Introduction

Feed intake is key to developing diet specifications, attaining target growth rates and has a significant impact on efficiency of production. Surveys have shown that feed intake varies by at least 25 percent among commercial farms.

This may in fact under-estimate the problem, since accurate data on feed intake is not readily available on many farms. Some of the data that is available, if estimated from long-term averages, or calculated on an inventory basis, fail to identify short-term deviations from this average.

Feed intake, viewed as a continuum from birth through marketing, is best described by a curvilinear function. It is not a straight line. That is why they are referred to as growth “curves.”

While total feed intake for a given group of pigs, calculated over the total growout period, provides a useful reference point against which other groups of pigs can be compared, it fails to generate the time course of feed consumption, which is essential to precise diet formulation. The three feed intake curves, illustrated in Figure 1, demonstrate the point very well.

The curve labelled “Farm A” represents the “theoretical” feed intake of a group of pigs from 20 through 115 kg, as predicted by NRC (1987), assuming a dietary DE content of 3.3 Mcal/kg. The other two curves result in identical overall feed consumption, but obviously arrive there in different ways.

Farm B achieved outstanding feed intake during the early to mid growout period, but suffered a serious drop in intake during the finishing period. Clearly, a lost opportunity exists here, as the loss in intake experienced late in life negates possible gains earlier in growout.

On the other hand, Farm C experiences a lower intake during initial growout, but achieves excellent intakes later on. Average daily feed intake, calculated for the complete growout period, would result in the conclusion that these three herds are all experiencing equivalent performance in terms of feed intake, when this is clearly not the case.

Figure 1: Three feed intake curves which all
result in the same total feed intake for the period

Turning to Figure 2, we see two actual feed intake curves, generated on two commercial farms during the same season. The NRC “standard” is presented as a reference. Farm A experiences problems with feed intake during the initial grower period, but once the pigs enter the finishing barn, feed intake accelerates and exceeds NRC by a considerable margin, something which is not normally observed in commercial practice.

Farm B experiences feed intake which is consistently below NRC, and in fact, is not too dissimilar from what is often observed on many confinement units. Again, the advantage of having completed feed intake curves is illustrated, because it focused attention on where shortfalls occur, and thus made diagnosing the problem so much easier.

Once feed intake has been identified as a problem, the next step, obviously, is to resolve it. Where does one look and what does one do to assist farm clients in overcoming shortfalls in feed intake?

Figure 2: Two feed intake curves determined on
commercial farms, with NRC presented for reference

Feed intake is influenced by genetics “The intensive selection programs for pig genotypes with better feed efficiency (FE) and carcass leanness have inadvertently selected pigs with reduced voluntary feed intake” (Webb,1989). Consequently, on most commercial swine farms, adequate feed intake is hardly realized and this is now generally recognized as a major factor limiting production.

Feed intake in pigs is also influenced by a number of factors related to the environment, management practices, as well as the pig itself. This means that a clear understanding of the key factors involved in determining voluntary FI in pigs is an important prerequisite for designing diets to ensure adequate nutrient intake under different production systems. Such information will also make it possible to impose nutrient intake restriction when desired.

The effects of many of the factors affecting voluntary FI in pigs have been previously measured. Most of these data have been derived from smaller studies often involving small groups or individually housed pigs and designed to evaluate a single factor at a time.

Whereas data obtained under such conditions provides some indication of how various factors influence voluntary FI in pigs, it is often difficult to extend such information to commercial production systems. This is particularly so considering that in a typical commercial farm, several factors occur simultaneously and interact with each other to determine overall voluntary FI levels.

Some of these factors are under study at the Prairie Swine Centre, and additional studies, particularly with large groups, will be conducted at the new research Centre at Elstow, Saskatchewan to be opened in March 2000.

This presentation provides an overview of the various factors that have been shown to affect voluntary FI, growth performance, and carcass characteristics in pigs and identifies some key areas where management can improve feed intake, particularly in grower-finisher pigs.

Factors Affecting Feed Intake in Pigs
Various factors have been shown to influence FI in pigs. This includes such factors as the environment (temperature, humidity, heat radiation, and air circulation), social factors (stocking density, group size, regrouping, etc.), genetics, health status, feeding frequency, dietary nutrient density, and presentation of food.

