MSU Extension Sheep & Goats

There are several strategies for feeding lambs to market weight, and the choice is largely influenced by your production system and the type of lamb market you are pursuing. Lambs can be produced in confinement, solely on pasture, or grown on pasture then finished in confinement. There are also many markets available for lamb in North America, each with unique market specifications. These range from selling lightweight lambs right at weaning (sometimes called hot house or milk-fed lambs) to growing them to high weights over a long period and selling them at greater fatness and maturity. There are basic principles that relate to animal nutritional requirements and growth that are important to understand in whatever system you employ or market that you seek.

Nutritional requirements of post-weaning growth

The nutritional requirements for growth are highly related to lamb maturity, which is best expressed as a percent of mature size. Lambs have similar nutritional requirements when expressed on a degree-of-maturity basis, regardless of breed. The percentage of protein required in a lamb diet is therefore highly related to their maturity when offered unlimited access to a diet. Lambs at 0.4 maturity (expressed as a fraction of maternal mature weight, which is the average maternal weight of sire and dam; mature size = 1) require 15-16% crude protein (CP) in a complete diet. This declines to 12-13% at 0.6 maturity and then further to 9-10% at 0.8 maturity.

Lambs are typically weaned at different degrees of maturity depending on the production system. In confinement operations, lambs are weaned younger and at lower degrees of maturity than they are in extensive range operations. In confinement, lambs are often weaned at maturity of 0.35 to 0.40 whereas in range operations they are more mature, often between 0.40 and 0.50.

Lambs are marketed at different degrees of maturity depending on the target market. The ideal degree of maturity to produce a yield grade 2 lamb is 0.7 maturity. This degree of maturity gives the appropriate amount of carcass fat (20-25%) to create the degree of marbling or intermuscular fat (IMF) that provides the best eating quality (IFM of 4%) without excessive fat, which worsens meat quality and feed efficiency. For non-traditional markets, lambs are usually sold at lower weights and degree of maturity, as these markets prefer smaller and leaner carcasses. The degree of maturity sought in these markets may vary from 0.3 for milk-fed/hot house lambs to 0.5-0.6 for lean, medium-sized lambs as preferred in the Halal market.

Protein is expensive and should be used judiciously, since excessive dietary protein is degraded and lost in digestion, with this loss of nitrogen waste creating environmental problems. Overfeeding protein is a common mistake observed in lamb production. Feeding excess protein to a lamb that is either too mature and/or that lacks the genetic potential for growth will not enhance its growth or carcass quality, and it will simply end up as nitrogen lost in the urine (majority) and feces. It is best on a whole-flock level to allocate these expensive protein resources to the animals in your flock that may need it, which on a mass basis, would be your ewes during lactation.

In practice, feeding lambs a diet in the range of 14-16% CP at weaning (more if weaning early and less if weaning late) and then dropping it down to 10-12% CP as lambs approach market weight (more if marketing smaller lambs and less if marketing larger lambs) is a good general recommendation for protein nutrition during the post-weaning lamb growth period. To keep things simple, some producers choose a diet of 13-14% for the entire post-weaning period, which is a reasonable compromise.

The focus on protein here was intentional, as it is commonly fed in excess. However, in most lambs grown in the US, the energy content of the diet is far more limiting for growth performance. Therefore, a high-energy diet is sought to make sure protein can be utilized and that the energy-to-protein ratio optimizes rumen health and function. In ruminants, we are essentially feeding microorganisms within the giant fermentation vat of the rumen, so providing the correct energy-to-protein ration in the diet is critical to maximize rumen microbial production of metabolizable energy and protein. One index of dietary energy is total digestible nutrients (TDN). To maximize growth and feed efficiency, a diet energy density of >80% is advised during the post-weaning lamb growth period.

Dietary fiber is also an important consideration in feeding market lambs. Diets that are too high in fiber will not have sufficient energy to maximize growth and will limit lamb performance and feed intake. Diets with very low fiber (such as pure grain diets) are not ideal either, since they can result in digestive upsets (acidosis) and excessive variation in feed intake as the rate of passage of feed through the digestive system is excessive. Adding fiber to the diet is important, but the key is to get the right amount and type of fiber. Fiber varies in its digestibility, with some fiber sources, such as the by-product feed soy hulls, having very highly digestible fiber. These fiber sources are ideal for lamb diets, as they provide fiber without compromising the energy content of the diet. Michigan State University Extension recommends feeding soy hulls at an inclusion rate of 20 to 25% to most grain-based lamb diets. Providing a fiber source that will promote rumination is also beneficial for growing lambs because it slows the rate of passage in their digestive system, optimizing rumen health and the extent of digestion. In addition to inclusion of a highly digestible fiber source such as soyhulls in the diet, inclusion of a small amount of hay or straw that is unprocessed or chopped to >1 inch satisfies the lamb’s need for rumination and decreases the rate of passage adequately for optimal lamb digestive health and growth performance. Providing this at 5-10% of the total diet is recommended. One way to monitor if lambs are receiving an adequate amount of forage fiber of sufficient chop length is to look at fecal consistency. Fecal consistency is very sensitive to dietary fiber/rate of diet passage. Provide more of this fiber source if fecal consistency too loose.

