Effects of physiological age on residual feed intake of growing heifers

doi:10.4236/ojas.2011.13011

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Vol.1, No.3, 89-92 (2011)

doi:10.4236/ojas.2011.13011

Open Journal of Animal Sciences

Effects of physiological age on residual feed intake of

growing heifers

Andrea N. Loyd, Charles R. Long, Andrew W. Lewis, Ronald D. Randel*

Texas AgriLife Research, Overton, TX, USA; *Corresponding Author: [email protected]

Received 22 July 2011; revised 6 September 2011; accepted 20 September 2011.

ABSTRACT

Using a retrospective evaluation of feed effi-

ciency, this study investigated the effects of

physiological age on residual feed intake (RFI)

in growing heifers. Data were collected during

1973 and 1974 at the McGregor location of the

Texas Agricultural Experiment Station. Heifers

(n = 77) were obtained from a large crossbreed-

ing program utilizing a five-breed diallel mating

scheme using Angus, Brahman, Hereford, Hol-

stein and Jersey breeds. At approximately 6

months of age, pre-pubertal heifers were indi-

vidually penned and received ad libitum access

to a balanced diet. Individual feed intake and

body weight data were collected at 28-day in-

tervals for 84 days prior to puberty and for 90

days after puberty. The diet was changed at

puberty to provide a low er energy density. Con-

sidering all females as cohorts, RFI was calcu-

lated for each heifer for each period using

separate models for the pre- and post-pubertal

periods. A moderate, positive Pearson correla-

tion (r = 0.48; P < 0.001) was detected between

pre- and post-pubertal RFI. Furthermore, heifer

RFI rank was compared between the pre- and

post-pubertal periods using Spearman rank or-

der correlat ion and a si mila r co rrel ation (r = 0.46;

P < 0.001) was revealed. This suggests that RFI

determined during the pre-pubertal period may

only be a moderate predictor of post-pubertal

RFI. As a result, physiological age should be

considered when evaluating cattle for feed effi-

ciency using RFI.

Keywords: Cattle; Puberty; Residual Feed Intake

1. INTRODUCTION

Feed provided to cattle represents a significant portion

of the cost of producing beef [1]. Identifying and select-

ing cattle that are more efficient at utilizing feed re-

sources can potentially reduce feed usage. Selection

strategies for improving feed efficiency have been shift-

ing away from the conventionally used feed:gain ratio

(FG) and are incorporating residual feed intake (RFI) as

an indicator of feed efficiency in beef cattle. Many vari-

ables can influence the outcome of an RFI evaluation;

however, many of these factors can be controlled (i.e.

test duration, type and amount of diet provided, selection

of cohorts of animals, etc.). The Beef Improvement Fed-

eration (BIF) has developed guidelines to standardize the

methodology for RFI evaluation [2]. Specific to cohort

selection, it is recommended that cattle be at least 240

days of age and not differ by more than 60 days of age at

the beginning of an RFI feeding trial [2]. Although cattle

may be of similar chronological age, they may differ in

physiological maturity since an important biological phe-

nomenon (puberty) may occur during this time in bulls

and heifers.

Brody (1945) suggested that puberty coincides with

the inflection point of the growth curve, which corre-

sponds to a shift in body composition away from lean

accrual and towards fat deposition. Recent studies sug-

gest that differences in body composition may contribute

to differences in RFI [4-7]. Since gonadal steroids asso-

ciated with puberty alter body composition, pre-pubertal

RFI may differ from post-pubertal RFI. The objective of

this study was to investigate the potential consequences

of cohort selection, specifically physiological age, on the

outcomes of RFI evaluations. To accomplish our object-

tive, RFI was evaluated on heifers during two distinct

stages of development (pre- and post-puberty).

Since the expense associated with RFI research is

substantial, this study was conducted using feed intake

(FI) and body weight (BW) data previously collected

from heifers pre- and post-puberty. The FI and BW data

were initially reported as a component of an experiment

[8,9] that did not include RFI analysis. The current study

made use of these historical data to bring new informa-

tion to RFI research.

Furthermore, there are currently very few research

initiatives investigating feed efficiency of replacement

A. N. Loyd et al. / Open Journal of Animal Scienc es 1 (2011) 89-92

heifers. Most studies use steers and terminal heifers

when evaluating various facets of RFI and feed effi-

ciency [10-13]. Therefore, this study provides unique

and relevant RFI data pertaining specifically to heifers

that will be retained in the cowherd.

