The influence of age and environmental conditions on supplement intake by beef cattle winter grazing northern mixed-grass rangelands

Abstract

This study evaluated the influence of cow age and temperature adjusted for wind chill (T_windchill) on supplement intake behavior of beef cattle winter grazing northern mixed grass prairie rangelands. A commercial herd of 272 (year 1) and 302 (year 2) bred cows (Angus, Simmental × Angus) ranging in age from 1- to 12-yr-old grazed a 329-ha rangeland pasture (~1.5 ha animal unit monthˉ ¹) from November to January. Cows were grouped into seven age classes (1 yr old, 2 yr old, 3 yr old, 4 yr old, 5 yr old, 6 yr old, and ≥ 7 yr old) and were provided free-choice access to a 30% CP self-fed canola meal-based pelleted supplement with 25% salt to limit intake. The target daily intake was 0.91 kg cowˉ ¹ dˉ ¹. Supplement was provided in a SmartFeed Pro self-feeder system to measure individual animal supplement intake and behavior. An Onset HOBO U30-NRC Weather Station was placed near the supplement feeders to collect weather data for the entirety of the grazing period. Average daily supplement intake and the coefficient variation in supplement intake displayed a T_windchill × cow age × year interaction (P ≤ 0.02). There was a negative linear effect of age on supplement intake (kg cowˉ ¹ dˉ ¹) for days with below average T_windchill conditions in both years (P < 0.01). There was also negative linear effect of age on supplement intake (g kg of BWˉ ¹ dˉ ¹) at average T_windchill in year 1 and below average T_windchill in year 2 (P < 0.01). Cow age had a quadratic effect on supplement intake for days with below average T_windchill in year 1 (P = 0.02); however, this was a curvilinear response where yearlings and 2-yr-olds consumed more supplement per kilogram of BW than other age cattle (P < 0.01). Cow age had positive linear effects on variation in supplement intake at below average T_windchill conditions in both years (P < 0.01). Daily visits to the supplement feeders displayed a T_windchill × cow age interaction (P < 0.01), where there was a linear decrease in visits with increasing age at below average T_windchill conditions (P < 0.01). In summary, both cow age and the winter environmental conditions interacted to influence animal supplement intake behavior and, as a result, nutrient delivery efficacy in winter grazing beef cattle.

Introduction

Winter grazing at northern latitudes often exposes beef cattle to periods of severe cold, which increases energy expenditure to maintain homeothermy (Webster, 1970, 1971; Young and Christopherson, 1974). Animals respond to the increased metabolic demand by increasing intake to meet thermoregulatory needs (Baile and Forbes, 1974; Ames and Ray, 1983; Arnold, 1985). However, forage intake on winter rangelands at northern latitudes is often limited by forage quality. In order to meet the nutritional needs and maintain a desired level of productivity of beef cattle during winter months, supplemental protein is often provided to increase intake of dormant forages and, as a result, performance (Lusby et al., 1967; Bowman et al., 1995; Bodine et al., 2001). Supplementation strategies assume that all animals consume a target quantity of supplement and deviation from the target intake can have deleterious effects on animal performance, reflected as decreased profit for the producer (Bowman and Sowell, 1997). Cow age has been shown to be an influential factor effecting individual supplement intake and foraging behavior (Adams et al., 1986; Kincheloe et al., 2004; Walburger et al., 2009). However, data are limited relative to sources of variation with beef cattle supplemented in herd groups in extensive winter environments. The lack of information is related to the difficulty in measuring intake of free-choice supplements in extensive production environments (DelCurto and Olson, 2010). Therefore, research is needed to refine supplementation strategies that optimize nutrient delivery to diverse groups of animals in extensive environments.

Information evaluating the interactions of environmental factors with individual animal attributes for grazing beef cattle is lacking (Walburger et al., 2009). Potential changes in energetic requirements to maintain homeothermy could alter supplement intake during winter months. Short-term behavioral responses may be critical to the energy balance of domestic animals under extreme weather conditions (Senft and Rittenhouse, 1985). Therefore, the goal of this research is to examine the effects of cow age and winter weather conditions on supplement intake behavior of beef cattle grazing winter rangelands. We hypothesize that supplement intake behavior is altered by the interaction of cow age and winter environmental conditions.

Materials and Methods

The use of animals in this study was approved by the Institutional Animal Care and Use Committee of Montana State University (#2015-AA04).