ENVIRONMENTAL FACTORS

Temperature
Air temperature is the most studied environmental factor with respect to its impact on animal performance. Animals perform well within a certain temperature range referred to as the thermal neutral zone (or comfort zone). This range for growing pigs is within 12-23oC, temperatures above this decrease feed intake, while temperatures below this range increase feed intake. Under high temperatures, feed intake is reduced approximately 40 grams for every 1oC above the thermal neutral zone. The problem of heat stress on pigs increases with bodyweight: each degree above thermal neutral zone reduces intake by 1% for 20kg pigs and 2.5% for 100kg pigs. Although we understand how this will slow growth rate, the impact on body composition has not been determined for pigs raised under hot conditions.

The effects of cold stress have not been studied extensively. Pigs exposed to cold stress have a higher metabolic rate resulting in increased feed intake, and a reduction in feed efficiency and gain. The extra feed consumed for each 1oC below lower critical temperature has been estimated at 25 and 39 g per day for growing pigs and finishing pigs respectively, while average daily gain (ADG) is reduced by 10-22 g/d. These estimates are likely to be lower for pigs housed in larger groups. Pigs tend to become fatter under cold stress, while some researchers have also reported decreased protein deposition in the carcass. Practical experience of pork producers using shelter housing in winter would substantiate the suggestion that index falls under cold conditions, especially when large amounts of bedding are not available to allow the pig to regulate its thermal environment.

Recent studies at the Centre conducted by Dr. Lemay indicate that both feed intake and gain can be improved by approximately 6% when a summer strategy of lowering the setpoint temperature is followed. The research suggests that producers can offset the impact of low feed intake and gains associated with rising summer temperatures. By adjusting the lower setpoint temperature 6 oC below standard for the summer period, cool evening air can be used to lower room temperatures (observed about 2oC) and increase feeding activity over the evening hours to compensate for reduced feed consumption during the day.

Humidity and Ventilation Rates
The impact of relative humidity on swine performance is tied to the prevailing temperature and ventilation rate. The effect of high humidity on FI, ADG, and FE is more pronounced during periods of high rather than low ambient temperature. In a study with growing-finishing pigs (25 to 106 kg), average daily FI was significantly reduced when temperature was increased to 28°C at a relative humidity of 65 – 70%, the typical relative humidity in commercial barns. In the same study, increasing relative humidity from 45 to 90 % at a constant air temperature of 24°C caused a significant reduction in FI and ADG. High humidity severely minimizes the ability of pigs under heat stress to dissipate the extra body heat through evaporation.

Ventilation rate determines the effective temperature that the animal actually feels. Thus humid conditions combined with high airspeeds can create uncomfortably low temperatures for the animal although the thermostat setting may appear accurate. These conditions lead to increasing feed intake. Low ventilation rates leads to increased CO2 levels and microbial proliferation and this adversely impacts on FI and ADG.

SOCIAL FACTORS

Space Allocation
Space restriction in pigs causes significant reductions in FI and ADG compared to adequate space allowed pigs (Table 2). Although space restriction causes reductions in FI and ADG in pigs, the magnitude of response relative to the level of restriction is quite variable. For instance, in one study a 36.7 % reduction in space allowance for 18-55 kg pigs reduced FI and ADG by 11 and 18 %, respectively while a 50 % space reduction for young pigs (7.1-19.6 kg) reduced both FI and ADG by approximately 12 %. At Prairie Swine Centre, Dr. Harold Gonyou using grow-finish pigs subjected to crowded conditions (approx. 24% space reduction) discovered that both feed intake and ADG were reduced.

Furthermore, pigs subjected to space restrictions did not exhibit any compensatory gain upon being provided with additional space, increasing only to the level that was typical for their weight range. The negative effects of exposing pigs to reduced space on ADG are not corrected by feeding pigs diets with high nutrient density. These observations suggest that reduced space leads to chronic stress that eventually impairs the efficiency of feed utilization. Moreover, space restriction in pigs may alter biochemical mechanisms and cause behavioral changes (e.g. increased aggression), which in turn diverts dietary energy away from being used for growth.

Regrouping
Regrouping strange pigs is commonly practiced as pigs move through a production facility. Mixing strange pigs leads to reductions in FI and ADG and this impact seems to persist even after pigs are re-united with their previous pen-mates. Based on these observations at Prairie Swine Centre, we can conclude that market pigs should not be regrouped with strangers, 2 weeks before shipping. Regrouping 8-week-old pigs has shown not to have any long-term effect on production levels thus indicating that regrouping is a transient stressor that pigs can overcome if given sufficient time.

