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ANIMAL PRODUCTION |
* Department of Animal Science, University of California, Davis 95616 and
and
Dirección de Servicios Técnicosa la Cadena Agroindustrial, Instituto Nacional de Carnes, Montevideo, Uruguay
Abstract
Over three consecutive years, 180 (60/yr) fall-born steer calves were weaned in May (average initial BW = 238 kg, SD = 36.2 kg) and allocated to one of three groups: 1) calf-fed steers that entered the feedlot at weaning; 2) short yearlings that grazed irrigated pasture for another 4 mo and entered the feedlot in September; and 3) long yearlings that grazed with short yearlings during the summer, remained on annual California foothills range through the fall, winter, and spring, and entered the feedlot the following May. All steers were fed until the average group backfat (BF), determined by ultrasound, reached 11 to 12 mm. On pasture, short- and long-yearling steers gained weight in the summer; long yearlings then slightly lost weight in the fall and winter, and then gained weight again the following spring. Average days in the feedlot were 188, 158, and 94 (P < 0.10) for calves, short yearlings, and long yearlings, respectively. Feedlot DMI increased with age (and weight) at feedlot entry, with no difference among groups in gain:feed ratio. The gain of BF was nil on pasture, even when animals were gaining weight, and then increased rapidly when animals were placed on a high-energy diet. Final body weights were heaviest (P < 0.10) in long yearlings, followed by short yearlings and then calves, indicating that a prolonged growing period increases the apparent mature size of the animal. Moreover, total carcass fat contents and percentage of Choice or above were all lower (P < 0.10) in cattle that were older at feedlot entry (i.e., long yearlings) compared with the other groups. In conclusion, increasing the backgrounding period decreased time and total concentrate requirements in the feedlot of Angus-Hereford steers. Older cattle reached 10 mm of BF at heavier weights. Grazing animals gained weight without increasing BF; however, BF increased rapidly in the feedlot. Prolonged grazing may decrease quality grade, either by impairing the ability of the animal to deposit intramuscular fat or by decreasing the time during which dietary energy supply is adequate for intramuscular fat deposition to occur.
Key Words: Beef • Carcass • Feedlot • Performance • Stocker
Introduction
In most countries, beef is produced on pasture (Field and Taylor, 2003
). In the United States, feedlots rely on feed grains to maximize the quantity and quality of beef produced. Even so, stocker programs rely on forage to raise animals to about 1/2 to 2/3 harvest weight. Stocker programs also consolidate calf crops into larger lots of feeder cattle and ensure year-round beef production (Bennett and Williams, 1994). On the other hand, forage-based production systems may present some disadvantages in terms of product quality. However, limited or no information is available regarding the effects of the length of the grazing period and age at feedlot entry on performance and carcass composition of feedlot cattle harvested at the same backfat (BF) end point. Therefore, the specific goal of this experiment was to study the effects of various grazing and feeding periods on the performance and carcass traits of calf-fed, short-, and long-yearling steers.
