Dry Matter Intake

The nutrient values of feeds and the requirements of ani­mals are usually converted to a DM basis to simplify ration calculation. DM is also important in its own right. be broken down, the sooner it exits and the larger the vol­ume of feed that can be consumed at the next meal.

Concentrates

Concentrates are generally ingested and digested more rapidly than forages, such that the substitution on a DM basis of grams of grain per grams of forage is not 1 : 1. In moderate amounts, concentrates improve the digestibility of poor forages by supplying energy needed for growth of the microbial population. If consumed rapidly in large quantities, concentrates decrease the rumen pH level to a point at which activity of cellulolytic bacteria is compro­mised and digestion of the forage is slowed (see Indigestion and Rumen Acidosis, below). Dividing the concentrate portion of the ration into several smaller meals is thus desirable because acidosis is prevented. The consumption of hay can only be increased in a limited fashion (less than 10%) by offering multiple meals, but it is important that the forage be available for many hours each day to encour­age consumption (Jarrige et al. 1978). A total mixed ration is ideal, but few goat farms can afford the equipment required to combine hay or silage and grain intimately, and even then the goat may sort out preferred components of the ration.

Forages

The quantity of feed that an animal is actually capable of ingesting in a day often limits the nutritive value of a ration, even if it is offered free choice.

Intake Limits

Some feeds are eaten in smaller than expected quantities because the goat finds them unappetizing. This is often true of heavily fermented silages or moldy feeds. Many goats refuse to eat hay that has fallen to the ground and been walked on. When the ration is adequately ingestible and digestible for the ruminant to meet its nutritional needs, the level of production often becomes the factor lim­iting ingestion. High-producing animals voluntarily con­sume a larger quantity of DM. For this reason, tables ordinarily increase the TDMI for each kg of milk produced or gram of bw gained.

Voluntary DMI is often expressed based on bw or meta­bolic weight, depending on whether animals are of similar or very different sizes. This is not a very accurate approach because it ignores the variations in digestibility and substi­tution. It does, however, permit discussion of gross trends in DM intake. During growth of a goat kid, the TDMI increases as the bw increases. This is because both mainte­nance requirements and volume of the rumen increase. The TDMI relative to bw decreases during growth. This is especially true at the end of the finishing period when internal body fat limits rumen volume. It remains nearly constant when expressed in terms of metabolic bw. Similarly, in late pregnancy, the increased volume of the uterus results in a relative decrease in TDMI, although high estrogen levels are also involved. After kidding, TDMI increases as the volume available for the rumen is increased by the involution of the uterus and the rumen epithelium develops to improve absorption of the end-products of digestion.

The maximum DMI for goats approaches 5-6 kg/100 kg bw, and animals with such elevated intake are naturally the best producers in the herd (Morand-Fehr 1981a). In goats fed alfalfa hay and grain during lactation, the average DMI varies from 2.8 to 4.2 kg DM/100 kg according to the stage of lactation. During lactation, daily DMI as hay may reach 3-3.5% of live weight. Intake is decreased during gestation. Typically the total DM intake is 2.2-2.8 kg/100 kg bw dur­ing the last eight weeks of pregnancy. Intake of hay varies from 1.7 to 2.3 kg/100 kg bw during this time.

These figures were obtained from Alpine dairy goats. For the same type of goat, a regression equation has been con­structed based on 11 600 daily rations consisting of alfalfa hay and grain (Morand-Fehr and Sauvant 1978): where

DM = dry matter intake in g/day

Milk = kg/day of 3.5% milk Gain = weight gain in g/day

Forage = percent of roughage in the ration

Such an equation only applies to the particular forage for which it was constructed. Similar trends have been demon­strated in West African Dwarf goats, in which voluntary intake of pelleted alfalfa during peak lactation was almost double that of non-lactating goats of a similar age. Goats suckling twins had greater TDMI levels than goats suckling singles (Adenuga et al. 1991). More recently, many studies have been summarized to produce predictions of the vol­untary DMI of lactating goats, Angora goats, growing, and mature goats, independent of the forage consumed (Luo et al. 2004c).

Encumbrance

French nutritionists have proposed a unit of encumbrance or fill unit system as a better way of determining the TDMI of various rations (Demarquilly et al. 1978; Jarrige 1988; INRA 2007).

Tables have been constructed for goats of varying bw, weeks in lactation, and levels of milk production. Selected values have been compiled from the INRA (2007) tables to create Table 19.5. These numbers presume a refusal level of 15% and digestible forage.

Encumbrance values are not routinely included in American feed tables. They are most useful when forages of variable quality are fed. These principles can be demon­strated by several examples. A 60 kg goat at maintenance can consume 1.30 MFU. An early-cut grass hay might have an EV of 1.06 MFU/kg DM if it was cut in good weather. Hay from the same field cut late and rained on might have an EV of 1.25 for dairy cattle and goats (but 1.98 for sheep). For the first hay, the goat can eat 1.30/1.06 = 1.51 kg DM.

Table 19.5 Daily dry matter intake (in kg) and capacity for ingestion (MFU).

DMI, dry matter intake; MFU, capacity for ingestion in milk fill units.

not the capacity for TDMI, but the amount of DM to be consumed when a ration has a certain energy density, to meet the energy requirements of the goat at maintenance. The ability of the goat to do this is not considered.

Substitution

When feeding concentrates in addition to the forage, it is helpful to know by how many grams the ingestion of for­age DM decreases for each 100 g of concentrate DM con­sumed. This depends on the forage and the percent of the final ration that will be grain. The substitution rate is appreciably less than 1 when small amounts of concen­trates are combined with energy-poor forage. In fact, when feeding weathered grass hay with an EV of 1.9 and an energy density of 0.76 Mcal NE_lZkg DM, the substitution rate varies from 0.02 to 0.21, depending on whether 10 or 70% of the diet is grain. Thus, the TDMI increases substan­tially. The substitution of one feed for another on a DM basis also seems to depend on stage of lactation. For instance, if 100 g more grain is fed to a goat in gestation, she will eat 62 g less alfalfa hay. In early lactation, for 100 g more grain the goat will eat 78 g less hay. After the first two months of lactation there is almost an equivalent substitu­tion of grain for good hay on a DM basis.

Conclusions Concerning Dry Matter

Out of all this confusion comes one clear message. Anyone constructing on paper or computer a ration for goats must subsequently verify that the animals in question will actu­ally consume it. This means weighing forage fed, refusals, and the grain consumed. If the animals are not eating as much forage as they need, the amount of grain must be decreased or the forage quality improved. Of course, the situation with pastured goats is trickier. Here, the goat raiser needs both experience and experimentation to feed the herd properly.