Balancing Milk Production, Fertility
WHILE the use of Holstein genetics has certainly improved milk output on farms throughout the British Isles, it has also had an impact on other aspects of dairy farming management and economics including: cow nutrition, fertility, longevity and bull calf values.
So, with this in mind, a research team at the Moorepark Institute in Co Cork, undertook a series of trials, designed to quantify the impact of black and white genetics on all aspects of commercial milk production.
BACKGROUND: The introduction of milk quotas in the European Union in 1984 has led to intensification of dairy farming with increasing milk production per cow, achieved by wide scale usage of Holstein-Friesian (HF) sires. Genetic improvement of North American and European HF cows has mainly focused on increasing milk production. However, selection for milk production alone can lead to deterioration in reproductive performance.
"Increased milk production has been associated with a lower first service conception rate, a greater number of services per conception and an increased incidence of silent heats and ovarian cysts," confirmed Moorepark's Dr Seamus Crosse.
"It has also been reported that the introduction of HF genes has been associated with decreased reproductive performance in New Zealand."
While genetic improvement of the HF dairy cow is mainly focused on increasing milk production, in dual-purpose breeds other traits of economic importance are also considered - eg, meat production. Energy partitioning may be different in dual-purpose cows compared with HF cows, which may result in less body condition loss in early lactation.
However, results from studies that compare production and reproductive performance between HF and dual-purpose cows have, to date, been inconsistent.
"The effect of high milk production on the incidence of reproductive disorders may be related to the extent of negative energy balance immediately after calving,"said Seamus Crosse.
"During early lactation, many high producing cows are unable to consume enough feed to meet their energy demands, resulting in a period of negative energy balance. Early post calving negative energy balance may delay the interval from calving to first ovulation, which has detrimental effects on later reproductive performance."
In addition changes in body condition score (BCS) during early lactation follow changes in energy balance. This makes the use of BCS an accurate and repeatable method to estimate body energy or fat reserves of dairy cows.
"We know that severe body condition losses after calving have been associated with lower first service conception rate," confirmed Seamus Crosse.
"However, there is a time lag between when a cow experiences a severe negative energy balance or nutritional disturbance and when the greatest impact is seen in regard to reproductive performance. This time lag occurs because of the 60- to 100-day period required from the time a follicle within the ovary is activated to begin development until the time the follicle is mature and ready to ovulate."
The overall objective of the trials carried out at Moorepark was, therefore, to determine, courtesy of four experiments, the effects of genetic merit for milk production, dairy cow breed and pre-calving nutrition on reproductive physiology and performance.
TRIAL STRUCTURE: In the first experiment groups of high genetic merit Holstein-Friesian, medium genetic merit Holstein-Friesian, and dual-purpose Montbeliarde and Normande cows were studied in each of four years. The dual-purpose cows were selected for milk and beef production.
"The objective of the study was to determine the effect of dairy cow breed on reproductive performance and associated production parameters,"said Seamus Crosse.
"The high genetic merit Holstein-Friesian cows had higher cumulative milk production, lower BCS at calving, larger body condition loss between calving and first service and lower overall pregnancy rate compared with the medium genetic merit Holstein-Friesian, Montbeliarde and Normande cows. …