Materials and Methods
Sixty Angus-Hereford steer calves from the University of California Sierra Foothill Research and Extension Center (Browns Valley, CA) herd were allocated at weaning to three groups (20 steers per group) during each of three consecutive years (1997, 1998, and 1999). Calves were born in the fall (October-November) and weaned in late spring (May-June); average initial BW was 238 kg (SD 36.2 kg). Animals were stratified by BW and age at weaning so that the three outcome groups were as similar as possible at the outset. At weaning, calf-fed steers were shipped immediately 136 km to the University of California-Davis feedlot. Short yearlings remained on irrigated pasture until September and were then sent to the feedlot. Long yearlings grazed on the same irrigated pasture with the short yearlings until September, and then remained on native range until May to June of the following year and finally sent to the feedlot. Irrigated pasture consisted of annual and perennial ryegrass (Lolium spp., approximately 45%), orchardgrass (Dactylis glomerata, about 30%), and a mixture of clovers (Trifolium spp., about 25%). Native range was typical California foothills annual range, consisting of a mixture of grasses (e.g., Bromus, Avena spp.) and forbs (e.g., Erodium, Medicago, Trifolium spp.). In this Mediterranean climate, fall rains (October to November) cause germination of the seed bank, but growth is slow due to cool temperatures. Warming temperatures in the spring produce rapid growth (February to May), and the cessation of rains (June) coincides with seed set and senescence. Therefore, forage quantity and quality decline through the summer and fall. Annual range was stocked at a rate that aimed to result in a residual mass of about 1,200 kg DM/ha. Standard management practices were observed, including vaccination, deworming, and implantation, and were approved by the University of California-Davis Campus Animal Use and Care Committee. Steers were vaccinated at weaning against Clostridia spp. and Haemophilus somnus (Ultrabac-8; Pfizer Animal Health, Exton, PA) and infectious bovine rhinotracheitis, bovine viral diarrhea, parainfluenza3, bovine respiratory syncytial virus, vibriosis (Campylobacter fetus), and five Leptospira spp. (Cattlemaster-4+VL5; Pfizer Animal Health) and dewormed with injectable doramectin (Dectomax, Pfizer Animal Health) at weaning and in the feedlot. Each animal was implanted (Synovex-S; Fort Dodge Animal Health, Fort Dodge, IA) twice, at weaning and then after 70 d in the feedlot. When placed in the feedlot, all animals were fed a low-concentrate diet for a 19-d period and then switched gradually (i.e., 50:50 low- and high-concentrate) over 14 d to a high-energy corn-based finishing diet (Table 1). Each group of 20 steers was fed in a single pen (12 x 21 m), twice daily, and bunks were allowed to become nearly empty (but not "slick") at least once weekly. Sequential measurements on live animals taken every 28 d included unshrunk BW and BF (by ultrasound). Ultrasound measurements were taken between the 12th and 13th ribs, at 3/4 of the width of the longissimus muscle, using an Aloka 500-V instrument with a 17-cm 3.5-MHz transducer (Corometrics, Wallingford, CT) and vegetable oil as the coupling agent. Average daily gain was determined for each animal as the slope of the linear regression of BW on days of age during the period of interest. A similar procedure was used to estimate rates of BF gain. At the end of the feeding period (i.e., when the average ultrasound BF for the group reached 11 to 12 mm), all steers were weighed after 18 h of feed withdrawal and then slaughtered at the University of California-Davis Meat Laboratory. Carcasses were chilled for 48 h and separated between the 12th and 13th ribs, and two trained carcass evaluators assigned USDA quality and yield grades. Carcass characteristics evaluated were as follows: hot carcass weight (HCW); percentage of kidney, pelvic, and heart fat (KPH); longissimus muscle area at the 12th to 13th rib (REA); BF depth at 3/4 of the width of the longissimus muscle; and marbling score. Dressing percentage, yield grade, quality grade, and percentage of retail cuts were calculated using standard equations (Boggs et al., 1998). Carcass specific gravity was determined by weighing the right side of each carcass in air and under water, and carcass fat percentage was calculated using the equation proposed by Garrett and Hinman (1969). A 2.5-cm slice of the longissimus muscle was taken at the 11th rib and stored at -20°C pending analysis of fat content by ether extraction (AOAC, 1990).
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Results
Growth Performance
Table 2 summarizes the growth performance data for the grazing and feedlot phases. No treatment differences between the short- and long-yearling groups in summer ADG were found. The long yearling group lost BW (-0.038 kg/d) and BF (-1.3 µm/d) in the fall, but had improved gains of weight (0.863 kg/d) and BF (6.0 µm/d) during winter and spring.
