The relationship of body size, lactation or gestation with ration intake, nutrient digestibility and digestible nutrient intake in beef cattle

A total of 285 observations from 72 beef cows and 117 yearling cattle were used to determine relationships of cattle characteristics such as body weight, height, length, width, depth, fatness, level of milk production and gestation length with diet intake, nutrient digestibility and digestible nutrient intake. Digestibility was determined using acid-insoluble ash as an internal indicator. In one study, five observations per cow were obtained as the physiological functions of these animals were changing. Cows were fed grass silage while yearlings were fed corn silage plus concentrates. Relationships were determined using multiple regression techniques.

In beef cows, body weight was positively related (partial regression coefficient (b) = .0072, P<.05) and body width negatively related (b=-.0801, P<.05) to digestible dry matter (DDM) intake. A curvilinear relationship (P<.05) of gestation length with DDM intake existed. Intake increased from 0 to 60 d of gestation but then de creased at an increasing rate from 60 to 90 d. Digestible crude protein intake and digestible hemicellulose intake were similarly related (P<.05) to the above characteristics. Diet dry matter (DM) intake was positively related (b=.0198, P<.05) to body weight. Curvilinear relationships (P<.05) of DM intake with body width, body depth and gestation length also existed.

An increase of one kg of DM intake resulted in a 2.1 (P<.05), 2.0 (P<.05) and 0.8 (P<.10) percentage unit decline in DM, crude protein (CP) and hemicellulose (HC) digestibilities, respectively. Digestibility of CP and HC was positively related (P<.10) with body width (b=.3736 and .4456, respectively), independent of intake effects. Angus cows digested more HC than did Hereford cows (81.7 vs 77.3%, P<.01).

In yearling cattle, DDM intake was positively related (b=.0096, P<.05) with body weight. Digestible CP and HC intakes were also related (P<.10) with body weight (b=.0006 and .0018, respectively). Heavier (b=.0147), longer (b=.0316) and younger (b=.0120) animals at weaning consumed more (P<.05) DM. As DM intake increased one kg, DM and HC digestibility decreased 1.2 and 2.7 units, respectively. Crude protein and acid-detergent fiber digestibilities were not affected (P>.10) by changes in DM intake. At constant intake, char acteristics of the yearling cattle were not related (P>.10) to digestibility of DM or individual nutrients.

In cows used in the five successive observations, DDM intake was positively related (P<.05) to milk production in light weight cows but was negatively related in heavy weight cows. A curvilinear relationship (P<.05) of gestation length with DDM intake existed. Intake increased from 0 to 70 d and then decreased from 70 to 160 d. Backfat was negatively related (b=-.0935, P<.05) to DDM intake. Dry matter digestibility was related to many characteristics of the cows. Curvilinear relationships (P<.05) existed with milk production, gestation length and body weight. Backfat was negatively related to DM digestibility, however an interaction (P<.10) between weight and backfat also existed. Several other interactions were present. These were milk production with gestation (P<.05), milk production with body weight (P<.05) and milk production with DM intake (P<.10). These relationships were represented graphically utilizing three dimensional figures and indicated the dynamic relationship of factors affecting digestibility.