2. MATERIALS AND METHODS

2.1. Animals and Experimental Design

Data used for this study were collected during 1973

and 1974 from the McGregor location of the Texas Ag-

ricultural Experiment Station as previously described

[8,9]. Heifers (n = 77) were obtained from a large cross-

breeding program that utilized a five-breed diallel mat-

ing scheme. Heifer breeds included straightbred Angus

(n = 7), Brahman (n = 5), Hereford (n = 5), Holstein (n =

6) and Jersey (n = 5) and F1 Angus × Brahman (n = 3),

Angus × Hereford (n = 4), Angus × Holstein (n = 4),

Angus × Jersey (n = 5), Brahman × Hereford (n = 6),

Brahman × Holstein (n = 6), Brahman × Jersey (n = 6),

Hereford × Holstein (n = 5), Hereford × Jersey (n = 5)

and Holstein × Jersey (n = 5) crosses (reciprocals

pooled). Pre-pubertal heifers were individually penned at

approximately 6 months of age in 3 m by 10 m open,

dirt-floored pens and were allowed ad libitum access to a

balanced diet. After reaching puberty, the diet was

changed for each heifer to reduce energy density (Table

1). Feed intake and BW data were recorded at 28-day

intervals for 84 ± 6 days prior to puberty and for 90 ± 4

days after puberty for each heifer following procedures

described by Long et al. (1979). Puberty was defined as

the first ovulatory estrus. Heifers were exposed to

marker bulls during overnight exercise periods to aid in

estrus detection. Heifers were also examined by rectal

palpation every 3 weeks and when marked by a bull to

determine ovarian activity [9].

2.2. RFI Calculation

In order to compare RFI during two distinct physio-

logical periods, the pre-pubertal and post-pubertal peri-

ods were considered separately for RFI calculation. As

Table 1. Pre- and post-pubertal diet compositions.

Ingredient (as fed) Pre-pubertal Post-pubertal

Sorghum, % 48.5 33.0

Cottonseed meal, % 20.0 10.0

Cottonseed hulls, % 25.0 50.0

Vegetable fat, % 4.0 4.0

Vitamin/mineral supplement, % 2.5 3.0

such, a separate model was fitted for each period to de-

termine pre- and post-pubertal RFI. All heifers were

considered as cohorts for each of the pre- and post-pu-

bertal RFI calculations. Initial BW and average daily

gain (ADG) of each period were computed from linear

regression of BW on day of test using the PROC REG

function of SAS (2002). Mid-test BW was estimated for

each period using the intercept and slope from the re-

gression and adjusting for 3% shrink. Metabolic mid-test

BW (MMBW) was calculated as mid-test BW0.75 for

each period. Expected daily FI was predicted for each

period by linear regression of average daily feed intake

(ADFI) on MMBW and ADG using the GLM procedure

of SAS (2002). The model fitted for each period was

Yi = β0 + β1MMBWi + β2ADGi, (1)

where Yi = expected daily feed intake, β0 = the regres-

sion intercept, β1 = the partial regression coefficient of

FI on MMBW, and β2 = the partial regression coefficient

of FI on ADG. Residual feed intake was computed for

each heifer for each period by subtracting actual feed

intake from expected feed intake.

2.3. Statistical Analysis

Pearson correlation coefficients were determined us-

ing the PROC CORR function of SAS (2002) to corre-

late pre- and post-pubertal RFI. Spearman rank order

correlation was used to evaluate changes in heifer RFI

rank from the pre- to post-pubertal periods [15].

3. RESULTS AND DIS CUSSION

Age, BW, ADG, ADFI, and RFI summary statistics are

presented in Table 2. As expected age, BW and ADFI

were greater for the post-pubertal period than for the

pre-pubertal period. Average daily gain was reduced in

the post-pubertal period, likely as a result of the de-

creased energy density of the diet. By definition, mean

RFI was 0.00 kg/day for both the pre- and post-pubertal

periods. The range between the most efficient and least

efficient heifers was 2.79 kg/day for the pre-pubertal

period and 6.04 kg/day for the post-pubertal period. De-

spite these heifers being individually fed, this range is

consistent with previous reports of RFI for growing cat-

tle receiving ad libitum access to feed in groups using

GrowSafe® [12] or Insentec [13,16] electronic feeding

equipment. Current BIF guidelines suggest that feed

intake data obtained from individually-penned cattle are

inadequate [2]. However, the initial concept of RFI was

published using data from cattle fed in individual pens

[17]. Furthermore, recent results suggest that feeding

cattle in individual pens yields similar results to group-

feeding cattle as long as the cattle are stimulated to come

to the bunk more than once daily [18]. In our study,

A. N. Loyd et al. / Open Journal of Animal Scienc es 1 (2011) 89-92

Table 2. Pre- and post-pubertal summary statistics for growing heifers (n = 77).

