However, this direct RZN has the disadvantage of late availability since reliable proofs require culling information of second lactation daughters. By combining the direct RZN with information on additional traits correlated to productive life, reliability and time of first release can be optimized.
Scientific research in Germany and abroad has revealed substantial genetic correlations between productive life and the following traits:• Somatic cell score (RZS)
• Functional conformation traits
Surprisingly, the proof for body depth was found to be negatively correlated to productive life (gen. corr.: –0.32). Up to now, there is no final physiological explanation for this finding yet. However, a lot of body depth at the time of a heifer’s first classification can be seen as an indication of an early maturing type of cow that is quite often not as long lasting as the later maturing kind.
Regarding udder quality, proofs for udder depth and fore udder attachment showed the strongest correlation to productive life with +0.45 and +0.36 respectively. Since udder depth has a negative correlation to milk production, fore udder attachment was chosen as the udder trait supplying additional information on a cow’s productive life.
Besides, the proof for feet&legs score is also used as an auxiliary trait (gen. Corr.: +0,32) within the new evaluation procedure for productive life.As a measurement for udder health, the proof for somatic cell score is closely correlated to productive life (gen. corr.: +0.45)
New breeding value for
functional productive life
German Holstein breeders put a lot of emphasis on longevity. Consequently, Germany was one of the world’s first countries to introduce a breeding value for functional productive life (RZN) in 1996. From August 2001 on, additional information from auxiliary traits correlated to productive life will further improve the reliability of RZN values.
Up to now, the sire index for functional productive life (RZN) is based on the culling information of the bulls’ daughters. In contrast to some other countries, the estimation routine also considers the effect of the daughters’ milk production, thus avoiding that the RZN of top production sires is biased by a special treatment of their high yielding daughters. Considering production figures when estimating proofs for productive life is because, in general, high yielding heifers receive more care and treatment than their herdmates with lower production records.
Somatic cell score, conformation, and maternal calving ease are used as auxiliary traits
Female fertility is another trait influencing a cow’s productive life (gen. corr.: +0.18). Considering the limited number of first crop daughters and the low heritability of fertility traits, it is not possible to estimate sufficiently reliable proofs for female fertility at an early time. However, the proof for the daughters’ calving ease (gen. corr.: +0.16) can be used as an auxiliary trait.
The combination of those traits with the direct culling data of daughters provides reliable and early information on a sire’s transmission ability for productive life, the new RZN. From August 2001, RZN proofs are available when a bull receives his first crop proofs for production, conformation, and somatic cell score. As soon as additional culling information is available, it gets a higher weight within this new RZN until the actual culling data provide the total information for functional productive life. Graph 1 shows how the weight of the different sources of information changes with a growing number of direct daughter information.
• Calving ease
Weighting of the different sources of information within the new breeding value for productive life with a growing number of daughters:
When a sire comes out with his first production and conformation proofs, the new RZN is based on a combination of pedigree RZN, proof for somatic cell score, functional conformation proofs (considering body depth, fore udder attachment, and feet&legs score), proof for calving ease and direct RZN based on early daughters’ culling information.At this time, the sire’s RZN already has a reliability of 50%. With a growing number of first and second crop daughters, their direct culling data continously gain weight until they make up 100% of the information included in the RZN resulting in a reliability of 99%.