Estimation of herbage intake of Angus heifers from growth rate and milk production selection lines

Abstract

Herbage intake was measured using n-alkanes (at age 10 and 13 months) and the pre- and post-grazing technique (at age 12 and 14 months) in 80 Angus heifers selected for high (Hi-Gr) or low (Lo-Gr) growth rate or high (Hi-Milk) or low (Lo-Milk) milk production. Average liveweight and estimated herbage intake at 10 months were 240.2±0.4 kg and 3.47±0.1 kg dry matter (DM)/head per day respectively. The respective values at 13 months of age were 287.2±1.9 kg and 6.50±0.36 kg DM/head per day. No differences in estimated herbage intake among the genetic lines were detected using n-alkanes at 10 or 13 months of age. Average liveweight at 12 months of age was 247.1±0.7 kg and mean estimated intakes were 4.86±0.26, 4.17±0.26, 4.37±0.26 and 3.00±0.26 kg DM/head per day for the Hi-Gr, Lo-Gr, Hi-Milk and Lo-Milk lines, respectively. The Lo-Milk line had lower (P<0.05) estimated intake than the other three lines. At 14 months of age, average liveweight was 272.5±0.6 kg and estimated intakes were 6.99±0.18, 6.52 kg±0.18, 6.71±0.18 and 7.28±0.19 kg DM/head per day for the Hi-Gr, Lo-Gr, Hi-Milk or Lo-Milk lines, respectively. Lo-Milk heifers had higher (P<0.05) intakes than heifers from the Lo-Gr and Hi-Milk lines. No statistical difference in intake between the Lo-Milk and the Hi-Gr lines was detected. In general, the Hi-Gr heifers outperformed the heifers from other lines.

Keywords:

• Angus
• estimated breeding value
• herbage intake
• n-alkanes

Introduction

The New Zealand beef industry is based on animals grazing pasture and the conversion of nutrients into quality saleable products. In the world scene, it is important that this is done efficiently and at low cost to maintain competitive advantage. In grazing systems it is advantageous to determine the amount of feed ingested by animals because feed intake measurements can be used to predict animal performance (Dove & Mayes 1991) and nutritional status (Mayes & Dove 2000). Feed intake also represents a major input cost to the production system (Archer et al. 1999) that is seldom quantified.

New Zealand beef cow breeding systems are characterised by replacement rates of approximately 20% per year (Charteris et al. 1998). This represents 305 000 heifers coming into the production system annually. Commercial breeders usually face a situation in which they have to choose between maternal traits or growth traits. Estimated breeding values (EBVs) for growth (EBV 600 Day Weight) and milk production (EBV 200 Day Maternal) represent two output traits. The current literature presents no information on herbage dry matter (DM) intake for animals selected for high or low growth or milk production under pastoral conditions.

The EBV 600 Day Weight is presently considered to be the best indicator of the likely maintenance cost of a mature animal, while EBV 200 Day Maternal reflects the fact that a higher milk yield is associated with higher metabolic requirements throughout the year (Graser et al. 2005). Both EBVs are ultimately related to the animals’ maintenance requirements, which is an economically important trait considering that 65–75% of total feed consumption is required by a breeding herd and 50% of this intake is required just for maintenance (Montaño-Bermudez et al. 1990).

The objective of this research was to measure herbage intake in four lines of Angus heifers divergently selected for high or low EBV 600 Day Weight or EBV 200 Day Maternal.

Materials and methods

Four experimental groups were formed with progeny derived from 520 industry-bred two-year-old and three-year-old Angus dams that had been fertilised by artificial insemination (AI) with semen from bulls (five per group) selected for either high (top 10%) or low (bottom 50%) EBV 600 Day Weight (Hi-Gr or Lo-Gr respectively), or EBV 200 Day Maternal (Hi-Milk or Lo-Milk). The dams were evenly distributed between four farms on the east coast of North Island of New Zealand and were randomly assigned to bulls within farms. The heifer progeny were transferred to Massey University at approximately six months of age. Herbage DM intake was estimated on four occasions—twice using the n-alkane technique and twice using the pre- and post-grazing pasture cover technique.

