2 April 2013

4 minute read

_{E.A. Bailey, E.C. Titgemeyer, R.C. Cochran, T.J. Jones,
and KC Olson}

Introduction

Pre- and postpartum deficiencies of metabolizable protein have been identified as potentially limiting to productivity of beef cows and calves. Pre-partum supplementation of forage-based diets with ruminally undegraded protein has increased weight gain and breeding performance in prior studies, but the level of ruminally degraded protein fed was not known. Feeding adequate ruminally degraded protein to beef cows maximizes the productivity of microbes in the rumen, so any benefits shown in prior work could have been attributed to increased ruminal fermentation.

Our objectives were (1) to determine the value of supplementing ruminally undegraded protein when dietary ruminally degraded protein supply was estimated to be adequate to support normal ruminal fermentation, and (2) to monitor the changes in intake and digestion that precede parturition in beef cows fed low-quality, warm-season forage.

Experimental Procedures

Pregnant Angus × Hereford cows were used in 2 experiments that measured intake, digestion, and performance of beef cows and calves when provided ruminally undegraded protein in addition to ruminally degraded protein needed for optimal ruminal fermentation. Cows used in both experiments were fed 1 of 3 supplements daily that supplied similar amounts of ruminally degraded protein (0.09 per cent of body weight) and increasing amounts of ruminally undegraded protein: 0.05 per cent (LOW), 0.07 per cent (MOD), or 0.09 per cent of body weight (HI). Supplement composition is shown in Table 1.

Experiment 1

Late-gestation cows (n = 18; body weight = 940 lb; body condition score = 4.5 [1 = thin, 9 = very fat]) were used in a 3-treatment, randomized complete block experiment. Cows were housed individually and assigned to be fed 1 of the 3 supplements described previously. Each cow had free-choice access to low-quality prairie hay (2.1 per cent crude protein) and supplements were fed daily. Fecal grab samples were collected daily at 8:00 a.m. Sample collection corresponded to the period spanning 14 through 5 weeks pre-partum. Hay intakes by individual animals were summarized as 10 weekly means. Proportional intakes (percentage of body weight) were expressed using individual animals’ average body weight for each month of the trial.

Experiment 2

Pregnant Angus × Hereford cows (n = 17; body weight = 1,160 lb; body condition score = 5.2) were used in a randomized complete block experiment. Cows were stratified by weight and body condition score and assigned to receive 1 of the 3 supplements evaluated in Experiment 1.

Within treatment, cows were assigned randomly to graze 1 of 3 native tallgrass pastures. Cows were gathered from the pastures each morning and sorted into treatment groups. Supplements were group-fed. This process was repeated daily from November 25 until all cows had calved (average calving date = March 7 + 13 days). Treatments were discontinued when calving occurred. Cows were weighed and assigned a body condition score at 4-week intervals until calving was complete. Performance of calves was monitored from birth until weaning the following fall.

Implications

Pregnant cows consuming low-quality tallgrass forage and supplemented with common feeds to provide ruminally degraded protein at 0.09 per cent of body weight daily appeared to have been fed sufficient protein to maximize performance within the constraints of energy supply. Therefore, altering supplemental protein composition to provide additional ruminally undegraded protein under such conditions is not warranted.

February 2013