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Feedlot ADG was not different (P > 0.10) among treatment groups but showed a tendency to increase with longer backgrounding times (Table 2
). In contrast to BW gain, BF gains in the feedlot were similar (P > 0.10) among groups. In addition, the coefficient of determination between BF gain and BW gain was very low (r2 = 0.5%; P = 0.338). Feedlot DMI was lowest (P < 0.10) for calves (8.58 kg/d), intermediate for short yearlings (10.33 kg/d), and highest for long yearlings (12.42 kg/d). The observed increase in feed intake was strongly associated with an increase in average BW (r2 = 73.7%). Feed conversion efficiencies, expressed as the gain:feed (DM basis) ratios, were not different among groups (0.142, 0.140, and 0.124 for calf-fed, short yearlings and long yearlings, respectively).
Figure 1 shows the BW gain patterns averaged across the 3 yr. In general, the overall growing and finishing phase was longest for long yearlings, intermediate for short yearlings, and shortest for calves. At the end of the finishing phase, long yearlings were consistently heavier than the other two groups. The development of the subcutaneous fat depots is depicted in Figure 2. Regardless of treatment group, it is evident that the increase in the subcutaneous fat depot was mainly associated with the feedlot phase, irrespective of age. Table 2 also shows the gains of BF relative to BW gains throughout the pasture and feedlot phases. This ratio is an indicator of the proportion of BW growth that is deposited as subcutaneous fat. In the feedlot, BF gain:ADG was similar in calves, short yearlings, and long yearlings and was two to eight times greater during the feedlot phase than during the pasture phase.
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The average BW at slaughter were 458, 489, and 538 kg for calves, short yearlings, and long yearlings respectively (P < 0.10, Table 3). The same numerical trends were found for HCW, because the dressing percentage was not different among groups (P > 0.10).
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Discussion
Short yearlings and long yearlings had similar ADG in the summer (Table 2). This was expected because both short yearlings and long yearlings grazed the same irrigated pastures together. In the fall, the long yearling group lost weight and BF. Fall is the worst feed season on California annual foothill range, because prolonged dry weather (May to October) results in an inadequate supply of dry forage of poor quality (George et al., 1985). Winter and spring rains are followed by germination and growth of new plants, so that forage quality and quantity improve dramatically. This was reflected in the improved gains of weight and BF during this season.
The National Research Council (NRC, 1996) equations for predicting feedlot DMI contain different coefficients specific for growing calves or yearlings, and several adjustment factors (effects of breed, empty body fat, anabolic implant, etc.). However, the predicted values (6.78, 7.95, and 9.95 kg/d for calves, short yearlings, and long yearlings, respectively) were 16 to 20% lower than the overall feedlot DMI observed in this study.
Feedlot ADG of long yearlings tended to increase with age at feedlot entry. This suggests that exposing animals to some kind of feed restriction allowed the expression of compensatory growth during the ad libitum feeding phase. Increased DMI accounted for most of the ADG differences because there were no significant changes in gain:feed ratio. This is consistent with the findings of Sainz et al. (1995), in which steers were first exposed to a nutritional restriction on a forage diet and then placed on full feed from 300 to 481 kg (115 d). In that previous study, restricted-refed animals showed an increase in ADG relative to those animals fed an uninterrupted high-concentrate diet on an ad libitum basis. This increase in ADG was associated with both an increase in DMI and gain:feed ratio. In the present study, the response was more pronounced in long yearlings than in short yearlings, perhaps due to the longer time the latter were on feed.
It is well established that the relationship between BW and composition of gain can be modified by previous plane of nutrition (Rompala et al., 1985). According to the NRC (1996), net energy requirements for growth are reduced in cattle exhibiting compensatory growth, implying leaner tissue growth. As shown in Table 3, consistent with the increase in ADG, long yearlings tended to be leaner in all 3 yr. However, it is not clear whether this increase in leanness was due to compensatory growth, to differences in maturity, or to differences in the rate of gain. Byers and Rompala (1980) and Ayala (1974) concluded that increasing the rate of gain above 0.9 kg/d in early-maturing steers enhanced fat deposition without significantly increasing protein deposition rates. Moreover, Owens et al. (1995) showed that increasing empty BW increased fat deposition relative to protein mass (r2 = 0.89). The average BW and ADG during the feedlot phase were greatest in long yearlings compared with calf-fed and short-yearling steers. These differences could partially explain the differences in carcass fat among groups.