Pre-pubertal Post-pubertal

Tr a it1 Mean SD2 Min3 Max4 Mean SD2 Min3 Max4

Initial age, months 5.6 0.6 3.9 7.6 11.8 2.0 8.6 15.9

Age at puberty, months 10.9 2.0 7.7 15.4

Test duration, days 84 6 78 119 90 4 84 94

Initial BW, kg 116.1 25.7 51.3 191.1 273.2 54.6 175.9 377.0

Final BW, kg 196.0 33.7 108.6 285.2 333.1 60.9 223.3 454.2

MMBW, kg 43.0 6.1 26.1 59.3 70.8 10.1 52.3 88.8

ADG, kg/day 0.95 0.17 0.41 1.45 0.67 0.17 0.21 1.17

ADFI, kg/day 6.0 1.0 3.9 8.5 9.0 1.6 5.9 13.5

RFI, kg/day 0.00 0.57 –1.47 1.32 0.00 1.11 –2.08 3.96

1BW = body weight; MMBW = metabolic mid-test body weight; ADG = average daily gain; ADFI = average daily feed intake; RFI = residual feed intake; 2SD

= standard deviation; 3Min = minimum; 4Max = maximum.

feed was transferred to the troughs several times daily

such that the cattle were frequently stimulated to con-

sume feed [8].

Openly accessible at

A Pearson correlation coefficient of 0.48 (P < 0.001)

was detected between pre-pubertal and post-pubertal RFI.

Spearman rank order correlation revealed a similar cor-

relation (r = 0.46; P < 0.001) between pre-pubertal and

post-pubertal RFI. These moderate correlation coeffi-

cients suggest that RFI determined prior to puberty may

be only a moderate predictor of post-pubertal RFI. These

results are consistent with other studies that evaluated

RFI on the same cattle at different ages. Archer et al.

(2002) reported a moderate phenotypic correlation of

0.40 between RFI measured in heifers during the post-

weaning period and again as non-gestating, non-lactating

cows after two parities. Arthur et al. (2001) compared

RFI of weanling and yearling bulls and observed a phe-

notypic correlation of 0.43. A genetic correlation of 0.55

was reported between steers evaluated for RFI during the

growing and finishing phases [11]. A repeatability esti-

mate for RFI of 0.62 was reported between the growing

and finishing phases of Limousin x Friesian heifers [13].

One of the proposed benefits of using RFI as a meas-

ure of feed efficiency in beef cattle is that it accounts for

between-animal variation in maintenance and growth

[20]. Furthermore, RFI is phenotypically independent of

its component traits (ADG and MMBW) and should

allow for comparisons between animals at different

stages of production during the RFI evaluation [21].

Under this principle, the evaluation of RFI using animals

of different ages should be valid. However, physiologi-

cal maturity has been implicated as a source of variation

when determining RFI [16] as feed efficiency measure-

ments are dependent on the stage of maturity of the ani-

mals at evaluation [22].

As cattle grow, composition of their gain shifts from

protein accretion to fat deposition [23]. Since the ener-

getic expense associated with protein accrual is less than

for fat deposition [24], the efficiency with which cattle

convert feed into BW gain is reduced as they mature. A

substantial shift from protein accretion towards adipose

deposition occurs around the time of puberty [3]. These

changes in body composition associated with advancing

physiological maturity could partially explain the mod-

erate correlations between pre- and post-pubertal RFI

reported in this study. This suggests that RFI determined

during the pre-pubertal period may be only a moderate

predictor of feed efficiency during the post-pubertal pe-

riod.

Admittedly, it is not possible to separate the effects of

puberty status from the effects of the diet change from

the pre-pubertal to the post-pubertal feeding periods in

the current study. To definitively attribute differences in

pre-pubertal and post-pubertal RFI, the same diet would

have to have been fed throughout the experiment. As this

is a retrospective analysis of previously collected data,

this could not be controlled. Nonetheless, previous ex-

periments that have evaluated RFI on cattle fed the same

diet at different ages observed similar correlations as

reported here [10,19]. As a result, physiological maturity

should be considered when selecting cohorts of cattle for

RFI evaluation.

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