Herbage DM intakes were estimated in heifers at average ages of 10, 12, 13 and 14 months. Herbage mass targets for the beginning of the experiment were set according to the group's liveweight (LW) and liveweight gain (LWG) during the previous month (Reardon 1977; Marsh 1979; Reid 1986). For intake estimates at 10 (June) and 13 (September) months of age, animals were dosed (Day 0) with n-alkane capsules (CRC, Captec Ltd, Auckland, New Zealand). After a stabilisation period of eight days, faecal collection began over two consecutive four-day periods (Dove & Mayes 1991). Herbage sampling was performed according to the methodology described by Vulich et al. (1993). The relative recovery of odd-chained internal (herbage alkane) and even-chained external (dosed) n-alkanes were compared in order to obtain estimated intakes as proposed by Dove & Mayes (1991). Intakes were estimated using the C32:C33 n-alkane pairs.

Herbage intake measurements, as estimated by the difference between pre- and post-grazing pasture cover (Frame 1993), were undertaken when heifers were at average ages of 12 months (August) and 14 months (October) using a 3.3 ha paddock divided into four lanes of approximately the same area (0.834 ha). Twenty animals per line were randomly selected and allocated at random to each of the lanes. Intake was measured during six consecutive 24-hour periods. A fresh area of herbage was offered to the animals each day and a back fence was used to avoid re-grazing. The pre-grazing herbage mass was measured using a rising plate meter (RPM; Jenquip, Feilding, New Zealand) immediately before the animals entered a new strip of grass. The post-grazing herbage mass was measured immediately after the animals were moved to the next strip.

The RPM readings (n=100) were used in calibration equations to predict herbage mass (L'Huillier & Thomson 1988). Briefly, regression equations were obtained by cutting the herbage within a 0.09 m² metal quadrat at ground level with a battery-operated shearing handpiece. The samples were then washed and oven-dried at 80°C until constant weight (Khadem et al. 1993; Realini et al. 1999). Prior to cutting, a reading was performed using the RPM. 48 randomly selected sites, representing the whole range of herbage heights, were measured (Frame 1993; Murphy et al. 1995; Realini et al. 1999). The measurements were used to develop a simple linear regression relating RPM readings with herbage mass (kg DM/ha). Intake was then calculated as the difference between the pre- and post-grazing values (Reeves et al. 1996). A single regression equation was used to determine pre- and post-grazing values at 12 months (August).

The coefficient of determination, R ², for the equation was 0.88 and the coefficient of variation was 15.71%.

At 14 months of age (October), separate regression equations for pre- and post grazing events were obtained:

The R ² and coefficient of variation for the pre-grazing equation were 0.84 and 11.16%; the respective values for the post-grazing equation were 0.68 and 24.93%.

During the four herbage intake measurements, the heifers grazed pastures composed of predominantly perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). A pre-grazing pasture mass of at least 2500 kg DM/ha and a residual mass of 1800 kg DM/ha were set as constraints to ensure that intake was not limited by herbage mass availability (Reardon 1977; Marsh 1979; Reid 1986). The heifers were weighed every 28 days within two hours of being removed from the pasture. Theoretical intakes, based on LW and animal performance during the experiment, were calculated using published formulae (AFRC 1993). The estimated herbage intakes were compared against the theoretical intakes as a measure of the accuracy of the estimations.

Statistical analysis

All data were analysed using a commercially available computer package (SAS 2001). Herbage intake data determined by the n-alkane technique were analysed using the general linear model (GLM) procedure. Estimated herbage intake using the pre- and post-grazing technique was analysed using a repeated measures analysis in the GLM procedure. Estimated herbage intake for the n-alkane technique was analysed using LW as a covariate considering the differences in LW between animals from different lines. Fixed effects were selection line and farm of origin. Differences between estimated and theoretical intakes were analysed by t-test. LW and LWG were analysed using the GLM procedure in repeated measures analysis in which individual animals were nested within a selection line (Gill & Hafs 1971; Small et al. 2000).

