Carcass traits of Brangus young bulls fed four concentrate levels evaluated by ultrasound

S L Silva, P R Leme, S M Putrino and D P D Lanna*

Faculdade de Zootecnia e Engenharia de Alimentos, Universidade de São Paulo.
Caixa Postal 23, Pirassununga-SP, Brazil - CEP: 13.630-000 -
sauloluz@usp.br
* Escola Superior de Agricultura Luiz de Queiroz,
Universidade de São Paulo. Piracicaba-SP, Brazil
(project supported by FAPESP, Brazil.)

Abstract

The objective of this research  was to study the effects of four concentrate levels, and days on feed, on live weight,  Longissimus muscle area (ULMA), back-fat thickness (UFT) and rump fat (URF) taken by ultrasound. Twenty four Brangus young bulls were fed four diets containing 20, 40, 60, or 80% of concentrate for 142 days. Ultrasonic measurements were taken at 0, 26, 53, 84, 109, and 142 with a Pie Medical Scanner 200 Vet device, using a 178 mm linear array transducer coupled to a standoff guide.

Live weight increased linearly and quadratically  with days on feed. ULMA increased linearly with days on feed for all diets,  with a positive quadratic effect according to level of of concentrate. UFT and URF increased in a linear way with days on feed, while URF showed a quadratic increase in treatments 40, 60, and 80% concentrate. URF was not affected by concentrate levels.

The 60% concentrate diet supported the best carcass traits in the Brangus bulls used in the trial..

Keywords: beef cattle, fat thickness, feedlot, ribeye area

Introduction

Variations in the composition of meat animals and their carcasses as affected by various breeding, production, and marketing practices have been investigated rather extensively for many years. A good estimation of body composition is very important in order to analyze the contiguous changes in body composition of the animal during experimental periods without expensive serial slaughtering. Based on these changes, the nutrient requirements during successive growth stages can be determined and the effect of feeding below or above requirements on the final carcass can be investigated (Campeneere et al 2000). This information could also enhance the status of selecting animals by carcass merit and properly rank cattle in the feedlot for improved feeding efficiency and intended marketing (Williams 2001).

Not enough information is available on body composition changes during the growing and finishing stages for certain breeds, especially with respect to Bos indicus and its crosses. This is probably due to the high cost and time-consuming procedures involved in the serial slaughter procedure used for these studies in the past.

Recent improvements in ultrasound technology have brought great advances, allowing information on the body composition of live animals to be gathered quickly and repeatedly, at a low cost and with good accuracy (Brethour 2000). According to Williams (2001), ultrasound technology will become an increasingly important tool in all segments of the beef industry. In addition, ultrasound measurements are useful to sort animals for slaughter at the same degree of finishing, decreasing losses due to fat excess or discounts imposed by lack of finishing, which is a common practice by Brazilian processing companies. These live evaluations can help producers to adapt their production system to yield more profitable and uniform products, which is particularly important for Bos indicus breeds and their crosses, especially with regard to the Brangus breed. In Brazil, this breed is produced from Nelore cows with contemporary Angus semen, and is becoming of great importance in the Brazilian herd. Recently, Nash et al (2000) monitored the effects of time on feed on carcass traits in Limousin × Angus heifers, as well as Crews et al (2003), who measured British × European composite steers, bulls, and heifers at the weaning, yearling, and preharvest stages, using ultrasound.

The characteristics evaluated by ultrasound are Longissimus muscle area and back-fat thickness between the 12^th and 13^th ribs, rump fat thickness, and intramuscular fat. This last trait is not normally evaluated in cattle raised under Brazilian conditions, with low degrees of fatness. The Longissimus muscle area is the most common estimator of total carcass muscle, and in the USDA grading system it is used to estimate yield grade. The longissimus muscle area has been associated with various measures of carcass lean, where the excess fat was either removed or trimmed to a uniform thickness (Hedrick 1983).

Fat is the most variable component of the carcass (Luchiari Filho 2000), and according to Hedrick (1983), fat depth measurement at three-quarters of the distance from the medial to the lateral edges of the muscle is a reliable index of body fatness. Rump fat is an alternative site for fat measurement, and similarly to back-fat thickness, it is negatively related to the percentage of retail product. This measurement has additional importance in Brazil and in many situations where lean beef is produced, because it is a better fatness index than is fat thickness over the 12^th rib.

Management of the nutritional program is an important tool to control the rates of tissue deposition. Feeding for moderate growth rates can restrict fat deposition, while feeding high concentrate diets maximizes growth rate and fat deposition. In many parts of the world, animals are fed for the highest rates of gain, but in developing countries concentrates are relatively more expensive and lower rates of gain can be more profitable.

The objective of this work was to evaluate the effects of four concentrate levels on the rates of tissue deposition of Brangus bulls using ultrasound measurements, and to report simple correlations between ultrasound and carcass measurements in typical end points of Brazilian breeds.

Materials and Methods

This research was carried out at Faculdade de Zootecnia e Engenharia de Alimentos, Universidade de São Paulo ,  FZEA/USP, Pirassununga, SP, Brazil ( 21º 59' S, 47º 26' W, at an altitude of 634m).

