Strategies for genetic selection in pasture-based dairy production

Keegan D Gay, Purdue University

Abstract

Dairy producers throughout the US continue to struggle with economic and social changes. Many of these producers are turning to pasture-based production systems which have significantly distinctive cost, revenue and management challenges. These differences cause some graziers to question whether the US Lifetime Net Merit (NM$) index which is based mainly on records from conventional herds, adequately facilitates selection of bulls to enhance pasture-based breeding programs. The gathering of data and subsequent development of indexes more closely aligned to pasture-based preferences and economic values attempts to answer this question. A survey aimed at gathering information on genetic preferences, selection criteria, production methods, feeding and cow health was developed and distributed to producers in 23 states. Questions regarding herd and production history, breed utilization, and breeding practices were included. Graziers were also asked to rank genetic traits on a Leikert-like scale of negative 5 to positive 5 to indicate their preferences for various selection criteria. Respondents averaged 18.8 ± 18.6 years of grazing history, 133.4 ± 129.7 milking head, and grazed 232.5 ± 61.1 d per year. Production was 21.3 ± 5.1 kg of milk per cow per day, 4.0 ± 0.4% milk fat, and 3.3 ± 0.3% milk protein. Also, 47.5% of producers practiced seasonal calving, defined as 75% of cattle calving in any 3 mo. window. Seventy percent of producers indicated the use of the Holstein breed, thus the indexes were created based upon Holstein characteristics. Two indexes were created using average ranks of traits in a grazing environment. Grazing preference index 1(GP1) weights were the relative ranking of each trait. Grazing preference index 2 (GP2) weights were created using a desired gains approach. GP1 was strongly correlated with Net Merit $ (NM$) (r=0.95), while GP2 was less strongly correlated with NM$ (r=0.56). Despite high correlations with previously existing indexes, producer desires are not being fully addressed, thus suggesting that there may be a benefit in an index specific to graziers' needs. The results of the survey were combined with economic data from the literature to develop three grazing merit indexes (GM$1, GM$2, and GM$3), which were compared to current US indexes. The two indexes differed only in that GM$1 utilized a milk price based on average prices from the previous five years while GM$2 utilized milk price based on the NM$ and essentially restricted to no gain in milk volume. GM$3 utilized cull prices and interest rates based on NM$. The economic weights of the traits for GM$ were multiplied by the predicted transmitting abilities 584 active artificial insemination Holstein bulls to compare with NM$. Spearman rank correlations for NM$ were 0.93 (p <.0001) with GM$1, 0.98 (p<.0001) with GM$2 and 0.98 (p<.0001) with GM$3. Traits included (and their percentage of weight) in GM$1 and GM$2 respectively include: Milk Volume (24%, 0%, 0%), Fat Yield (16%, 21%, 21%), Protein Yield (4%, 17%, 17%), Productive Life (7%, 8%, 7%), Somatic Cell Count (-8%, -9%, -9%), Feet and Leg Composite (4%, 4%, 4%), Body Size Composite (-3%, -4%, -4%), Udder Composite (7%, 8%, 8%), Daughter Pregnancy Rate (18%, 20%, 8%), Calving Ability (3%, 3%, 3%), and Dairy Form (6%, 6%, 6%). This is compared to NM$ 0, 19, 16, 22, 10, 4, 6, 7, 11, 5, and 0 % for the same traits, respectively. Dairy Form was added to GM$ to offset the decrease in strength associated with selection to reduce stature through selection against a Body Size Composite. The large increase in the relative importance of Milk Volume in GM$1, was due to a difference in the milk prices used to calculate the index and the increased marginal value of milk yield because production is less in grazing systems. The substantial increase in the index weight on Daughter Pregnancy Rate in GM$ compared to NM$ reflects the increased focus on seasonal calving in grazing herds. Emphasis on Productive Life decreased substantially in GM$ because grazing cattle have been estimated to remain in the herd longer, therefore the marginal value of increasing length of Productive Life is diminished. While NM$ provides guidance for pasture-based dairy producers, a GM$ index based upon appropriate costs and revenues, will allow selection of cows and bulls to allow more optimal genetic progress.

Degree

M.S.

Advisors

Schutz, Purdue University.

Subject Area

Animal sciences

COinS