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Nitrogen balance (NB) experiments allow calculation of N retention in the body by subtracting N excreted in feces (NF), urine (NU) and milk (NM) from N intake (NI). In a previous study, we found that NB data from experiments with lactating dairy cows were generally high and, in the current meta-analysis, we update our earlier study with experiments from the last 2 decades and investigate probable causes of error. A total of 83 publications, with 86 experiments and 307 dietary treatments, were selected from top-ranked scientific journals that reported all NB components. The NB and NB components were analyzed by linear regression with a model that used NI as an independent variable and experiment as a random effect. The NF, NU and NM each represented 27 to 34% of NI, and the remaining N accumulated in the body was equal to 38.5 g/d (overall SD = 43.2 g/d). Retained N (as g/d or % of NI) increased linearly with NI, and this led to unlikely high N retentions, especially at high NI. Both NF and NU (g/d) increased with increasing NI, and we assume that some N in feces and urine were unaccounted. Only ~22% of experiments measured N in wet feces samples and, when analysis used dry samples, no mention of corrections due to potential volatile N losses during drying were reported. No experimentalists preserved feces immediately to prevent volatilization during collection. Moreover, ~27% of experiments estimated urine volumes by concentration of creatinine in spot samples, and in these experiments, NU was ~12% lower than those where total urine was collected (168 vs. 191 g/d). Only 40 experiments reported the volume and concentration of acids used for urine preservation, 33 furnished incomplete information, and the remainder did not describe the urine preservation method. In conclusion, the results of NB experiments using lactating dairy cows overestimate N retention, and the losses of N from feces and urine are the most probable reason. [ABSTRACT FROM AUTHOR]

Study aims were to investigate associations of hyperketolactia (HYKL) status of Holstein dairy cows between 6 and 60 d in milk (DIM), defined by milk acetone (mACE) and β-hydroxybutyrate (mBHB) content, with daily milk yield and composition. Milk samples (∼5.0 million) were collected over a 5-yr period (2014–2019) within the milk recording system in Poland. Concentrations of mACE and mBHB determined by Fourier-transform infrared spectroscopy were used to categorize samples into 4 ketolactia groups. Based on threshold values of ≥0.15 mmol/L mACE and ≥0.10 mmol/L mBHB, ketolactia groups were normoketolactia (NKL; mACE <0.15 mmol/L and mBHB <0.10 mmol/L), BHB hyperketolactia (HYKL BHB ; mACE <0.15 mmol/L and mBHB ≥0.10 mmol/L), ACE hyperketolactia (HYKL ACE ; mACE ≥0.15 mmol/L and mBHB <0.10 mmol/L), and ACE and BHB hyperketolactia (HYKL ACEBHB ; mACE ≥0.15 mmol/L and mBHB ≥0.10 mmol/L). To investigate ketolactia association with production outcomes, a linear model was developed, including ketolactia group, DIM, parity, their interactions, year-season as fixed effects, and random effects of herd and cow. Among all milk samples, 31.2% were classified as HYKL, and of these, 52.6%, 39.6%, and 7.8% were HYKL ACEBHB , HYKL BHB , and HYKL ACE , respectively. Ketolactia groups differed for all traits studied in all parities and DIM. Among HYKL groups, lowest milk yield was found in HYKL ACEBHB cows, except for 6 to 30 DIM in first- and second-lactation cows. Milk yield of HYKL BHB cows was higher than that of NKL cows until 20 to 30 DIM, and then it was lower than NKL cows. Milk yield of HYKL ACE cows was mostly lower than NKL cows. Energy-corrected milk (ECM) yield of HYKL ACEBHB cows was higher than that of NKL cows until 30 to 35 DIM for second lactation and third lactation or greater, and in the whole study period for first lactation. The yield of ECM for HYKL BHB cows was mostly higher than that of NKL cows, whereas HYKL ACE cows had higher ECM than NKL cows until 15 to 25 DIM and then was lower for the HYKL ACE group. Milk composition differed among HYKL groups. Highest milk fat (MF) and lowest milk lactose (ML) contents were observed in HYKL ACEBHB cows. Cows in HYKL ACEBHB and HYKL BHB groups had higher MF and lower milk protein (MP; except in 6–8 DIM in first lactation) and ML content than NKL cows. Milk fat content was higher in HYKL ACE than NKL cows in first lactation and during the first 30 to 40 DIM in older cows. Lactose content was lower in HYKL ACE than in NKL cows within 30 to 40 DIM; afterward it was higher in NKL cows. Lower MP content was found in HYKL ACE than in NKL cows, except during 6 to 9 DIM for cows in first lactation and third lactation or greater. In conclusion, HYKL is associated with altered milk production in all parities, but a range of these negative relations depends on ketone status addressing both ACE and BHB contents. Further research is needed to ascertain underpinning biochemical defects of HYKL from elevated ACE, alone or in combination with BHB, during early lactation. [ABSTRACT FROM AUTHOR]