Developing an Understanding of the Nutritional Requirements of Grazing Sheep and Lambs
by A. Richard Cobb, Extension Specialist / Sheep
Much of this material was taken from the Pasture Management Guide for Livestock Producers published by Iowa State University.
To develop grazing systems that optimize profitability, the nutrient needs of the animal and factors affecting plant growth and performance must be considered. Animal performance will be optimized if animals receive a balanced diet regardless of the system of production. Balancing the nutrient needs of animals with the forage supply is a challenge, however, because the quality and availability of different forages will vary throughout the year, and the nutrient requirements vary considerably among breeds and for individual animals at different times during their life cycle.
It is possible to improve the use of pastures and increase their production by carefully managing the grazing of the forage plant. To do so a producer must be an observer of both the pasture and the animal. The idea of watching forages grow may not seem very appealing, but it is the key to successful grazing management. A producer must also realize that legumes need a higher level of management than grasses to remain productive.
The energy requirements of female ruminants are primarily affected by their physical size and which stage of their yearly biological cycle they are in. Producers need to know what stage of that cycle their ewes are in for successful management of their flock. This is easier to do if the entire flock is on the same lambing schedule and if rams are added and removed at the required times with specific lambing seasons in mind rather than allowing rams to run with the ewes all year. More complicated, management intensive lambing schemes can coexist with once a year lambing. This does however increase the necessity for the producer to keep accurate records as to which ewes belong to which group. Failure to keep accurate records in any system of production can result in over or under feeding during critical times of the year. This will increase the cost of maintaining the flock or can decrease the potential of ewes to excel within the system due to poor nutrition at critical times.
A ewe, if she breeds and lambs, has a Biological Cycle that she travels through throughout the year. The nutritional needs of the ewe vary greatly with the stage of production. Their cycle can be divided into five well-defined periods with different nutritional requirements. The duration of lactation and post weaning maintenance periods varies with management systems. Ewes can be expected to loose almost 20% of their weight at lambing. In addition, she will loose five to seven percent of their body weight during lactation, recover this during the post-weaning (maintenance) period, and then gain weight during gestation.
Biological Cycle of the Ewe
||35 to 52
(first 15 to 17 weeks)
|105 to 119
(last 4 to 6 weeks)
|28 to 42
(first 6 to 8 weeks)
|42 to 60
(last 4 to 6 weeks)
|28 to 42
|Maintenance (dry, open)*
||116 to 176
* Length of lactation and maintenance vary with system of production
By observing this table, it can be seen that very clearly defined breeding seasons in terms of number of days can have a significant impact on the stage of production ewes within the flock can be in during the year. This is an area producers can work in to increase their level of production by decreasing the amount of time ewes are exposed to the ram.
Observation and Condition Scoring of the Ewe Flock
The statement: “A good Shepherd walks among his sheep everyday” has a great deal of merit. By doing so, a shepherd will gain an understanding of each ewe in the flock and should be able to detect when she is not feeling or performing well. To accurately determine if she is thriving, develop a system whereby you determine the condition score of each ewe a number of times each year. The exact number of times you condition score is up to you but a minimum would be four. These would be 4-6 weeks prior to breeding and birthing as well as at weaning and half way through the maintained period of the ewe. We recommend 4 to 6 weeks prior to breeding and birthing so that if the ewe is not thriving, there is time to do something about her lack of condition and to improve it. Ewes that are thin at weaning may be the most productive animals you have as they are usually the highest milkers and have taken the fat from their back and poured it into the milk they produce. You do not want to lose these ewes and you can if they are not properly taken care of.
Ewes are given condition scores of 1 for very thin animals to 5 for very fat animals. An average should be between 2 and 3 and the ewe should be the fattest she is all year on the day she lambs. Producers can do a visual appraisal but that can be misleading and so they should grip the loin area of the ewe to get an idea of how much fat she is carrying. Do not concentrate on exact measurements, but rather, if the animal is increasing or loosing score. Evaluate animals that have been in the same group for a period of time and realize that there is something wrong with the individuals that are not improving when everyone else is. There is a reason why she is not thriving and it is your responsibility to determine what the problem is. She may be parasitic or have a tooth condition or a structural problem that does not allow her to travel in order to graze. She may be old, or just a very young ewe that is still developing and needs more nutrition. There are any number of problems that cause her to fall behind. A ewe in poor condition needs to receive more nutrition and that may mean removing her from the flock and feeding concentrate.
Observation of Pasture
Perhaps the previous statement; “A good shepherd walks among his sheep everyday”, should be expanded to “A good shepherd walks upon his fields and among his sheep everyday.” In reality this is what we need to do. Every time we observe the sheep we should also observe the pasture they are grazing as well as the next pasture they will be move to and the one they will be moved to after that, and lets not forget the one we just took them out of. As a producer you should know how many sheep are in the pasture, the date they were turned in and the date you plan to remove them. Your daily assessment of the amount and quality of forage available as well as an estimate of how the sheep are doing and acting on the pasture are critical to the success of your program.
Sheep Grazing Behavior
An understanding of sheep behavior and habits may be a helpful in allowing producers to understand why controlled grazing can be an aid in increasing the income potential for a grazing flock.
