title: Effect of protein provision via milk replacer or solid feed on protein metabolism in veal calves.
abstract: The current study evaluated the effects of protein provision to calves fed a combination of solid feed (SF) and milk replacer (MR) at equal total N intake on urea recycling and N retention. Nitrogen balance traits and [(15)N2]urea kinetics were measured in 30 calves (23 wk of age, 180±3.7kg of body weight), after being exposed to the following experimental treatments for 11 wk: a low level of SF with a low N content (SF providing 12% of total N intake), a high level of SF with a low N content (SF providing 22% of total N intake), or a high level of SF with a high N content (SF providing 36% of total N intake). The SF mixture consisted of 50% concentrates, 25% corn silage, and 25% straw on a dry matter basis. Total N intake was equalized to 1.8g of N·kg of BW(-0.75)·d(-1) by adjusting N intake via MR. All calves were housed individually on metabolic cages to allow for quantification of a N balance of calves for 5 d, and for the assessment of urea recycling from [(15)N2]urea kinetics. Increasing low-N SF intake at equal total N intake resulted in a shift from urinary to fecal N excretion but did not affect protein retention (0.71g of N·kg of BW(-0.75)·d(-1)). Increasing low-N SF intake increased urea recycling but urea reused for anabolism remained unaffected. Total-tract neutral detergent fiber digestibility decreased (-9%) with increasing low-N SF intake, indicating reduced rumen fermentation. Increasing the N content of SF at equal total N intake resulted in decreased urea production, excretion, and return to ornithine cycle, and increased protein retention by 17%. This increase was likely related to an effect of energy availability on protein retention due to an increase in total-tract neutral detergent fiber digestion (>10%) and due to an increased energy supply via the MR. In conclusion, increasing low-N SF intake at the expense of N intake from MR, did not affect protein retention efficiency in calves. Increasing the N content of SF at equal total N intake decreased urea production, increased protein retention, and coincided with improved fiber degradation. Therefore, results suggest that low N availability in the rumen limits microbial growth and rumen fermentation in calves fed low-N SF (93g of CP/kg of DM), and this effect cannot be compensated for by recycling of urea originating from MR.
authors.affiliation: Animal Nutrition Group, Wageningen University, PO Box 338, 6700 AH Wageningen, the Netherlands. Electronic address: Harma.Berends@nutreco.com.
authors.forename: H
authors.initials: H
authors.lastname: Berends
authors.affiliation: Animal Nutrition Group, Wageningen University, PO Box 338, 6700 AH Wageningen, the Netherlands.
authors.forename: J J G C
authors.initials: JJ
authors.lastname: van den Borne
authors.affiliation: Department of Animal Health and Bioscience, Aarhus University, DK-8830 Tjele, Denmark.
authors.forename: B A
authors.initials: BA
authors.lastname: Røjen
authors.affiliation: Animal Nutrition Group, Wageningen University, PO Box 338, 6700 AH Wageningen, the Netherlands.
authors.forename: W H
authors.initials: WH
authors.lastname: Hendriks
authors.affiliation: Animal Nutrition Group, Wageningen University, PO Box 338, 6700 AH Wageningen, the Netherlands.
authors.forename: W J J
authors.initials: WJ
authors.lastname: Gerrits
mesh_terms: D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D001835:Body Weight; D002417:Cattle / Q000502:physiology*; D004032:Diet / Q000662:veterinary*; D004043:Dietary Fiber / Q000378:metabolism; D004044:Dietary Proteins / Q000008:administration & dosage / Q000032:analysis*; D008892:Milk / Q000378:metabolism; D011506:Proteins / Q000378:metabolism*; D012417:Rumen / Q000378:metabolism; D012820:Silage / Q000032:analysis*; D014508:Urea / Q000378:metabolism; D003313:Zea mays / Q000378:metabolism
chemical_list: D004043:Dietary Fiber; D004044:Dietary Proteins; D011506:Proteins; D014508:Urea
keywords: concentrate; digestibility; milk replacer; urea kinetics