A commercial herd of bred cows (Angus, Simmental × Angus) ranging in age from 1 to 12 yrs. of age were assigned to one of seven age classifications (1 yr old, 2 yr olds, 3 yr olds, 4 yr olds, 5 yr olds, 6 yr olds, and ≥ 7 yr olds) and winter grazed on a 329-ha rangeland pasture (~1.5 animal unit month haˉ ¹) for 2 yr (262 cows in the first year, and 297 cows in the second year with an average weight of 612.89 kg). The winter grazing season occurred from December 1, 2016 to January 12, 2017, and November 1, 2017 to January 3, 2018. Number of cows, cow weight, and body condition are listed by age class and year in Table 1. All cattle had free-choice access to a 30% CP self-fed canola meal-based pelleted supplement with 25% salt to limit intake (Table 2). The daily intake target was 0.91 kg cowˉ ¹ dˉ ¹. Each individual animal was equipped with an electronic ID tag (Allflex USA, Inc., Dallas-Ft. Worth, TX) attached to the left ear for the measurement of daily individual supplement intake and daily visits to the supplement feeder using two SmartFeed Pro trailers (C-Lock Inc., Rapid City, SD), with a total of 8 feeding stations (4 per trailer). Daily supplement intake was recorded in kg cowˉ ¹ dˉ ¹ and g kg of BWˉ ¹ dˉ ¹ to account for the correlation between BW and age.

Table 1.

Number of cows (n), cow weight (unshrunk), and body condition by age class for the 2 yr of grazing (December to January 2016 to 2017, November to January 2017 to 2018) at the Northern Agricultural Research Center Thackeray Ranch, Havre, MT

	Age
	1	2	3	4	5	6	≥7
2016 to 2017
N	60	43	27	26	31	40	35
Initial
BW, kg	559.4	584.4	656.7	682.9	688.2	706.8	701.2
Body condition	5.35	5.13	5.31	5.65	5.62	5.66	5.94
Final
BW, kg	510.7	546.2	608.2	635.1	637.6	653.0	659.8
Body condition	5.55	5.30	5.46	5.80	5.60	5.81	5.76
2017 to 2018
N	67	48	39	25	22	26	70
Initial
BW, kg	497.1	545.5	579.6	623.2	656.3	644.4	655.2
Body condition	5.88	5.27	5.37	5.42	5.65	5.44	5.44
Final
BW, kg	493.2	543.1	566.3	598.6	626.6	611.8	619.7
Body condition	5.57	5.30	5.16	5.19	5.42	5.26	5.25

Table 2.

Supplement composition for beef cattle winter grazing rangeland in December to January 2016 to 2017 and November to January 2017 to 2018 at the Thackeray Ranch, Havre MT (as-fed basis)

CP1	30.00%
Crude fat	1.00%
Crude fiber	8.00%
Ca	2.00%
P	1.00%
Salt	25.00%
K	0.75%
Se	1.50 ppm
Vitamin A	9,072.00 IU kgˉ 1
Vitamin D	907.00 IU kgˉ 1
Vitamin E	9.00 IU kgˉ 1

¹9.9% nonprotein N.

Supplement feeders were placed ~150 m apart and were centrally located in the pasture. All eight feed units were pinned open to allow cattle-free choice access to supplement for the entirety of the winter grazing period. All cattle age classifications consumed supplement near target intake with a normal distribution of feeding events beginning at dawn and ending at dusk with peak supplement intake occurring near mid-day. Results of a preliminary analysis to evaluate the length of time between supplement intake readings that constitute a new visit to the supplement feeder suggested that readings more than 30-min apart delineate a new visit. This 30-min duration of time was then validated by visual observations of cattle visits to the supplement feeder. Supplement intake variation, measured as coefficient of variation, was based on daily intake estimates for individual animals. Each cow was considered an experimental unit.

An Onset HOBO U30-NRC Weather Station (Bourne, MA) was placed near the supplement feeders and programmed to collect ambient air temperature and wind speed every 15-min for the entirety of the grazing period. Temperatures adjusted for windchill (T_windchill) were calculated using the National Weather Service formula modified for cattle (Osczevski and Bluestein, 2005; Tucker et al., 2007; Graunke et al., 2011). Daily average weather conditions were then paired with daily supplement intake readings for each individual animal for the duration of the grazing period. Each day was then classified as below average (<1 SD from the mean), average (±1 SD from mean), or above average (>1 SD from the mean) temperature, T_windchill, and wind speed within each year of the grazing trial to evaluate relative temperature change within the grazing period on supplement intake behavior (Table 3). Due to weather conditions being correlated, preliminary models were fitted for each variable and evaluated for relative support using Akaike’s information criterion adjusted for small sample sizes (Burnham and Anderson, 2002). T_windchill received virtually 100% of the support among candidate models and was retained to evaluate the effects of cow age and environmental conditions on supplement intake behavior.

