current understanding of physiological and physical factors driving feed intake and production.
Allen, M.S. 2011. Personal communication.
Allen, M.S., and B.J. Bradford. 2011. Control of energy intake through lactation. Proceedings 10th Western Dairy Management Conference. Reno, Nev.
Dann, H.M., G.A. Varga and D.E. Putnam. 1999. Improving energy supply to late gestation and early postpartum dairy cows. J. Dairy Sci. 82:1765-1778.
This article was originally published in April 2011 Feedstuffs issue and is reproduced with their permission.