Dairy cows are bred for the ability to produce large yields of milk, from which dairy products are made. To be able to maintain high milk production, a dairy cow must be bred and produce calves. High-producing dairy cows require large amounts of high-energy feeds to meet nutritional requirements for maintenance, milk synthesis and reproduction.(1) Therefore, there are many nutrition studies involving dairy cows.(2-5) High-starch–low-fiber diets, through shifting the pattern of end products of ruminal fermentation towards greater propionate and lactate, provide readily available sources of energy (i.e. glucose) for growing muscles. This will often lead to greater feed conversion efficiency and growth.(2) The changed nutrient partitioning involves decreased lipogenesis and increased lipolysis, and results in a state of negative energy balance (NEB). Although mobilization of body fat tissue is a physiological adaptation in the high-yielding dairy cow in NEB, this can have a negative effect on health and productivity.(6) In early lactation, cows with metabolic profiles indicating a severe NEB, such as high concentrations of NEFA, have an increased risk of diseases such as clinical ketosis, displaced abomasum, metritis, or retained placenta.(7) Furthermore, insulin is an important regulator of lipogenesis. It is shown that dairy cows selected for increased milk yield have greater insulin resistance, which is associated with increased body lipid mobilization and lower body condition score.(1, 6) Plasma concentrations of insulin generally decline postcalving and this decline is often associated with a more severe NEB, which makes it an important biomarker to understand the health of the animal.(8)
In warmer parts of the world reduced milk production results from a phenomenon called heat stress. It counteracts the genetic progress achieved in increasing milk production in cows.(9) Heat stress has an enormous economic impact on the global dairy industry. Factors contributing to this economic issue include decreased milk production, increased metabolic disorders and health problems, slow heifer growth, compromised milk quality, and reduced reproductive performance.(9) Lactating cows prefer ambient temperatures ranging between 5-25°C. At temperatures above that, the cow can no longer cool herself adequately and thereby enters heat stress.(7) Elevated respiration rates and rectal temperatures, impaired metabolism and poor reproductive performance characterize heat stress in cows.(7) Furthermore, high-producing cows have been found to be more affected by heat stress than its low-producing counterparts due to that the zone of thermal neutrality shifts to lower temperatures as milk production, feed intake and metabolic heat production increase.(7) The decreased milk yield caused by heat stress is partly caused by reduced nutrient intake and partly by an energy-intake independent change in nutrient partitioning, such as the metabolic profile.(10) In cattle, the heat stress increase insulin secretion and decrease insulin sensitivity, making insulin an important biomarker to measure insulin resistance.(10)
Dry period: From the time of lactation until the cow calves again. (The dry period is necessary to allow a cow time to replenish her body reserves, especially minerals that were depleted during the lactation period and also to give the udder tissue a chance to compress and renew before the next lactation).
Gestation period: From conception to calving (nine months).
Heat stress: Heat stress in dairy cattle is one of the leading causes of decreased milk production and fertility.
Heifer: Female calf.
Lactation period: Milk producing period (about 305 days).
Mastitis: A persistent, inflammatory reaction of the udder tissue.
Metritis: An inflammation of the wall of the uterus.
Negative Energy Balance (NEB): A severe negative energy balance occurs in early lactation and is characterized by reduced blood glucose and insulin concentrations and elevated blood GH concentrations.
Open period: The period between calving and re-conception.
Perparturient period: 3 wk precalving to 3 wk postcalving, which is a very critical period when innate and acquired defence mechanisms are low.
Transition period: The time between 60 days prior to and 60 days after calving; the most critical time within this period is the 21 days before and after calving.