Abstract
The target of this study was to describe the interactions between the development of body condition in course of lactation and various descriptors of yield and fertility. Taking other investigations into consideration it was aimed to identify an optimal conditional range to be used in herd management which combines high milk yield with acceptable fertility traits and minimal health disorders. For this purpose, backfat thickness was measured by ultrasound at 46111 dairy cows on 78 different farms and was subsequently related to production variables.
The development of body condition in course of lactation is influenced by parity, breed and average herd yield. Backfat thickness generally is lower for younger animals, especially in late lactation and in the dry period. With cows of third and higher parity growth effects disappear. On the other hand, those elderly animals are at higher risk of being overconditioned in late lactation. Negative energy balance is more intense and also more prolonged in Holstein-Friesians compared with crossbred german black pied x HF cows. Nevertheless those animals of high genetic merit generally manage to restore their energy reserves until subsequent parturition. In low-yielding herds attention particularly should be directed to adequate feeding and energy supply in relation to milk yield in late lactation and in the dry period. In some cases it can be beneficial taking the season-depending forage quality into consideration.
Negative energy balance is getting more intense and prolonged with increasing milk yield. However a conditional nadir below 10 mm leads to decreasing milk production. Adequate feeding adjusted to milk yield seems to be a problem within herds. After depletion of nearly all their energy deposits high-performance animals are not able to restore an adequate amount of fat reserves until next parturition. On the other hand, the low-producing herdmates are predisposed to becoming excessively fat in late lactation. To reach a high production level without an increasing incidence of health disorders, conditional nadir should not decline below 13 mm RFD on herd average. Lower values only lead to negligibly higher milk yields but cause a distinctively higher risk of culling and an extreme energy deficit. High herd yields do not have to be at expense of reproductive performance and can be achieved without extreme body condition losses.
The development of body condition in course of lactation is strongly related to the milk protein content. The average protein percentage of total lactational milk should range between 3,5 and 3,8%. Values below 3,2% refer to an extreme energy deficit in early lactation. Dairy cows experiencing an intense negative energy balance post partum start lactation with elevated fat percentages, which decrease to values lower than average later in lactation.
Intervals to first artificial insemination and to conception as well as calving intervals extend with increasing amounts of body fat losses in early lactation. Those animals losing the largest amounts of body fat and showing the longest intervals to conception conceive again about two months after returning to positive energy balance. On the other hand, those with only slight energy deficit become pregnant again around nadir of body condition. To achieve profitable reproductive performance, conditional nadir should not decline below 13 mm RFD on herd average, body fat loss in the first 4 weeks p.p. should not increase above 0,14 mm per day.
Body condition in the dry period and around parturition has a stronger influence on milk yield and solids in early lactation than on total lactational yield. Independent of dairy merit mean body condition in the dry period should range between 22 and 24 mm. Those cows show high reproductive efficiency and low culling risks later on. Condition scores above 35 mm RFD have to be avoided because of the highly increasing culling risk as well as decreasing milk production. Simultaneously excessive body fat reserves lead to longer intervals to first insemination. Conditional nadir should reach about 13 mm RFD. Lower values can increase individual milk yield related to herd average but at expense of higher culling risks and lower reproductive performance. Dairy cows should return to positive energy balance after a 100 days in milk maximum and another 100 days later a distinct regeneration of body fat reserves should be realised. At drying off, animals should have regained calving condition. However high producing dairy cows moderately have to gain body condition in the dry period, because replenishment of body energy reserves during late lactation becomes increasingly difficult.
The validity of body condition scoring is limited concerning the individual, but it is a powerful management tool to assess the nutritional status of a dairy herd. It is a practical instrument in controlling the physiological loss of body fat reserves in early lactation and to allow the cow an adequate replenishment of body tissue reserves in anticipation of the subsequent lactation.
An efficient herd management can offset depression in fertility, which commonly is combined with an increasing milk yield. Standard curves throughout lactation to be used in dairy herd management are in figure 57 and 58. |