Group size
The number of pigs to be kept in a single pen is an important consideration in a swine farm not only because of its influence on barn design but also because of its possible influence on FI and overall performance of pigs. The effect of group size on FI in pigs has not been observed consistently. Whereas some studies have reported a decline in FI as the number of pigs per group increases, others have not. Recent research at the Centre shows that overall feed intake and ADG were not effected by group size. However significant differences in ADG were observed during the first six weeks of the study. Dr. Gonyou notes that in these trials with groups of 10, 20, 40 and 80 pigs per pen, feeder location, and space were optimal and that different results could be expected under other circumstances. In this case feeders were positioned across one end of the pen and one single-space feeder accommodated 10 pigs. Social interactions among group-housed pigs together with the extra effort required to access feed when pigs are housed in a larger space may be responsible for the observed reductions in FI. This is an important area of research emphasis at the new Elstow Research Farm where large groups can be accommodated in all ages of pigs.

HEALTH

The health status of an animal is an important determinant of overall performance. In general, the immune system responds to the presence of pathogenic agents by synthesizing and releasing compounds known as cytokines, which in turn activates cellular and humoral components of the immune system. High activation of the immune system represents a form of stress (i.e. immunological stress) and pigs use physiological and behavioral strategies to maintain homeostasis during a disease challenge. During disease infection, potential anabolic hormones are inhibited and voluntary FI, ADG and FE are reduced from 5-24%. Recent research shows that pigs with activated immune systems have lower voluntary FI, FE, and body protein accretion compared to those with low immune system activation.

In addition to compromised FI and growth performance, disease infection also influences how animals use dietary nutrients for various body functions. Diseased animals exhibit a shift in the partitioning of dietary nutrients away from lean muscle accretion towards metabolic responses that support the immune system and also accelerates the breakdown of muscle proteins.

GENETICS

The genetic potential for gain varies with different genetic lines. Feed intake levels and FI patterns differ between pigs of divergent genetic lines. A variety of experiments have demonstrated that certain breeds of pigs, notably the Duroc, have a higher capacity for ad libitum feed intake than other breeds. While such data is interesting, one must recognize that variations within a breed are often greater than among breeds. Pigs selected for faster gain exhibit higher FI levels compared to those with slow gain potential. In general, daily FI level is directly related to the respective daily amounts of lean and fat deposited (about 3 to 4 times more energy is required to deposit fat compared to lean tissue) and the efficiencies for utilization of dietary energy for the accretion of body components. Pigs with a high potential for lean tissue growth tend to have a lower voluntary FI compared to those with low muscle accretion rate.

FEED

Feed Composition
Feed composition in terms of nutrient content and nutrient balance is an important determinant of FI in swine. In general, pigs consume feed to meet their nutrient requirement and therefore, the energy content of a diet has a great influence on FI. The pig has evolved to utilize a wide range of feed ingredients and although pigs can adjust their FI to compensate for low dietary nutrient density, the actual voluntary FI may be limited by the physical nature of the diet, is perhaps related to gut fill or passage rate before adequate nutrients are consumed.

Dietary crude protein content and the balance of dietary amino acids have also been shown to influence FI in pigs. Pigs fed on low protein diets or diets deficient in one or more essential amino acids respond by consuming more feed in an attempt to meet requirements for the limiting nutrients. Recently, varying Lysine:DE ratios were found to increase ADG in gilts but showed no effect on FI in either sex.

Feed Presentation
Feed intake in pigs is also influenced by the way feed is presented. Providing feed all the time is important. If feeders are empty or plugged for part of the day feed intake will suffer. Feeder ease of adjustment is thus an important feature of feeder design. The feeder should allow easy access to the feed at all times, without the pig having to assume awkward positions to eat. Spending a few minutes in the barn watching pigs eat will determine how long they spend eating, how comfortable they are eating and whether feeder access is a problem. Feeder design can affect ad libitum feed intake by as much as 15-20%.

Offering feed in pellet form as opposed to mash form improves FI by 3-12%. The wide range of results has been explained by the fineness of grind. That is where feed is finely ground to make the pellet (avg. 450 micros in diameter) and this same feed is fed in mash form the difference attributed to pelleting is 3-5% improvement in intake. If however a coarse ground mash is compared to a finely ground feed that is pelleted, the differences would favour the pellet diet with 8-12% better FI.

Wet feeding increases feed intake by 6% compared to dry feeding over the grow-finish period (12 weeks). The largest part of this effect is seen in the late grower and finisher periods (weeks 5-12 in grower barn) where wet feeding improved ADG by 50 grams/day over dry feeding, and Feed Intake increased by 9% (250 g/da).

WATER

Pigs require at least two times as much water each day as feed by weight. Since water is required for swallowing and normal digestive processes, we can assume that if pigs are not drinking they are not eating. One functioning nipple drinker is required per pen of 12 pigs. Water flow rate is also important, and must be greater than 700ml/minute but no more than 1,500 ml/minute.