Table 3.

Average temperature (°C), wind speed (m sˉ ¹) and temperature adjusted for windchill (T_windchill; °C) below average, average, and above average weather conditions, overall year means (±SE) and total precipitation (cm) for the 2 yr of grazing (December to January 2016 to 2017, November to January 2017 to 2018) at the Northern Agricultural Research Center Thackeray Ranch, Havre, MT

	Weather classification			Overall conditions
	Below average	Average	Above average	Mean	Precipitation, cm
Year 1					2.90
Temperature, °C	ˉ18.97 ± 0.65	ˉ9.35 ± 1.12	0.32 ± 0.59	ˉ9.34 ± 1.33
Wind speed, m sˉ 1	1.69 ± 0.33	6.25 ± 0.50	12.08 ± 0.63	6.48 ± 0.65
T windchill, °C	ˉ31.66 ± 0.61	ˉ19.08 ± 1.07	ˉ8.63 ± 0.82	ˉ19.47 ± 1.41
Year 2					4.06
Temperature, °C	ˉ17.88 ± 1.36	0.24 ± 0.66	10.57 ± 1.20	ˉ2.44 ± 1.24
Wind speed, m sˉ 1	1.91 ± 0.20	6.34 ± 0.27	11.34 ± 0.68	5.91 ± 0.41
T windchill, °C	ˉ27.09 ± 1.36	ˉ8.37 ± 0.78	3.98 ± 1.40	ˉ10.33 ± 1.29

This study was conducted at the Thackeray Ranch (48° 21′ N 109° 30′ W), part of the Montana Agricultural Experiment Station located 21 km south of Havre, MT. Climate is characterized as semi-arid steppe with an average annual precipitation of 410 mm. Vegetation is dominated by Kentucky bluegrass (Poa pratensis L.), bluebunch wheatgrass (Pseudoregnaria spicata [Pursh] A. Love), and rough fescue (Festuca scabrella Torr.). Precipitation was higher in the winter of 2017 to 2018 than 2016 to 2017 (4.06, 2.90 cm); however, temperatures during the 2016 to 2017 winter were substantially cooler than the winter of 2017 to 2018 (ˉ9.60, ˉ2.90 °C), resulting in higher amounts and prolonged periods of snow in the first year of the study (Table 3). The production and quality of pasture vegetation was estimated by sampling 75 randomly located plots prior to grazing each year. Clipped samples were placed in a forced air oven at 60°C for 48 hr and then weighed. Vegetation samples from each plot were ground to pass a 1-mm screen in a Wiley mill. Samples were then analyzed in duplicate for nitrogen (Leco CN-2000; Leco Corporation, St. Joseph, MI) and fiber (NDF and ADF; Ankom 200 Fiber Analyzer, Ankom Co., Fairport, NY) as indicators of vegetation quality (Table 4).

Table 4.

Average annual grass production (±SE, kg haˉ ¹), CP (±SE, %), NDF(±SE; %), and ADF(±SE; %) of the experimental paddock for the 2 yr of grazing (December to January 2016 to 2017, November to January 2017 to 2018) at the Northern Agricultural Research Center Thackeray Ranch, Havre, MT

	Grass production (kg haˉ 1)	CP, %	NDF, %	ADF, %
Year 1	3128.03 ± 21.78	6.85 ± 0.03	70.46 ± 0.08	43.92 ± 0.05
Year 2	2709.42 ± 23.71	7.07 ± 0.03	70.09 ± 0.08	44.46 ± 0.05

Daily individual supplement intake (kg cowˉ ¹ dˉ ¹ and g kg of BWˉ ¹ dˉ ¹), the CV of supplement intake, and daily visits to the supplement feeder were analyzed using ANOVA (car; Fox, and Weisberg, 2011) with a generalized linear mixed model (lme4; Bates et al., 2015) including cow age, year, T_windchill, T_windchill × cow age, cow age × year, T_windchill × year, and T_windchill × cow age × year as fixed effects, and individual cow as a random intercept to account for the autocorrelation of repeated measurements of supplement intake variables for each individual. Data were plotted and transformed if needed to satisfy assumptions of normality and homogeneity of variance. An α ≤ 0.05 was considered significant. Orthogonal polynomial contrasts were used to determine linear and quadratic effects for each analysis and means were separated using the Tukey method when P < 0.05 (emmeans; Lenth, 2019). All statistical analyses were performed in R (R Core Team, 2017).

