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Smith, Katherine Mae

Animal and Veterinary Sciences

2021

M.S.

In the dairy industry, a common way of adding energy to the diet is in the form of fermentable starch. However, an overabundance of fermentable carbohydrates can cause a buildup of volatile fatty acids that exceed the buffering capacity of the rumen. These changes can lead to negative health consequences such as subacute rumen acidosis and milk fat depression. Not only does a cow need readily fermentable sources of energy in her diet, but she also needs physically effective structural carbohydrates.Fiber has been a difficult portion of the diet to characterize as both the chemical and physical properties are important for the maintenance of animal health and to support requirements for proper production. The physical nature of fiber can be described using the measurement physically effective neutral detergent fiber (peNDF). This describes the fraction of dietary fiber that stimulates chewing and forms the rumen digesta mat. The chemical nature of fiber can be described in multiple terms, but to describe the undegradable fraction, a measure named undegradable neutral detergent fiber after 240-h of fermentation (uNDF240) was developed. This fraction is important to characterize because of its effect on gut fill, degradation, and passage dynamics in the rumen. Research has observed a relationship between uNDF240 and peNDF. Combining these two characteristics into one measurement could give nutritionists a more useful measurement when balancing diets for dairy cattle. This measurement is the physically effective uNDF240 (peuNDF240). But, in order to fully implement this measurement in ration balancing, there must be more research on how peuNDF240 affects dry matter intake (DMI) and milk responses. The primary goal of this thesis research was to observe the interactions between rumen fermentable starch (RFS) and peuNDF240 in the rumen. The focal study (Chapter 2) investigated the effects of RFS and peuNDF240 on DMI, lactation performance, behavior, and the rumen environment of lactating Holstein dairy cows. The four diets were: 1) low peuNDF240 (6.4% of DM), low RFS (16.7% of DM); 2) low peuNDF240 (6.1% of DM), high RFS (19.2% of DM); 3) high peuNDF240 (8.6% of DM), low RFS (16.9% of DM); and 4) high peuNDF240 (8.0% of DM), high RFS (19.0% of DM). Cows fed higher peuNDF240 diets consumed less DMI as a percentage of body weight than cows fed lower peuNDF240 diets. Cows fed diets containing higher RFS produced less milk fat and 3.5% fat-corrected milk (FCM) than cows fed lower RFS. Lower peuNDF240 diets resulted in lower de novo and mixed origin fatty acids in milk fat and a greater degree of unsaturation. Overall, higher RFS diets tended to reduce the efficiency of FCM production (FCM/DMI) compared with lower RFS diets. There was no effect of peuNDF240 or RFS on eating or ruminating time per day, but higher RFS diets reduced meal length and increased daily meal bouts. The lower RFS diets reduced acetate:propionate ratios, but there was no effect of treatment on measures of rumen pH. Diets with higher RFS tended to increase rumen pool size of starch while higher peuNDF240 diets increased pool size of uNDF240 although there were few dietary effects on rumen turnover of starch or uNDF240. Lower peuNDF240 diets resulted in greater total tract NDF digestibility than higher peuNDF240 diets. Even low to moderate RFS can elicit milk fat depression when dietary peuNDF240 is low and ranging between 6.0 and 8.6% of DM.