During the transition period – the six to eight weeks between late pregnancy and early lactation – cows are under immense metabolic challenge as they transition from non-lactating, late pregnancy to milk production. It coincides with a time when the cow’s immune system is at an all-time low, leaving them susceptible to infection.
Metabolic diseases like milk fever (parturient hypocalcaemia), ketosis, and fatty liver have secondary impacts and the occurrence of one often opens the door to another. They also crack the door open for infectious diseases like mastitis.
DairyNZ data suggests that for every downer cow, two more cows have milk fever and 16 more cows have subclinical milk fever.
Dr Jean Margerison, a specialist in dairy cow nutrition at Massey University, said paying attention to the dietary cation-anion difference (DCAD) of the springer diet in herds prone to metabolic diseases could lead to significant gains.
The DCAD is calculated from the amount of positively charged (cations) potassium and sodium and negatively charged (anions) chlorine and sulphur in the cows’ diet. The balance between the two influences the blood pH, which in turn affects calcium absorption and magnesium availability.
“If a cow has a positive blood pH, she stores calcium. When she calves and needs that calcium for colostrum and milk production, she takes it out of her blood, leaving her deficient as it takes time to start mobilising calcium from other stores in the body.”
Normally, a cow’s blood pH sits at about neutral, a pH of 7. A negative DCAD would reduce the blood pH, increasing calcium and magnesium absorption.
Margerison said both pasture, particularly in effluent paddocks, and pasture silage were high in potassium – a cation. In herds genetically prone to milk fever, feeding too much pasture or pasture silage could trigger milk fever.
Margerison recommended feeding a maximum of just 3kg DM/cow of pasture or pasture silage a day to reduce the cation proportion in the springer diet. Offering 2-3kg DM/cow of straw or hay would contribute to gut fill and dilute the overall energy of the diet. Cereal silages such as maize offered a low DCAD option to make up the rest of the diet. Palm kernel was another low DCAD option but was high in phosphorous which could interfere with the production of hormones used in calcium absorption.
Supplementing with anion salts like magnesium chloride, magnesium sulphate, or magnesium oxide would also have an impact, though Margerison warned the unpalatable nature of these meant they should not be used in an attempt to correct a poorly designed diet.
“You cannot fix a diet that is completely wrong just by putting in a lot of minerals – the cows just won’t eat it.”
In the New Zealand context, Margerison said springer mobs could effectively be treated as the transition mob. Two weeks pre-calving on a low DCAD diet was enough time for any potential benefits to be felt.
The transition diet usually had similar elements to what was fed in early lactation.
“The profile of the microbes in the rumen changes according to what you are feeding, so you are preparing the rumen for the milking diet.”
