Influence of low-level supplementation with a high-protein feed on Influence of low-level supplementation with a high-protein feed on performance of beef cows grazing tallgrass-prairie range during performance of beef cows grazing tallgrass-prairie range during the fall the fall

,


Introduction
Forage quality in most of the western United States declines during the late summer and fall and is quite low as the plants reach vegetative maturity.This is especially true in the tallgrass-prairie regions that are dominated by C4 grass species.Previous research at Kansas State University has demonstrated that cattle grazing low-quality tallgrass prairie respond very positively to supplementation with ruminally degradable protein (the protein available to rumen microbes) and that the greatest efficiency is achieved from the first increments of supplemental protein.The nutrient requirements of spring-calving cows are typically lowest during the fall and it has been demonstrated that the efficiency of metabolizable energy use to promote body condition gain is greater during late lactation than during the dry period.Together, these factors may provide a unique opportunity to realize efficient range cow weight and body condition gains prior to entering the winter grazing season.This could be important for the maintenance of reproduction in beef cows in poor body condition and also could moderate subsequent winter supplement dependency by building mobilizable reserves during a period when such reserves are established efficiently.Therefore, the objective of our study was to evaluate the impact of delivering limited quantities of a hand-fed, high-protein supplement during the fall grazing period on fall and subsequent winter beef cow performance.The provision of supplement prior to weaning versus after weaning was also evaluated to determine if performance differences existed due to the time of initiation of supplementation.

Experimental Procedures
An experiment was conducted from August 15, 2001 through the beginning of the 2002 summer grazing season that used 136 mature, pregnant, spring calving Hereford x Angus cows.The treatments were as follows: 1) control with no fall supplementation; 2) fall supplementation during the entire fall grazing period, both before and after weaning (August 15 to December 14); 3) fall supplementation beginning after weaning (October 15 to December 14).Initial body weights of the cows and calves and body condition scores of the cows (1 to 9 scale) were recorded on August 14, 2001 and repeated approximately every 60 days and within 48 hours of calving.Additional body weight and condition scores of the cows and calf weights (for the 2002 calf crop) were collected at the beginning of the summer grazing season.Treatments were randomly assigned to 12 fall pastures of tallgrass prairie with 3 replications per treatment.Four groupings of the treatment/fall pasture combinations were then assigned to one of four winter pastures of tallgrass prairie (each fall treatment was represented in each winter pasture).The cattle were stratified by body condition score and pair weight and assigned to one of the three fall supplementation treatments.The pastures varied in size from 60 to 100 acres; therefore, the randomization procedure was designed to allow a consistent number of cows across treatments and a stocking rate of approximately 7.5 acres per cow/calf pair.
All fall-supplemented cows received 0.14% of their average initial body weight per day (as-fed basis) in supplement during their designated supplementation period, and all treatment groups received 4 lbs/day of the same 40% crude protein supplement in meal form during the winter grazing period (December 15 to calving).The supplement used throughout the experiment was comprised of approximately 52% cottonseed meal, 30% soybean meal, 15% sunflower meal, 2.5% molasses, and 0.5% grease.All supplementation occurred 3 days per week (Monday, Wednesday, and Friday) and was prorated to deliver the designated daily amount.To ensure that only cows consumed the supplement fed during the period before weaning, calves were separated from their dams before bunk feeding the supplement.During the entire fall period, all cows were fed as groups in their respective pastures.On supplementation days during the winter period, cows within each of the four pastures were separated into their respective treatment groups and bunk fed their allotment of supplement.Adequate forage was available in all pastures during the course of the study, and the approximate quality of the forage available in those pastures was characterized.Five samples, randomly distributed throughout each of the experimental pastures, were collected in each time period (total samples = 160) utilizing 1.08 square foot frames (Table 1).A commercial mineral mix was provided free choice to all cattle throughout the experiment.To evaluate the effect of fall supplementation on subsequent reproductive performance, two blood samples were collected from the tail vein of each cow prior to the breeding season (May 10, 2002 andMay 20, 2002) and assayed for progesterone levels to determine whether cows were cycling prior to the breeding season.Pregnancy was confirmed by rectal palpation on September 12, 2002.

Results and Discussion
Cows receiving supplement prior to weaning tended (P=0.16;Table 2) to increase in body condition a bit more than nonsupplemented cows.This observation was corroborated by a higher (P=0.03;Table 3) weight gain in that group.However, weight change in calves (Table 4) nursed by these cows during this period was not different (P=0.33)from the calves nursed by nonsupplemented cows.All cows lost body condition during the period after weaning (October 15 to December 14) even though weight gain was positive (due to growth in the products of conception).
Cows receiving fall supplementation lost less (P=0.02)body condition and gained more (P=0.02)weight than the control cows.Cumulative weight and body condition scores were affected as well.Weight and body condition change between the two supplemented groups was not significantly different during the period after weaning, which suggests that neither compensation nor adaptation (i.e., adaptation to having been supplemented previously) were important under these circumstances.
In contrast, at calving the cows receiving fall supplementation tended (P=0.12) to be only slightly heavier with no significant differences in body condition score when compared to the control cows.This suggests that the cows that were not supplemented during the fall exhibited some ability to compensate for the earlier nutritional restriction.No significant differences in calf birth weights were observed among the treatments for the 2002 calf crop.However, calves produced by cows that had received supplementation during the previous fall gained faster (P=0.03)than calves from control cows during the period from birth until the start of the summer grazing season (May 20).Likewise, calves from cows that had been supplemented both before and after weaning gained faster (P=0.02)than those calves whose dams only received supplement during the period after weaning.No significant differences were observed among treatments in either the proportion of cows that were cycling prior to the breeding season or in the number of cows that ultimately became pregnant.
In conclusion, feeding beef cattle a limited amount of a high-protein supplement during the fall period can elicit positive changes in body weight and body condition scores, particularly during the period after weaning.Similarly, this practice also may positively affect the performance of calves born to these dams.However, it also appears that cows that do not receive fall supplementation have some potential to compensate during the winter if they are appropriately supplemented during that period.It seems likely that low-level fall supplementation would have greatest applicability in cows that enter the fall grazing season in a compromised state of body condition.

Table 2 . Influence of Low-Level Fall Supplementation on Beef Cow Body Condition Scores
NA = not applicable.Statistical comparison under consideration was not applicable to the designated period.
b c SEM = standard error of the mean.d Average calving date = March 7, 2002.

Table 3 . Influence of Low-Level Fall Supplementation on Beef Cow Body Weights
aN A = not applicable.Statistical comparison under consideration was not applicable to the designated period.b SEM = standard error of the mean.c Average calving date = March 7, 2002.

Table 4 . Influence of Low-Level Fall Supplementation on Calf Body Weight and Cow Reproductive Performance
Statistical comparison under consideration was not applicable to the designated period.b SEM = standard error of the mean.