Browsing by Person "Heering, Ruth"

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    Impact of physically effective fiber on protein metabolism in dairy cows
    (2022) Heering, Ruth; Dickhöfer, Uta
    Fiber is a prerequisite in the nutrition of dairy cows, because it promotes rumen motility and mat formation, while stimulating chewing activity (i.e. eating and ruminating). Chewing and salivation ensure a stable rumen environment for the microbial symbionts by providing an even release and recycling of nutrients as well as by buffering the end products of rumen fermentation, and thus reducing the risk of acidosis. The concept of physically effective neutral detergent fiber (peNDF) has been introduced to evaluate the adequacy of fiber supply to dairy cows. The peNDF combines both, the physical and the chemical characteristics of fiber, namely the dietary particle size and the neutral detergent fiber concentration. Various studies have been conducted to understand the effects of peNDF, particularly those concerning rumen health issues and the prevention of acidosis. Although some studies have looked at the effect of peNDF on nitrogen (N) metabolism, no studies are known that have evaluated the impact of peNDF on the partitioning of N excretion in dairy cows. Given that the peNDF can be considered as that fraction of the feed that stimulates chewing and salivation in ruminants, it may enhance recycling of circulating N and therefore increase the efficiency of N use by the animal. In this doctoral project, it was hypothesized that increasing the peNDF concentration of a diet stimulates chewing activity in dairy cows, thereby promoting salivation up to a certain peNDF concentration, after which chewing activity declines as a result of lower dry matter (DM) intake. It was further hypothesized that greater chewing activity and salivation may stimulate rumen N recycling and microbial protein synthesis (MPS), compensating for potential negative effects of reduced rumen-degradable crude protein supply on feed intake, nutrient digestibility, and performance of dairy cows. The present dissertation uses the rumen N balance (RNB) to evaluate the availability of degradable crude protein in the rumen. Overall, three in vivo studies were conducted to investigate the proposed hypotheses. The first study tested the effects of four graded peNDF concentrations of a total mixed ration (TMR) on the chewing activity of eight lactating dairy cows to evaluate whether the selected peNDF concentrations have an effect on chewing activity. The second study evaluated the effects of different combinations of two dietary peNDF concentrations and two RNB (RNB0: 0 g/kg DM and RNB–: – 1.5 g/kg DM) in a TMR fed to twenty lactating dairy cows. The third study tested the effects of four different peNDF concentrations in a TMR fed to four rumen-fistulated lactating dairy cows. Overall, increasing the peNDF concentration did not always result in a reduction of DM intake by cows, which could be related to their stage of lactation and energy requirement. Total chewing time and total chews per kilogram of DM intake increased linearly with increasing peNDF concentration; however, the increment in chewing activity may have resulted only in a small increase in salivation and hence, in N recycling in the animal. Although, high peNDF concentration negatively affected the yield and efficiency of MPS and apparent total tract digestibility (ATTD) of nutrients at low RNB, there were no indications that these parameters had any effect on the yield or composition of milk produced by the cows. There was also an interaction between the peNDF concentration and the RNB in the diets of dairy cows, with the effects being more pronounced at low RNB. Therefore, recommended peNDF concentration for dairy cows will vary with RNB and vice versa. The minimum RNB needed to sustain rumen functioning may differ depending on the dietary peNDF concentration. In this regard, more studies are needed for a better understanding of the interaction between RNB and peNDF concentration. Finally, offering dairy cows a slightly negative RNB diet at an optimum peNDF concentration may be a means of reducing the N emission from dairy milk production and hence, may provide economical and environmental benefits.