A final consideration in lamb-feeding programs is to minimize the risk of urolithiasis or urinary calculi. This condition leads to the formation of calculi (tiny particles/stones) in the urinary tract that result in blockage leading to lamb death. High-energy lamb diets are high in phosphorus, which is the major risk factor for this condition that impacts only male lambs due to the small diameter and greater length of their urinary tract. Increasing the calcium content of the diet is imperative to prevent this condition. The goal is to provide sufficient calcium supplement (feed grade limestone is a good source) to make the total lamb diet a ratio of 2:1 calcium to phosphorus. The addition of salt, ammonium chloride (max of 0.5% of dietary dry matter), unlimited access to clean water, and provision of dietary fiber are also important in reducing the risk of this condition.

Feeding strategies

One feeding strategy is to grow lambs as quickly as possible from birth to weaning. This is often the goal of confinement systems, as the overhead of lamb growth is high. Therefore, moving lambs to market as fast as possible allows for more efficient use of confinement housing. One can control the diet and environment tightly in confinement systems. In these systems, the cost of feed and facilities is high, but so is the growth performance of lambs, thus providing counteracting forces impacting the cost of a pound of gain. In pasture systems, the cost of feed is much lower, but the efficiency of feed use is much less, as is the growth performance, so these are also counteracting forces influencing the cost of gain. In any lamb growing program, it is important to assign a cost to the time it takes animals to reach market weight, as the cost of facility use and risk of mortality increase the longer lambs are fed. If the overhead and risk of mortality are factored into the cost of gain (often called the yardage) cost, then it becomes possible to compare the cost of gain of systems that differ in the time it takes lambs to reach market.

A common method used to grow lambs in the U.S. is to rear them with their dams on pasture and then bring them into a feedlot after weaning to finish them to market weight. Most of the lambs raised in the West are reared this way, and because most of the lambs produced in the US are born in the intermountain states of the West, this constitutes the dominant production system. Lambs reared in this manner may express high growth rates even on range under certain conditions and will often continue to do so after they adjust to the feedlot in their intermediate stage of maturity (0.5-0.6). As lambs reach 0.7 maturity, their growth rate slows, and their feed efficiency rises (worsens) quickly as their composition of gain changes to a greater proportion of fat as they reach finishing weight. Unfortunately, many lambs in this growing program are taken to weights exceeding 0.7 maturity, leading to overfat lambs that are expensive to produce, less environmentally friendly and have inferior eating quality that negatively impacts consumer demand.

Lambs can also be grown more slowly and then fed to achieve higher gains and accretion of fat just before marketing in their finishing phase. This is commonly called background feeding. It results in compensatory gain (or growth) which is a term for very efficient use of feed (fewer pounds of feed to produce a pound of gain), which occurs when lambs are transitioned from a low to a high plane of nutrition. We observed compensatory growth at Michigan State University during a recent trial (repeated over 3 years) in which we compared lambs finished only in the feedlot (fed a grain-based diet for 6 weeks) to those fed in a backgrounding system, which consisted of lambs grown on pasture for 4 weeks, followed by 4 weeks of feeding the same grain-based diet before slaughter. Backgrounded lambs (those fed on grain after 4 weeks of pasture feeding) had a feed:gain ratio of 4.2, whereas those fed on grain during the same period, were less efficient with a feed:gain of 6.0.

Background feeding systems can reduce the cost of gain and allow you to produce a leaner animal at the same market weight. These systems can be especially advantageous if, by delaying marketing, you sell during a period of lower supply and higher prices. Lamb meat quality does not appear to be appreciably impacted by background feeding. There may be a loss of tenderness and darker meat color than can be measured, but this is unlikely to be perceived by the consumer as an inferior product. The tenderness of lamb is high in general and rarely a limiting factor in consumer preference.

Lamb diets should be formulated carefully to ensure lamb health and to produce economical, efficient growth without excessive waste of protein that is harmful to the pocketbook and environment. Lamb feeding systems should be chosen with the target market and infrastructure requirements in mind. Feeding lambs quickly to market is a good strategy when facility costs are high and when this strategy matches periods of high market prices. Background feeding systems can promote efficient growth and have the potential to lower the cost of gain and produce a high-quality product.