Projection of Protein Yield to 305 Days

Abstract

Lactation records with at least 305 d in milk, information on milk and protein yields at each sample day, and completion dates during 1984 were obtained from the dairy records processing center in Provo, UT. Most of the 180,323 Holstein and 11,685 Jersey records were from Colorado, Idaho, Maryland, Ohio, Utah, and Washington. Protein yield between last sample day and 305 d was regressed on product of milk yield between last sample day and 305 d, protein percentage from partial record or at last sample day, and a quadratic function of days in milk. This function estimated a factor for each day of lactation to adjust last sample day or partial lactation protein percentage to expected protein percentage for remainder of lactation. Errors from projection with average percentage of partial records had smaller variance at less than 240 d in milk than did those from projection with last sample day percentage. For records with less than 240 d in milk, partial lactation protein percentage was adjusted by a quadratic function of days in milk jointed at 50 and 140 d. For records with 240 d or more in milk, last sample day protein percentage was adjusted by a quadratic without joint points. No difference between factors for first and later lactations was found. Holstein and Jersey factors were applied to records completed during 1982 and 1983 in addition to previously analyzed 1984 data to determine projection errors for partial records of 150 to 304 d in milk. Standard deviations of projection errors showed a linear decline from 15.1kg for Holsteins and 12.6kg for Jerseys for the 150 to 159-d group to almost 0 at 304 d.