Sheep do not graze continuously. They have specific stages, during the 24 hour daily cycle when grazing is punctuated by ruminating, resting and idling. Sheep graze from 6 to 9 hours out of every 24. They break their active grazing time into about 5 to 6 separate grazing periods, with time required for ruminating and resting between grazing periods. The grazing periods change with the season of the year. In summer, grazing the first few hours after daybreak is normally the largest single meal of the day. In this early morning grazing, animals tend to eat a lot and are less selective in their diet. A second grazing occurs in the late afternoon until about sunset, with minor grazing periods during other parts of the day and even at night. In winter, most grazing occurs from midmorning to midafternoon when temperatures are warmest.
The number of rumination periods may amount to fifteen during the 24-hour cycle. Although the total time of rumination may be from 8 to 10 hours, the length of each may differ greatly; from 1 minute to anything up to 2 hours. The adult intake of water is from 3 to 6 liters, and the number of urinations and defecations total approximately 9 to 13 and 6 to 8 respectively. The average grazing intake of an individual sheep may differ greatly from that of the main flock, and the amount ingested may also be affected by the presence of lambs. The fertility of the soil, the use or non-use of fertilizers, the geographical situation of the grazing land and the nature of the climate, all affect the grazing behavior of the flock.
In cases where there is restricted vegetation, sheep nibble the grass very close to the ground and feces are often deposited where there is good quality grazing. Sheep do not normally consume plants or grass which has been contaminated with feces, but when circumstances prevail, the sheep do consume the good herbage in spite of contamination.
Grazing animals prefer to see each other at all times. Unlike a predator whose eyes are round and located on the front of the head, a sheep’s eyes are oval and located on the side of their head. This allows for excellent horizontal and peripheral vision and allows the animal to maintain visual contact with their flock mates without having to lift their heads. Because sheep are able to detect motion easily this is their chief defense against predators. Sheep will not usually raise their heads for a better look until whatever is moving is close to entering their flight zone. Sheep are flock animals and contact with and inclusion in the flock is necessary for their mental wellbeing. We all know that most sheep become overly stressed when separated from their flock.
Sheep use their upper lips and lower teeth to graze. They actually bite the grass off unlike a cow that uses it tongue to bring the forage into their mouths and tearing or shearing it off with the teeth on their bottom jaw. Sheep do not graze as close to the ground as horses, but they do graze closer than cattle. Therefore sheep can eat short forage more easily than cattle and their most efficient height for grazing is from 2 to 6 inches. It is difficult for them to get sufficient bites if the forage is very short. Also, they have more difficulty grazing forage that is much taller than their optimum height of 2 to 6 inches. In either case, sheep can have difficulty consuming enough during the time they will graze each day to meet their nutritional needs, Grazing animals cannot greatly compensate for inefficient bite size by grazing more hours during the day.
Most grazing animals can get all their energy, protein and some vitamin and mineral requirements from forage. Generally in a pasture system, the limiting nutrient for production will be the amount of available energy in the forage.
Digestibility of forages is important because it largely determines the amount of energy (food value) the animal can get from the forage. Immature, vegetative forages are more concentrated energy and protein sources with higher digestibility than mature forages. As forage grasses and legumes mature, the stems and plants as a whole become fibrous and unpalatable to livestock. Quality declines to the point where ruminants are simply unable to consume enough digestible energy to meet their requirements for production potential. Ruminants cannot make large changes in intake to compensate for poorer-quality forage as these stay in the ruminant digestive system longer, reducing the amount that can be consumed.
Total Digestible Nutrients, (TDN), is a common measurement related to forage quality. During the growing season, the TDN content of forages often range from 50 to 70%. At 50%, TDN is barely sufficient to meet maintenance requirements of most animals. As TDN increases, animals can more easily meet their production potential. Ideally, TDN content of forages should be 60 to 70% to achieve high milk production or gain.
The age and maturity of forage tissue are the most important factors determining the amount of available energy in forages. Grasses are typically more fibrous and slightly lower in nutritive value than are legumes. Legumes are often higher in protein and maintain quality longer with maturity than grasses.
The biggest challenge for produces doing a good job of managing pastures is to control grass growth in May and June so that it does not develop past its most nutritive stage. Another challenge for producers who rotate pastures is the tendency to leave animals on a pasture or paddock too long in the spring, waiting for the animals to graze it down uniformly. Meanwhile forage in other pastures or paddocks quickly matures and becomes less palatable.
Sheep are like people in that they have preferences about what they eat first. If given the opportunity, grazing animals will eat their favored plants over and over. If not given proper rest, the plants selected and re-grazed every few days will decline in vigor or persistence. In many cases, the legume plants will be the first species to disappear. As pastures lose plant diversity, they also will lose productivity and weeds and brush will invade the stands.
One of the greatest advantages of rotational grazing is that the managed rotation and rest prevents excess selectivity and re-grazing by the animals, allowing faster recovery and improved plant vigor.
Animal behavior can be useful when deciding to move animals. Because the average nutritive quality of the forage declines the longer a group on animals is in a pasture, the early morning “quantity” grazing is a good time to get the animals to eat more of the lower quality forage in the paddock. Under ideal conditions, when the flock is a period of low nutritional demands, (dry, early gestation), moving the group after the morning grazing period is a good use of the lower-quality forage on the last day of the grazing period. But if the animal group is one that requires a high-quality diet for lactation or gain (ewes with lambs or weaned lambs), then turning the group onto the next high-quality paddock or pasture before a big grazing, (daybreak or mid-afternoon) will permit a better level of nutrition in the diet.
A. Richard Cobb, Extension Specialist, Sheep, 217-333-7351