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Our aim was to determine the effects of P intake on P balance, serum parathyroid hormone (PTH) levels and bone resorption during the final 4 weeks prepartum and the first 8 weeks of lactation. Sixty pregnant multiparous Holstein Friesian dairy cows were assigned to a randomized block design with repeated measurements and dietary treatments arranged according to a 2 × 2 factorial design. The experimental diets contained 3.6 (high phosphorus, HP) or 2.2 (low phosphorus, LP) g P/kg DM during the dry period (Dry-HP and Dry-LP, respectively) and 3.8 or 2.9 g P/kg DM during 56 d after calving (Lac-HP and Lac-LP, respectively). The level of dietary P, expressed as g/kg DM, of Dry-LP was 18% greater than recommended by the Dutch Central Bureau for Livestock feeding, while the P content of Lac-LP was, across the 8 wks of lactation, 24% lower than recommended. Both P intake and fecal P excretion decreased in the dry period to increase again in subsequent lactation. Cows fed high dietary P excreted more P in the feces than cows fed low dietary P pre- and postpartum. Cows in both Dry-HP and Dry-LP were in positive P balance in the dry period. Cows in both Lac-LP and Lac-HP were in negative P balance after calving, with the negative P balance being more pronounced in Lac-LP than in Lac-HP. Serum concentrations of PTH, and apparent total-tract OM and NDF digestibility, were neither affected by any 2-way or 3-way interaction between time of sampling and dietary treatments nor by the P concentration of the experimental diets during the pre- and postpartum period. Before calving, serum carboxy-terminal collagen crosslinks (CTX) concentrations were basically similar between Dry-HP and Dry-LP. After calving, serum CTX concentrations increased, with a more pronounced increase when Lac-LP was fed compared with Lac-HP. The results suggest that when feeding diets containing low P (2.9 g/kg DM) postpartum, cows excreted less P in the feces than at recommended dietary P (3.8 g P/kg DM) without negative impact on OM and NDF digestibility. The increase in serum CTX concentrations, without increasing serum PTH concentrations upon feeding low P, indicates a prominent role of bone resorption to meet P demands in the first 8 weeks postpartum. The present trial focused on the final 4 weeks of gestation and the subsequent 8 weeks of lactation, but long-term effects of low dietary P during early lactation on serum PTH and on bone P dynamics in mid and late lactation need to be further investigated., (© 2025, The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).) more...

Omitting or shortening the dry period may result in a fairly constant ration throughout the transition period of dairy cows, reducing the need for adaptation of cow metabolism and rumen function to a new lactation. The objective of this study was to determine the effect of dry period length (DP) on rumen adaptation and cow metabolic state during the transition period. Twelve pregnant, rumen-cannulated Holstein Friesian dairy cows at the end of their first lactation were assigned to 1 of 3 treatments: a conventional (60 d), short (30 d), or no dry period (0 d). At dry-off, cows received a dry-cow ration until calving. Lactating cows received a lactation ration. Cows were monitored from 8 wk before calving until 8 wk after calving for milk yield and DMI. Rumen biopsies were taken from 3 locations in the rumen at 60, 40, and 10 d before calving and 3, 7, 14, 28, and 56 d after calving to assess papillae dimensions. Blood was sampled weekly from 3 wk before until 8 wk after calving, and liver biopsies were taken at wk -2, 2, and 4 relative to calving. Prepartum, DMI, and milk yield were greater for cows with a short or no dry period, compared with cows with a conventional dry period. Postpartum, DMI was greater for cows with a short dry period compared with cows with a conventional dry period. Plasma glucose concentration was greater for cows without a dry period, compared with the other DP postpartum. Plasma concentrations of nonesterified fatty acids and BHB, and liver triglyceride content, did not differ among dry period. Rumen papillae differed in size based on biopsy location, but there was no interaction between biopsy location and the effect of DP. Rumen papillae surface area for cows managed for a 30- or 60-d dry period decreased toward calving. At 40 d prepartum, papillae surface area was greater for short and no dry period treatment compared with a conventional dry period. At 10 d prepartum, papillae surface area was greater for the no dry period treatment compared with both other treatments, and this difference was still present 3 d postpartum. Cows managed for a short dry period showed faster increase in papillae dimensions after calving compared with cows managed for a conventional dry period. From d 28 onwards, no differences in papillae surface area were observed. The faster rumen adaptation postpartum may be related to the increased DMI during the first weeks postpartum for cows managed for a short dry period. However, this did not result in improved metabolic status or milk yield. The results from the present study demonstrate that the dietary changes related to a conventional DP affected rumen papillae development, not only prepartum but also early postpartum. Further optimization of DP as well as dietary composition throughout the transition period may support cows in their adaptation to a new lactation., (The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).) more...