Changes in rumen capacity of dairy cows during the periparturient Changes in rumen capacity of dairy cows during the periparturient period period

Four-ruminally fistulated, multiparous, pregnant Holstein cows were studied to characterize ruminal adaptations during the transition from gestation to lactation. Cows were fed typical far-off and close-up diets, a late lactation diet containing wet corn gluten feed (20% DM), and an alfalfa hay, corn silage based early lactation diet. Ruminal measurements were obtained 72 (late lactation), 51 (far-off dry), 23 and 9 (close-up dry) days before expected parturition and 6, 20, and 34 days postpartum. Measurements included total fill, dry matter fill, fluid fill, and water-holding capacity of the rumen. Dry matter intake and milk production data were collected daily and body weight and body condition were determined weekly. Body weights and condition increased during the dry period, whereas intake as a percentage of body weight decreased.


Introduction
Dry matter intake g ener ally decreases by 20 to 30% in dairy cows during the last 3 wk before parturition.This decrease in intake coincides with increasing feto-placental weight and is tho ught t o be par tly due to a decrease in rumen capacity because of space limitations in the abdominal cavity.Physical measurements of the space available in the abdominal cavity obtained from frozen cross sections of pregnant and nonpregnant cows indicated a decrease of 30% due to the presence of t he fet us, placenta, and associated fluids.The problem with this technique is that it does not account for the expandability of the abdominal cavity or repositioning of the uterus.The purpose of our study was to measure changes in rumen capacity and function in vivo dur ing late gestation and ear ly lactation in Holstein cows in order to determine if the observed decrease in dry matter intake is associated with decreased rumen capa city.

Procedures
Fou r-ruminally fistulated, multiparous, pregnant Holste in cows were utilized in this study.The cows were impregnated to sires of similar calving ease and had similar projected calving dates, body weights, body condition scores, previous 305-day mature equivalent milk yields, and frame sizes (Table 1).Frame size was define d using hip and wither height as well as width between the hoo ks.Cow s were housed in a tie-stall barn during an experiment that was initiated 94 days befo re c alving and t erminated 34 days after calving.Cows were fed typical late lactation, far-off, close -up , and early lactation diets (Table 2).Daily feed intakes and milk weights were recorded for individual cows.Bod y weights and condition scores were determined weekly.
Ruminal total fill, dr y matt er ( DM) fill, liquid fill, and water holding capacity were determined on days 72 (late lactation), 51 (far-off dry), 23 and 9 (close-u p dr y) prepartum and 6, 20, and 34 days postp art um.Ruminal water holding capacity was defined as the weight of water removed from a rumen filled with water to a pre-set mark on an air outlet hose and was assumed to estimate rumen capacity.The pre-set mark on the air outlet hose was aligned with the topline of the cow.Liquid fill represents the difference between to tal fill and D M fill.On sample days, rumens were emptied, contents weighed, and sampled in t riplicate for dry matter determination.Rumens were then rinsed and filled with water to a pre-set mark on an air outlet hose.At this point t he scales were zeroed and the water removed from the rumen was weighed.

Results and Discussion
The initial characteristics of the study cows are presented in Table 1.Cows of similar size, co ndition, bodyweight , and milk production potential were selected to reduce variation in the pre-determined measurements.The cows were impregnated to sires of similar calving ease scores (9.8 ± 0.2%) and projected calving dates (± 4 days).The average birth weight of the calves was 80.8 ± 10.3 lb and the cows calved between December 21 and 29, 2000.None of the cows experienced health diso rders during t he experimental period and milk yield reached 88 lb by day 34 of lactation.All cows were offered a common total mixed ration (TMR) consist ent with their state of lactation or gestation (Tables 2 and 3).
The cows gained approximately 132 lb of body weight and their BCS increased from 2.43 ± 0.21 to 2.94 ± 0.21 between days 72 and 9 prepartum.Dry matter intake expressed as a per centage o f body weight (Figure 1) decreased during this time but nutrient intake was sufficient to support fetal growth and improve nutritional status as evidenced by th e increase in body weight and condition score.Ruminal dry matt er fill exhibited the same pattern as dry matter intake on a percentage of body weight basis.Rumen water holding capacity increased linear ly (P<0.01)from late lactation to ear ly lactation (Figure 2), which clearly refutes the theory that the observed decrease in dry matter intake prepartum is due to a decrease in rumen capa city.These results indicate that the expandability of the abdominal cavity is sufficient to accommodate the developing fetus without undu ly restricting rumen capa city.
Evidence that rumen capacity does not restrict intake is further supported by the fact that ruminal dry mat ter fill expressed as a percentage of rumen water holding capacity decreases pre par tum and increases postpartum (Figure 3).Dietary neutral detergent fiber (NDF) content has been advanced as a regulator of dry matter intake.Our results do not support t his concept because NDF intake expressed as a percentage of rumen water holding capacity increased when cows co nsumed the high NDF far-o ff diet, whereas dry matter intake decreased (Figure 4).Intake of NDF mimicked DMI when cows consumed the close-up and lactation diets.In summary, the depression in dry matt er intake o bser ved p rep art um is not due to limite d rumen cap acit y.

Figure 3 .
Figure 3. Dry Matter Fill as a Percentage of Rumen Water Holding Capacity.

Figure 4 .
Figure 4. Neutral Detergent Fiber (NDF) Intake as a Percentage of Water Holding Capacity.