Results

Intake at 10 months

Data from three animals were deleted from the analysis because they were outside ±3 standard deviations from the mean. Estimated herbage intake was analysed with and without the animals that failed to produce at least three faecal samples (n=5). Deleting the five animals from the analysis did not influence the outcome; the results reported are from the analysis of 77 animals.

Heifers from the high growth (Hi-Gr) line were heavier (P <0.05) than heifers from all the other lines (Fig. 1(a)). Heifers from the low growth (Lo-Gr) line were heavier than the high and low milk production (Hi-Milk and Lo-Milk) heifers (P<0.05) and heifers from the Lo-Milk lines were heavier than heifers from the Hi-Milk line (P<0.05). There were no differences in LWG over the 16-day period (i.e. eight days of adjustment and eight days of measurement) (average across lines was 0.16 kg/day) among heifers from the different selection lines during the intake measurement at 10 months of age.

Fig. 1  (a) Liveweights and (b) estimated daily herbage intakes of heifers from lines selected for high (Hi-Gr)

or low (Lo-Gr)

600 day liveweight or high (Hi-Milk)

or low (Lo-Milk)

200 day maternal liveweight. Different letters denote statistical difference at P >0.05 level.

No differences were detected in estimated intakes between lines (Fig. 1 (b)). There was also no statistical difference between theoretical and estimated intakes (Fig. 2).

Fig. 2  Comparison between theoretical dry matter intakes (black line) and intakes estimated by the n-alkane method (solid line) at 10 and 13 months of age and by pre- and post-grazing pasture covers (dashed line) at 12 and 14 months of age in heifers from lines selected for high or low 600 day liveweight or high or low 200 day maternal liveweight. Standard error bars are shown.

Intake at 12 months

Differences in LW (Fig. 1(a)) were significant (P<0.001) across the lines. LWG during the 12-month intake period was affected by selection line (P<0.05). Heifers from the Hi-Gr and Hi-Milk line gained weight while heifers from Lo-Gr and Lo-Milk line lost weight. There was no difference in LWG between the Hi-Gr and Hi-Milk lines and between the Lo-Gr and Lo-Milk lines.

Heifers from the Hi-Gr, Lo-Gr and Hi-Milk lines had higher (P<0.001) estimated herbage intake (from pre- and post-grazing herbage masses) than animals from the Lo-Milk line (Fig. 1(b)). The difference between estimated and theoretical intakes was different (P <0.05) across lines (data not shown). Herbage intake estimated using the pre- and post-grazing technique overestimated intake by 36.6% compared with the theoretical method.

Intake at 13 months

Data from eight animals were deleted from the analysis because they were outside ±3 standard deviations from the mean. All the animals produced at least three faecal samples during each of the collection periods. Heifers from the Hi-Gr line were heavier (P<0.05) than heifers from all the other lines (Fig. 1(a)). Heifers from the Lo-Gr line were heavier (P<0.05) than the Hi-Milk and Lo-Milk heifers. There were no statistical differences in LWG (overall mean across lines 1.2 kg/day) among heifers from the different lines during the 13-month intake measurement. There were no statistical differences in estimated intake between the genetic lines (Fig. 1(b)). However, estimated intakes tended to be lower than the theoretical intakes (Fig. 2).

Intake at 14 months

Selection line affected (P<0.05) LW at the time of the 14-month measurement (Fig. 1(a)). Heifers from the Hi-Gr line were heavier (P <0.05) than heifers from all the other lines. There was no statistical difference between Lo-Gr and Hi-Milk heifers, but both were heavier (P <0.05) than heifers from the Lo-Milk line. Heifers from the Hi-Gr line gained weight at a higher rate than heifers from the Lo-Gr line (P<0.05). Differences in growth rate between the Hi-Gr, Hi-Milk and Lo-Milk animals were not statistically significant. Animals from the Lo-Milk line had higher estimated herbage intake than animals from the Lo-Gr or Hi-Milk lines (P<0.05) (Fig. 1(b)). There were no differences in estimated herbage intake between Lo-Milk and Hi-Gr lines. The pre- and post-grazing technique underestimated herbage intake by 20.2% over all lines when compared against theoretical intakes.

