Analysis of carcass characteristics and fat deposition of Merino, South African Mutton Merino and Dorper lambs housed in a feedlot

Abstract

The objective of this study was to determine the effect of time spent in a feedlot on the size of the various fat depots and the distribution of the main tissues (muscle, bone, and fat) of three common South African sheep breeds. Lambs were supplied with a balanced diet (16% protein, 10 MJ ME/kg feed) ad libitum and had free access to water. Lambs from each breed were divided into six groups, which were slaughtered at 21-day intervals until a production period of 105 days had been reached. During carcass dressing, visceral and renal fat was removed and expressed as a percentage of carcass weight. The subcutaneous fat (SCF) depth was measured on the longissimus lumborum at the 13th rib position using an electronic calliper. The proportion of muscle, bone and fat was determined by dissecting a three-rib cut made on the prime rib between the 9th and 11th ribs. The percentage of visceral and renal fat increased throughout the production period for all breeds. The percentage of renal fat was up to 2.9% higher than the percentage visceral fat in both SAMM production groups. Dorper lambs tended to have high SCF levels (5.6 mm fat after 42 days) due to the early maturing nature of the breed. They reached a maximum fat depth of 20.4 mm fat after 105 days. The SAMM lambs tended to deposit SCF at a slower rate and the late maturing Merino breed was found to be much leaner, and did not reach the high fat levels of the SAMM or Dorper lambs. The percentage of muscle and bone in all carcasses decreased with an increase in the number of days in the feedlot, while the percentage of carcass fat increased during this period. The increase in late maturing adipose tissue in all breeds as they become older is amplified by the restricted movement in the feedlot and high energy diet that the lambs receive.

Premium lamb production of South African sheep breed types under feedlot conditions

“…Intact rams also presented higher growth rates and were leaner than castrated lambs at slaughter (Schanbacher et al, 1980), whereas the fat depths of ewe lambs and wethers were similar when slaughtered at the same live weight (Dimsoski et al, 1999). Brand et al (2018) found higher rates of fat deposition in Dorper lambs than in SAMM lambs, which were in turn higher than those of Merino lambs. Cloete et al (2012) observed that the subcutaneous fat depths of Dormer and SAMM carcasses were greater than those of Dohne Merino and Merino carcasses.…”

“…R. 863, 2006). Feedlot operators tend to be prejudiced against finishing early maturing breeds, because they deposit fat at an earlier age and therefore have to be slaughtered at a lower live weight (Brand et al, 2018), thereby reducing the income per carcass. However, in harsher climatic regions, in which the earlier maturing Dorper, Meatmaster, and Namaqua Afrikaner are used predominantly and demand for lamb is high, operators may decide to enter these lambs in the feedlot.…”

Slaughter Characteristics of Feedlot-Finished Premium South African Lamb: Effects of Sex and Breed Type

“…Breeds developed from indigenous fat-tailed or fat-rump breeds tend to be early maturing and tend to deposit fat at an earlier age, as is the case with the Dorper, Meatmaster, and Namaqua breeds. These breeds also show differences in fat partitioning between the various fat depots, as well as the distribution of subcutaneous fat [9,17]. Breeds also vary in their growth potential in terms of frame size and growth rate.…”

“…When considering that lambs of different breeds start to deposit fat at different stages as well as exhibit different growth capacities, it is important to implement precision finishing in maintaining production sustainability. Early maturing breeds deposit fat at an earlier stage and must, therefore, be slaughtered at a lighter live weight compared to later-maturing breeds for carcasses to obtain the same classification [9]. With the differences in slaughter weight, it is possible that the different breeds will not only differ in terms of carcass size and shape but could possibly differ in terms of meat quality characteristics, as it is not clear from previous studies [3,4] whether the effects of different breeds are accounted for by the quality descriptors.…”

“…While the proposed slaughter weights of ~36 kg for Dorper lambs and 42-45 kg for dual-purpose and Merino lambs [9] have conventionally been used based on slaughter information, in an era of precision farming, technology can be used to give more accurate indications. Ultrasound scanning can be used to measure back-fat cover [10], and hence determine the optimal point of slaughter to achieve the desired classification in terms of subcutaneous fat cover.…”