MEASURING FEED INTAKE

Daily feed intake - the single most important piece of information that a nutritionist can use to minimize feed costs, while ensuring performance is maintained. Knowing how much feed pigs consume each day provides the key to determining amino acid and other nutrient levels required in the diet. In the nursery this is relatively easy, low feed intakes and hand-feeding practices have allowed interested stockpeople to easily track feed disappearance for a few test pens. This becomes a much greater challenge in the grower-finisher barn with large volumes of feed and automatic feed delivery.

Included with this paper is a procedure for collecting and analysing feed intake in growerfinisher pigs (Standard Operating Procedure - Feed Intake Protocol). Accurate weighing, tracking of days in the barn and the use of a computer program (GrowthMaster, or PorkMaster) will result in unique feed intake curves being established for your farm. These feed intake curves determine not only the amounts of feed that disappear (consumed plus wasted) each day but they illustrate at what stage of production feed intake lags behind target. With this information, the following checklist can be used to determine how to increase feed intake.

A side benefit to this exercise is that the optimum shipping weight for your farm can be determined more accurately using this procedure since the added cost of each extra day in the barn (composed of feed plus barn housing costs) can quickly be determined.

A simple estimate of feed intake can also be found by measuring feed disappearance in 4 to 6 pens at various points over the growth period. To create a mixed sex curve ensure 50% of each sex in the pen, or create sex specific curves using all males or all females in the test pens. You can then plot the feed weight fed in those pens against the feed intake curves included at the end of this paper. Note that diet energy has an influence on feed intake and the attached feed intake curves for the grower-finisher period show three lines representing three different feed energy levels.

Use the attached Feed Intake Checklist to determine if feed intakes are reducing profitability in your barn.

ECONOMICS OF FEED INTAKE

The cost of poor feed intake varies among farms. Typically a 10% reduction in feed intake will result in pigs taking 2 weeks longer to reach market weight and require 15 kg additional feed, which represents about $2.80 per pig. Slower growth is harder to place a value on, but is likely to be well in excess of $2 per pig sold. Thus a 10% reduction in feed intake, typically attained under commercial production conditions, robs the bottom line of nearly $5 per pig sold.

SUMMARY and ACTION PLAN

Feed intake has been determined to be one of the important economic performance factors in pork production. Several factors related to the environment, social interactions, diet, health status and pig genotype are reported to influence voluntary feed intake. The influence of these on feed intake is mediated through their respective and/or interactive effects on the physiological status, immune system activation, nutrient requirements, growth potential and composition of growth.

On a commercial swine farm, feed intake is influenced by a number of factors. This complex interaction requires an integrated research approaches in which key factors are evaluated concurrently. Research in this area will continue to be an important part of the Centre’s program and will increase in importance with the establishment of the new Elstow farm.

Commercial pork producers can benefit from what we have determined to-date, to increase feed intake. By monitoring feed intake and using the Checklist provided, feed intake can be better managed and growth rates and efficiencies improved.

FEED INTAKE MANAGEMENT CHECKLIST

Grower-Finisher Barn

• Do you have a feed intake target?
  • How serious is the problem?
  • Where in the growth period does the problem exist?
• Do the pigs have continuous access to feed?
  • Does the feed system ensure a continuous supply of feed to the feeder?
  • Is there adequate feeder space given the number of pigs in a pen?
  • Are feeders properly adjusted to easy access, while preventing excessive wastage?
  • Are the feeders designed to maximize feed intake?
  • Are the feeders properly placed within the pen (room)?
• Do the pigs have a continuous supply of fresh water?
  • Are drinkers functioning properly?
  • Is the flow rate adequate for nipple drinkers?
  • Are the drinkers accessible to the pigs?
  • Are there sufficient drinkers in the pen?
• Is the barn environment supporting maximum intake?
  • Is room temperature too high, even in cool weather?
  • Is humidity too high?
  • Is air movement high and or mist coolers used during warm weather?
  • Are you using a nocturnal cooling strategy to reduce barn temperatures?
• Is the health status affecting feed intake?
  • Is immune system function minimized?
  • Is AIAO or age segregated rearing practiced or possible?
  • Is depop-repop practical?
• Has the feeding program been reviewed?
  • Are there nutrient imbalances that could affect feed intake?
  • Is the mixed diet consistent with the formulation?
  • Are unpalatable ingredients present?
  • Is feed separation a problem?
  • Is the diet contaminated with mycotoxins?
  • Is the diet contaminated with noxious weed seeds?
  • Has the diet been formulated to offset summer heat stress?