Discussion

This study aimed to measure differences in herbage intake among different selection lines of Angus cattle. There were no differences in intake among the four selection lines when intake was measured in animals aged either 10 or 13 months using the n-alkane technique.

Khadem et al. (1993), using intraruminal chromic oxide controlled release capsules in Hereford×Friesian heifers weighing 365 kg and with a growth rate of 0.04 kg/day, reported estimated DM intakes of 1.56 kg/100 kg LW. This is similar to the average estimated intake of 1.55 kg/100 kg LW found across all lines at 10 months of age in the current study. Khadem et al. (1993) also reported that the same heifers (412 kg LW) gaining 1.70 kg/day had an estimated herbage intake of 2.42 kg/100 kg LW. This is also similar to the results obtained at 13 months of age in the current experiment, in which heifers (average 287.2 kg LW, gaining 1.2 kg/day) had an average estimated herbage intake across lines of 2.26 kg/100 kg LW. Morris et al. (1993), using intraruminal chromic oxide controlled release capsules in 223 kg Charolais×Angus steers with a LWG of –0.42 kg/day, reported estimated DM intakes of 1.3 kg/100kg LW. Subsequently, the same steers (334 kg LW), when gaining 1.47 kg/day, had an estimated herbage intake of 2.6 kg/100 kg LW. The estimated herbage intakes reported in the present study are therefore in accordance with previous assessments performed under New Zealand pastoral conditions. No intake estimations for the selection lines used in the present trial were found in the literature.

Using the pre- and post-grazing technique and averaged across all lines, estimated intakes at 12 and 14 months were 36.6% higher and 20.2% lower than the respective theoretical intakes (Fig. 2). These differences were statistically significant at both measurements periods; therefore, if the theoretical intakes are to be taken as accurate, the pre- and post-grazing technique was not accurate in estimating herbage intake in the current trial.

There are several potential sources of error in estimation of herbage mass using the RPM method. Operator variability, paddock contour, percentage of dead material and treading damage are important factors that can affect the accuracy of estimations (Lile et al. 2001). In the current experiment, the same person operated the RPM throughout the study period, the paddock was reasonably flat and the amount of dead material was, on average, 15.3% and 10.8% when intake was measured at 12 and 14 months, respectively. One of the factors contributing to the inaccuracy of the measurements was related to treading damage, which was exacerbated by rainfall during the experiment and by the small grazing area allocated to the heifers every 24 hours. The difference between the pre- and post-grazing herbage mass determined by the RPM increases as grass in the post-grazed paddocks were covered in mud and abnormally compressed due to trampling. The use of a single regression equation for the pre- and post-grazing readings may have also contributed to a systematic error.

Herbage intake measured by the difference technique at 12 months of age resulted in heifers from the Lo-Milk line having significantly lower apparent intakes than heifers from the other selection lines, while at 14 months of age heifers from the Lo-Milk line had statistically higher intakes than heifers from the Hi-Milk or Lo-Gr selection lines. The lack of consistency between the results of the four intake measurements was probably caused by differences in LWG among the selection lines during the measurement periods. At 10 and 13 months of age, there were no statistical differences in LWG among animals from the different lines during the measurement period whereas LWG differences existed among the selection lines at 12 and 14 months of age.

Conclusions

Although the Hi-Gr heifers had the highest LWG, in a self-replacing beef cow herd, the higher maintenance cost and potentially lower milk production of dams from the Hi-Gr line should also be considered. Combining the growth characteristics of the Hi-Gr lines with an appropriate level of milk production into a selection index would produce heifers with an adequate combination of genes for a self-replacing beef cow herd.

Acknowledgements

This project was funded by Meat and Wool New Zealand. Dr Nicolas López-Villalobos is acknowledged for statistical advice.