Consequences of dry period length and dietary energy source on physiological health variables in dairy cows and calves

During the transition period, dairy cows experience a negative energy balance (<strong>NEB</strong>) caused by the high energy requirement for milk yield, while feed intake is limited. Severity of the NEB has been associated with an increased incidence of metabolic disorders and infectious diseases, inflammation, immunosuppression and oxidative stress. It is known that shortening or omitting the dry period or feeding a glucogenic ration improves the energy balance (<strong>EB</strong>) in dairy cows in early lactation. It can be expected that an improvement of the EB due to shortening or omitting the dry period results in reduced inflammation, immunosuppression and less oxidative stress in dairy cows in early lactation. The first objective of this thesis was to study the effects of dry period length and dietary energy source on immune competence, inflammatory biomarkers and oxidative stress in dairy cows over 2 subsequent lactations. The second objective was to study the consequences of maternal dry period length on colostrum immunoglobulin content and immune competence of calves in the first 12 weeks of life. In the current study, 167 cows were assigned to 3 dry period lengths (0, 30, or 60 d) and 2 early lactation rations (glucogenic or lipogenic). Cows were planned to have the same dry period length and ration over 2 subsequent lactations. Omitting the dry period reduced plasma bilirubin levels compared with a conventional dry period, which is line with the better EB in cows with a 0-d dry period. Effects of dry period length on inflammatory biomarkers, oxidative stress variables and natural antibodies (<strong>NAb</strong>) titers were, however, less consistent. Omitting the dry period increased not only negative acute phase proteins (<strong>APP</strong>) in plasma, but also positive APP, oxidative stress variables in plasma, and NAb in milk. Shortening the dry period to 30-d did not influence inflammatory biomarkers and oxidative stress compared with a conventional dry period of 60-d. Occurrence of clinical health problems did not differ between cows with different dry period lengths. In the current study, changes in positive APP and oxidative stress variables in plasma and NAb in milk could be explained by the occurrence of clinical health problems related to inflammation (clinical mastitis, fever, metritis and retained placenta), rather than a better EB due to a shorter or no dry period. Moreover, a higher titer of IgG binding lipopolysaccharide in plasma was associated with decreased odds of high somatic cell count and occurrence of clinical mastitis. In the first lactation after implementation of dry period length and dietary treatments, feeding a glucogenic ration in early lactation increased NAb titers in milk compared with a lipogenic ration, which could be explained partly by a better EB. In the second lactation after implementation of dry period length and dietary treatments, feeding a lipogenic ration in early lactation increased cholesterol levels in plasma compared with a glucogenic ration, which could be related to the high fat content in this ration. Cows with a 0-d dry period had a lower colostrum production and less immunoglobulins in colostrum compared with cows with a 30-d or 60-d dry period. After colostrum uptake, NAb titers in plasma of calves from cows with a 0-d dry period were lower during the first week of life compared with calves from cows with a 30-d or 60-d dry period. Levels of specific antibodies in calves, after immunization in week 6 and 10, in calves were not affected by the maternal dry period length. Birth weight of calves from cows with a 0-d dry period was lower compared with calves from cows with a 30-d dry period, but not compared with calves from cows with a 60-d dry period. Growth of calves until 12 weeks of life was not affected by dry period length. In conclusion, although shortening and omitting the dry period improved the EB in early lactation, this did not result in clear consistent effects of dry period length on inflammation or oxidative stress. Changes in inflammation biomarkers, oxidative stress variables and NAb in milk were a reflection of the occurrence of health problems related to inflammation in particular clinical mastitis and compromised uterine health. Furthermore, albeit omitting the dry period compared with shortening or conventional dry period cows resulted in a reduced immunoglobulin content in colostrum and reduced NAb titers in plasma of their calves in the first week of life, but did not affect specific immune response of the calves in the first 12 weeks of life.