Carcass fat percentage was greatest for calves and short yearlings, and lowest for long yearlings. Previous research has shown that the amount of fat in the carcass depots increases with DOF (Burson et al., 1980; McCaughey and Cliplef, 1996). However, for this particular experiment, the correlation between DOF and percentage of carcass fat was very low (r2 = 9.3%). The reduced carcass fat content in long yearlings occurred despite the use of a constant backfat end point and indicates that the growth path had profound effects on the distribution of fat.
The rapid increase in BF thickness was associated with the high energy intake of the animals during the feedlot phase. This indicates that the intake of energy above the animals’ maintenance requirements was the most important factor affecting the deposition of subcutaneous fat, especially considering that animals placed in the feedlot differed in age, BW, and previous plane of nutrition. When we analyzed the ratio during the summer grazing period, no differences were found between short and long yearlings.
Previous findings regarding the intramuscular fat contents of forage-fed and grain-fed animals have not been consistent. Although some studies found no effect of forage vs. grain feeding (Oltjen et al., 1971; Reagan et al., 1977), others (Schaake et al., 1993; Mandell et al., 1997) found grain feeding to increase intramuscular fat content relative to forage feeding. In the present study, the percentage of intramuscular fat in the longissimus muscle showed a tendency similar to the marbling score, although no differences were detected. A relatively high coefficient of determination (r2 = 64.1%) between intramuscular fat and marbling score explains the similar trend observed for both variables. Overall, the percentage of carcasses grading Choice or higher for calf-fed, short yearlings, and long yearlings was 46, 48, and 30%, respectively. Calves and short yearlings generally had higher amounts of intramuscular fat and quality grades relative to long yearlings, suggesting the possibility that prolonged nutritional restriction after weaning may impair subsequent development of the intramuscular fat depot. Alternatively, the deposition of fat could be inherently slower in the intramuscular depot relative to the subcutaneous depot, so that long yearlings may not have had sufficient time on feed to achieve the same degree of marbling as calves and short yearlings.
The present results confirm the conclusions of Owens et al. (1995) that animals backgrounded on forage have elevated apparent mature sizes and therefore must reach heavier weights to achieve acceptable market finish. Our data also provide detailed evidence that days on feed is an important factor affecting the amount and distribution of carcass fat. For example, the KPH and subcutaneous fat depots were relatively static within this age and weight range, even though carcass fat differed. In contrast, Burson et al. (1980) reported that increased time on feed from 56 to 175 d increased both BF thickness and KPH fat. Similarly, Schoonmaker et al. (2002) found that delayed feedlot entry increased carcass weights but reduced quality grades. Dolezal et al. (1982) grouped carcasses by fat thickness and were able to predict the expected beef palatability, as well as and perhaps slightly better than the USDA quality grades. More work is required to define the relationship between subcutaneous, internal and intramuscular fat, and how it may be affected by genetics, nutrition, and management.
Implications
Increasing the backgrounding period decreased the time and total concentrate requirements in the feedlot of Angus-Hereford steers. Older cattle reached market finish at heavier weights. Grazing animals gained weight without increasing backfat, which only increased in the feedlot. Prolonged backgrounding may decrease quality grade, either by impairing the ability of the animal to deposit intramuscular fat or by decreasing the time during which dietary energy supply is adequate for intramuscular fat deposition to occur.
1 Correspondence: 1 Shields Ave. (phone: 530-752 0526; fax: 530-752 0175; e-mail: rdsainz{at}ucdavis.edu).
Received for publication October 16, 2002. Accepted for publication September 8, 2003.
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