Publikationsdatenbank

Inter-individual variation in nitrogen and phosphorus metabolism and excretions in lactating Holstein dairy cows (2022)

Art

Hochschulschrift

Autor

Müller, Carolin Beatrix Maria (WE 2)

Quelle

Berlin, 2022 — VI, 115 Seiten

Sprache

Englisch

Verweise

URL (Volltext): https://refubium.fu-berlin.de/handle/fub188/33861

Kontakt

Institut für Veterinär-Physiologie

Oertzenweg 19 b
14163 Berlin
+49 30 838 62600
physiologie@vetmed.fu-berlin.de

Abstract / Zusammenfassung

Cattle husbandry constitutes a major source of anthropogenic nitrogen (N) and phosphorus (P) losses associated with negative effects on human, animal and environmental health. Thus, particularly in light of a growing global human population and dwindling global resources, there is a demand to develop holistic emission mitigation approaches that concurrently promote a sustainable and efficient handling of available resources. Initial mitigating attempts primarily focused on nutritional strategies such as reducing N and P content of feed, while maintaining a constant energy level, in order to ameliorate utilization efficiency of ingested N and P for milk production in dairy cows. Milk urea N (MUN) concentration was shown to linearly correlate with urinary urea excretions, thus enabling the prediction of urinary N emissions and the assessment of dietary N supply. A higher P utilization efficiency (Peff), defined as the ratio between milk P secretion and P ingested, was assumed to reduce the fecal P load in dairy cows, since more P is channeled into milk. So far, variation in MUN concentration and Peff appear to be a result of a combinatory effect between animal genetic and feed composition. Thus, selectively breeding for low MUN concentration and high Peff dairy cows may potentially contribute attenuating urinary N and fecal P load and facilitating a more sustainable dairy industry. However, a significant portion of inter-individual variation in MUN concentration and Peff and underlying physiological mechanisms are largely unknown. Thus, the objective of the present thesis was to elucidate physiological and molecular mechanisms affecting inter-individual variation in MUN concentration and Peff in lactating Holstein dairy cows. Results are implicated to contribute in assessing the mitigation potential of selectively breeding on MUN concentration and Peff, while concomitantly precising feeding strategies to attenuate the environmental N and P footprint of cattle husbandry. The first study focused on clarifying physiological mechanisms involved in the regulation of urea metabolism and associated N excretions in dairy cows with divergent MUN concentrations. For this purpose, 20 German Holstein dairy cows were purchased in pairs, comprising one cow high (277 mg/L; n = 10) and one low (189 mg/L; n = 10) in MUN concentration, but were comparable in lactation period, body weight and milk production. After transported to the institutional barn of the Research Institute for Farm Animal Biology (FBN, Dummerstorf, Germany), cows were fed two different planes of dietary crude protein (13.8% vs. 15.9% in dry matter). After a two-week feed adaptation period within a loose housing system, we conducted a 2-day indirect calorimetry and a 4-day N balance with 13C urea isotope tracer study in tie stalls. Our study revealed, MUN groups did not differ in either fecal N, urinary urea or urinary N excretions despite fed the same feed composition. The latter results have been attributed to differences in renal performance concerning the overall renal glomerular filtration rate and specifically the renal clearance rate of urea. Interestingly, high MUN cows excreted less urinary creatine independent of protein feeding and less urinary uric acid on the low protein diet than low MUN cows. However, since urinary urea accounts for 75% of whole urinary N and MUN groups did not differ in overall urinary N excretion, we concluded high and low MUN cows not to differ in terms of nitrous oxide or ammonia emissions. In contrast, a reduction in dietary crude protein level considerably attenuated urinary N excretions with simultaneously improving N utilization efficiency for milk production. The second study aimed to elucidate underlying physiological and molecular mechanisms affecting P and N excretions of dairy cows differing in Peff. To this end, 20 German Holstein dairy cows were fed a feed ration comparable in dietary P (0.37% in dry matter) and crude protein (14% in dry matter) content, both located in the lower range of recommendations by NRC (National Research Council; Nutrient requirements of dairy cattle, 2001). After a three-week feed adaptation in a loose housing system we implied a 4-day P- and N balance study in tie-stalls. Cows were retrospectively grouped into a high (40%; n = 10) and a low Peff (33%; n = 10) group according to the results obtained from the P balance study. Finally, dairy cows were slaughtered to obtain tissue samples from jejunal mucosa, renal cortex and mammary gland with subsequently analyzing the expression of P transport proteins encoding mRNA by quantitative real-time PCR. Initially, our work revealed Peff and milk P yield to be strongly related to milk fat and milk protein yield. High Peff cows had an enhanced renal P reabsorption rate compared to low Peff cows, which was associated with reduced urinary P losses. The latter was paralleled by an upregulation of inorganic P transport protein -2 expression in renal tissue suggesting to be decisively involved in affecting individual differences in urinary P excretion. However, with respect to the much greater fecal P excretion, differences in urinary P excretion seem to be negligible. High Peff cows had a lower P apparent digestibility and a more negative P balance than low Peff cows, which has been attributed to a higher mobilization of body P reserves. Thus, taking the given results, in the presence of an insufficient P supply microbial protein synthesis in the rumen and related milk synthesis seem to be prioritized in expense of body P reserves. Beyond that, we found high Peff cows to excrete more fecal P relative to P ingested than low Peff cows compromising initial ecological benefits of a high Peff. In conclusion, results suggest reducing dietary crude protein supply rather than a selectively breeding for low MUN concentration to have a significant potential on mitigating N emissions deriving from cattle husbandry. In contrast, a selective breeding for high Peff and associated Neff was shown to be ecologically and economically reasonable. However, an insufficient dietary P supply stimulated the mobilization of body P reserves. A long-term mobilization of P reserves may negatively affect animal health and longevity and intensifies fecal P load. Thus, selective breeding for high Peff is reasonable, but dietary P supply should be geared towards the respective milk P yield and should be frequently verified by fecal P load within herd.