The effects of dry period versus continuous lactation on metabolic status and performance in periparturient cows

Effect of various dietary fat supplementations on liver lipid and glycogen of high-yielding dairy cows in the peripartal period

In a model experiment, Holstein-Friesian dairy cows were fed on a corn-silage-based diet supplemented with 11.75 MJ NE1 per day of calcium soaps of palm oil fatty acids (CAS) or hydrogenated triglyceride (HTG) or without fat supplementation (control). All diets were fed to the cows over a period from 21 +/- 3 days (d) prior to the expected calving to d 100 +/- 5 postpartum. On d 25 (basal sample) and d 14 prepartum as well as on d 5 and 25 postpartum liver samples were collected by percutaneous biopsy. Total lipid content, fatty acid composition and glycogen of liver tissues were determined. At d 5 postpartum, both control and CAS cows had higher liver lipid (P < 0.05) and lower glycogen (P < 0.05) concentrations than cows in the HTG group. No significant (P < 0.05) differences were detected in liver fat content among the groups at d 14 prepartum or d 25 postpartum. The glycogen concentration slightly decreased in the liver of cows in each treatment group from d 14 prepartum to d 5 postpartum; however, this decrease was more intensive in both the control and CAS groups than in the HTG group. The variations in liver lipid concentrations were accompanied by significant changes in the proportion of C16:0, C16:1n-7, C18:0, C18:1n-9, C18:2n-6 and C20:4n-6 fatty acids in the liver lipids. The results show that HTG supplementation exerted more advantageous effects on liver lipid and glycogen metabolism than did CAS supplementation.

Mechanisms and regulation of calcium absorption from the gastrointestinal tract in pigs and ruminants: comparative aspects with special emphasis on hypocalcemia in dairy cows

Adequate blood calcium (Ca) concentrations are a prerequisite to maintain several physiological functions of mammals such as pig and ruminants. Thus, blood Ca levels have to be regulated within very close limits. This is basically ensured through the coordinated effects of the calcitropic hormones parathyroid hormone (PTH) and calcitriol (1,25-dihydroxyvitamin D3). If plasma Ca decreases, one main effect of subsequently secreted PTH is the stimulation of renal reabsorption of Ca from the glomerular filtrate to reduce urinary Ca loss. Another important action of PTH is the induction of the renal enzyme 1[@@@]\rmalpha [@@@]-hydroxylase, which is responsible for the production of calcitriol. In most monogastric species, so far investigated, one of the most important effects of calcitriol is to stimulate active absorption of Ca from the gastrointestinal tract, particularly the upper small intestines, via a vitamin D-receptor-mediated genomic action. Whether this concept can be transferred without any constrictions to ruminants is still under discussion. Marked interspecies differences have to be recognized with respect to ruminant or non-ruminant animals, particularly with respect to individual segments along the gastrointestinal axis and with respect to vitamin D sensitivity of Ca transport mechanisms. This review will elucidate some of the current concepts related to the mechanisms and sites of Ca absorption in pigs and ruminants with special emphasis on dairy cows where Ca homeostasis is occasionally compromised at the time of parturition.

Reducing or eliminating the dry period of dairy cows

This review considers the research that has been conducted recently on reducing the length of the dry period of dairy cows, with particular emphasis on the effects of eliminating the dry period altogether. Milk yield in the subsequent lactation is reduced by up to 25%, but this loss is offset to some degree by the milk produced when cows would otherwise be dry. The lower subsequent milk yield in cows continuously milked is most likely to be a consequence of changes in the mammary gland during late gestation rather than insufficient feed or body condition to maintain milk synthesis. Shortening or eliminating the dry period may result in a lower incidence of metabolic problems post-partum, and a reduced negative energy balance in early lactation due to the maintenance of dietary intake while milk yields and body condition loss are reduced. The reductions in both body condition loss and negative energy balance may have a beneficial influence on reproductive performance. However, it is concluded that more research, particularly with cows that graze pasture during lactation, together with an economic appraisal, is needed before it could be recommended that Australian dairy farmers change their current dry period practices, particularly if continuous milking was to be considered.