Masters Theses

Date of Award

6-1984

Degree Type

Thesis

Degree Name

Master of Science

Major

Animal Science

Major Professor

B.R. Bell

Committee Members

M.J. Montgomery, J.W. Philpot

Abstract

Feed efficiency data were collected on 353 first lactation Jerseys at the Dairy Experiment Station (DES) from 1971-1982. Feed intake, milk yield, and bodyweight change were recorded for a 28-day period on individual cows during mid- to late lactation. Two-hundred, thirty-two of these cows were from a selection line, developed through use of the highest available A.I. PD milk sires. One-hundred, twenty-one were from a control line originating from 40 foundation cows bred to sires taken randomly from commercial A.I. sampling programs in 1967. The same control sires were used throughout the experiment.

The feeding regime included alfalfa-orchardgrass hay and corn silage fed ad libitum, while grain was fed according to production. Feed samples were taken, by trial, and subjected to ADF analysis to derive estimated net energy values (ENE). Gross feed efficiency was defined as 4% fat-corrected milk/total ENE in feed consumed.

Least squares means were employed to determine if differences existed between genetic lines for estimates of gross feed efficiency. Initially, bodyweight change was used as a covariate to adjust for tissue balance differences. The original model indicated no significant line effects for estimates of gross feed efficiency. However, bodyweight change was considered an inaccurate indicator of physiological status. Thus, a new variable (ENBAL_28) was created and defined as the difference between NRC recommendations and actual energy intake. After dividing the data set into two energy balance groups (positive and negative), analysis indicated an increase of 10% in efficiency estimates for cows in the negative state.

When ENBAL_28 was used as a covariate to adjust for physio logical status of cows, the residual variance was reduced and line differences for efficiency became significant (P .01). This indicated that selection line females used a lower percentage of energy intake for maintenance. ENBAL_28 displayed a strong negative correlation with estimates of feed efficiency. Negative relationships with feed efficiency estimates also appeared for intake variables. Results indicated a postive association between efficiency and milk production. It was concluded that gross feed efficiency is a complex trait under the influence of various genetic and environmental constraints.

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