Twenty-four Brangus young bulls with a mean initial weight of 236±6.0 kg and mean age of 322±3.9 days were allocated in two pens, with 12 animals each, in a complete randomized experimental design with four concentrate levels (Table 1). Pens were equipped with a Calan gate system to control individual feed intake. After 28 days of the adaptation period, and at every 28 days, animals were weighed after a period of 18 hours without food and water. At this time ultrasonic images were made of the Longissimus muscle area (ULMA) and back-fat thickness (UFT) between the 12^th and 13^th ribs, and at the rump fat (URF) over the Biceps femuris muscle. Ultrasonic images were obtained with a Pie Medical Scanner 200 Vet real-time device equipped with a 3.5 MHz 18 cm linear array and a coupled acoustic guide. Vegetable oil was used as a coupling to obtain adequate acoustic contact. Images were saved on a personal computer and interpreted using the EView^® software (Pie Medical Inc.). Measurements were taken at 0, 26, 53, 84, 109, and 142 days after the adaptation period.

At the end of the experiment all animals were slaughtered and simple correlations between ultrasound and carcass measurements were calculated. The effects of concentrate level and days on feed (time) were evaluated by polynomial regression and repeated measurements, respectively, using the SAS (2001) software.

Results and Discussion

The simple correlation coefficient of Longissimus muscle area measured by ultrasound and measured on the carcass was 0.88, while the correlation between back-fat thickness in the carcass and that obtained with ultrasound was 0.82.  These data confirm the good correlations observed in other studies, despite the fact that these animals had relatively little subcutaneous fat. Live weight increase during the trial was not the main focus of this work; despite this fact, results and statistical analysis are presented because of their importance for carcass trait comparisons (Table 2).

A significant interaction occurred between day of measurement and concentrate level (p<0.06) for LW. LW was not affected by concentrate level, even though there was a trend of linear increase (p<0.15) starting on feeding day 53 (Table 2). There was no difference among treatments from the beginning of the trial until 53 days of feeding, as expected, even though animals fed the 60% concentrate tended to be heavier than those fed the 20% concentrate in the last period. Differences increased and were significant at the end of the trial (p<0.05). A similar tendency (p<0.15) was observed between the 20% and 80% treatments. The other treatments were not different in any of the periods evaluated. LW increased linearly (p<0.0001) in all treatments and quadratically (p<0.01) for treatments 40, 60, and 80% concentrate with days on feed.

Similarly to live weight, a significant interaction between factors was detected (p<0.01) for ULMA (Table 3).   As days on feed increased, ULMA increased linearly (p<.0001) in all treatments, and a quadratic effect was also observed for the 60 and 80% treatments (P<0.05). Nash et al. (2000) monitored the carcass traits of Limousin × Angus heifers (11 months of age; 341 kg) fed during 120 days and scanned by ultrasound every 28 days. They also found a linear effect of days on feed over ULMA, similar to those reported by May et al (1992).

No differences between treatments in ULMA were observed at 0, 26, or 53 days, but bulls fed the 60% diet tended (p=0.06) to have a greater ULMA than those on the 20% treatment,. This difference was significant at 84 days until the last measurement. Also, the 40% treatment was greater than the 20% concentrate level at 109 days on feeding, but this difference decreased in the last measurement at 142 days (p=0.09). Treatments 20 and 80% were not different in any of the measurements.

ULMA was positively and linearly related to feed concentrate level up to 60%, with a trend  for a quadratic increase to become more pronounced as the feeding period increased. Mandell et al (1998) also found no differences in Longissimus muscle area of Hereford and Simmental steers fed high or low concentrate levels.

No significant interaction was observed for UFT measurement (Table 4). . Concentrate levels had a linear effect on UFAT at 26, 84, 109, and 142 days of feeding.. At day 53, this effect was not significant (p=0.14). The absence of difference in this period could be due to measurement problems, since the absolute values of UFAT were too low to be sufficiently accurate.

Until the third period there were no differences  between treatments; however, from 84 days of feeding until the end of the trial, the bulls fed the 20% concentrate diet had a thinner UFAT than those fed the 60% and 80% diets. The other treatments were not different throughout the trial. As expected, UFT increased linearly with days on feed, since the animals were at the growing stage, during which fat increase is linear. May et al (1992) and Nash et al (2000) also reported a linear increase in fat thickness as days on feed increased. URF also increased linearly with days on feed (Table 5). No studies were found in the literature that report the effects of days on feed or concentrate level on URF; even though this is also a measure of fat deposition. Similar results were expected for UFT, and this in fact happened, when days on feed are taken into account. However, differently from UFT, URF was not affected by concentrate level in any period of measurement. There was no apparent reason for this difference.

In general, URF measurements were greater than UFT measurements since the beginning of the trial, which implies that URF measurements may be more useful than UFT for detecting back-fat differences in young and/or leaner cattle. The lower performance and rates of gain for the 80% concentrate diet may be related to problems of sub-acute acidosis often associated with animals with Bos indicus blood when fed high concentrate/grain diets.

Conclusions

• Brangus bulls fed a 60% concentrate diet had higher rates of back-fat deposition than bulls fed lower or higher levels of concentrates. The effects of concentrate levels on longissimus muscle area tended to increase with time on feed.
  
  
• Fat thickness over the rump was always  greater than that of back-fat at all measurement periods since the beginning of trial, but was not affected by concentrate level.
  
  
• Ultrasound longissimus muscle area was positively and linearly related to feed concentrate level up to 60%, with a trend  for a quadratic increase to become more pronounced as the feeding period increased.

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