Factors related to voluntary dry matter intake by beef cattle fed primarily corn silage

The objectives of this study were to determine the influence of ration characteristics and body characteristics on voluntary feed intake of beef heifers and to determine the influence of voluntary ration intake on animal gains and feed efficiency when corn silage was fed as the primary ration constituent. Corn silage ad libitum and 2.72 kg of various concentrates per animal per day were fed to 432 beef heifers, initially weighing 204-227 kg, during three one-year periods. The experiments were from 110 to 140 days in length and each experiment was divided into four periods of approximately one month each. Body weights and sonaray measurements of fat thickness were determined initially and at monthly intervals throughout the experiments. These measurements provided the basis of the body characteristic variables (percent fat and percent lean) and for the calculation of average daily gain (ADG) for each experimental period. Daily amounts of feed offered, refused and consumed by each pen of animals provided the basis of the voluntary intake variables and for the feed efficiency variables for each experimental period. Factors known or suspected to affect voluntary intake (VI) were used, and their effects on voluntary intake were determined using simple correlation coefficients with period measurements used as repeat observations. The independent variables, percent dry matter (DM) from silage, percent digestible energy (DE) from silage, mid-period weight and elapsed days, were utilized to establish useful multiple regression equations to predict voluntary feed intake. Simple correlation coefficients between various voluntary intake measurements and other factors known or suspected to influence either ADG or feed efficiency (DM intake, kg per body weight gain, kg) were calculated. Multiple regression equations were also developed to determine the influence of voluntary intake and other variables (percent DM from silage, percent DE from silage, voluntary DM intake per day, voluntary DE intake per day, elapsed days and mid-period weight) on animal per-formance (ADG and feed efficiency) with period measurements used as repeat observations. Voluntary intake of dry matter (VI-DM) and of digestible energy (VI-DE) was highly correlated with either variable which characterizes the ration, namely percent of either DM or DE from silage (r = approximately 0.8). Therefore, VI increased as the proportion of silage in the ration increased. The other expressions of VI (VI per body weight, VI per metabolic size and VI above maintenance) were also positively correlated with percentage of either DM or DE contributed by silage. There were highly positive correlation coefficients between VI and body weight. When VI was expressed per body weight, it was negatively correlated with body weight. Expressing VI per metabolic body size reduced the influence of body weight to approximately 7% of the total influence of body weight. ADG was positively correlated with VI in some experimental periods only. This was due to lower than expected gains in one period while VI was as high as expected. Feed efficiency (higher numbers represent lower efficiency) was positively correlated with VI, VI per body weight, VI per metabolic size and VI-DE above maintenance. There was a high positive correlation between estimated fat percentage of the carcass and VI—DM and VI—DE. Almost all coefficients within periods were also positive (VI-DM, period 1-4: .60, .62, .31, and .46; VI-DE, periods 1-4: .59, .64, .20, and -.05). The fatter animals consumed more DM and more DE even within a period probably because they were also the heavier animals. The more meaningful prediction equations for VI were as follows: VI-DM, kg/day = -1.73 + 0.0806 (%DM from silage) + 0.00902 (mid-period weight, kg) - 0.00274 (elapsed days); VI-DI, kcal/day = -1321 + 327 (%DE from silage) + 3.04 (mid-period weight, kg) + 24.1 (elapsed days). The more meaningful prediction equations for animal performance were as follows: ADG, kg = 0.756 + 0.0167 (%DE from silage) - 0.0000530 (VI-DE, kcal/day) - 0.00278 (elapsed days) + 0.00136 (mid-period weight, kg); DM efficiency = -1,06 + 0.0608 (%DM from silage) + 1.01 (VI-DM, kg/day) + 0.0338 (elapsed days) - 0.0130 (weight, kg).