Results

Average daily supplement intake expressed as kg cowˉ ¹ dˉ ¹ displayed a T_windchill × cow age × year interaction (P = 0.01; Figure 1) where there was a negative linear effect of age on supplement intake for days with below average T_windchill for both years, as well as, average T_windchill in year 1 (P < 0.01; Figure 1A, B, D). There was no effect of age on supplement intake at above average T_windchill in year 1 (P ≥ 0.32; Figure 1C). Cow age displayed quadratic effects on supplement intake for days with average and above average T_windchill in year 2 (P < 0.01; Figure 1E, F). However, the quadratic effects of age on supplement intake in year 2 were limited to yearlings consuming less supplement than 2-, 3-, 4-, 5-, and ≥ 7-yr-old cattle at average T_windchill (P < 0.01; Figure 1E), and yearlings consuming less supplement than 3-, 4-, 5-, and ≥ 7-yr-old cattle (P ≤ 0.03) with 2-yr-old cattle consuming less supplement than 3- and ≥ 7-yr-olds at above average T_windchill (P ≤ 0.04; Figure 1F).

Figure 1.

Influence of cow age, year, and temperature classification (adjusted for windchill) on average daily supplement intake (expressed as kg cowˉ ¹ dˉ ¹; ±SE) by beef cattle grazing dormant northern mixed grass rangeland in December to January 2016 to 2017 and November to January 2017 to 2018 at the Northern Agricultural Research Center Thackeray Ranch, Havre, MT.

Average daily supplement intake expressed as g kg of BWˉ ¹ dˉ ¹ also displayed a T_windchill × cow age × year interaction (P = 0.02; Figure 2) with negative linear effects of age on supplement intake at average T_windchill in year 1 and below average T_windchill in year 2 (P < 0.01; Figure 2B, D). Cow age had a quadratic effect on supplement intake on days with below average T_windchill in year 1 (P = 0.02; Figure 2A), however, this was a curvilinear response where yearlings and 2-yr-olds consumed more supplement per kg of BW than 4-, 5-, 6-, and ≥ 7-yr-old cattle (P < 0.01; Figure 2A). Cow age also displayed quadratic effects on supplement intake for days with average and above average T_windchill in year 2 (P ≤ 0.01; Figure 2E, F), though, the only effects observed were that yearlings consumed less supplement per kilogram of BW than 2-, 3-, and ≥ 7-yr-old cattle (P ≤ 0.01) with 6-yr-old cattle consuming less supplement than 3-yr-olds at average T_windchill (P = 0.02; Figure 2E) and yearlings consuming less supplement than 3- and ≥ 7-yr-old cattle at above average T_windchill (P < 0.01; Figure 2F). There was no effect of age on supplement intake for above average T_windchill in year 1 (P ≥ 0.29; Figure 2C).

Figure 2.

Influence of cow age, year, and temperature classification (adjusted for windchill) on average daily supplement intake (expressed as g kg of body weightˉ ¹ dˉ ¹; ±SE) by beef cattle grazing dormant northern mixed grass rangeland in December to January 2016 to 2017 and November to January 2017 to 2018 at the Northern Agricultural Research Center Thackeray Ranch, Havre, MT.

Variation in supplement intake (% CV) displayed a T_windchill × cow age × year interaction (P < 0.01; Figure 3), where there was positive linear effects of cow age on variation in supplement intake at below average T_windchill conditions in both years and during average T_windchill conditions in year 1 (P < 0.01; Figure 3A, B, D). There was a quadratic effect of cow age on variation in supplement intake at average T_windchill conditions in year 2 (P = 0.02; Figure 3E), even though the only effect observed was higher variation in supplement intake for yearlings than 2- and 3-yr-old cattle (P ≤ 0.05; Figure 3E). There was no effect of cow age on variation in supplement intake at above average temperatures in either year (P ≥ 0.08; Figure 3C, F).

Figure 3.

Influence of cow age, year, and temperature classification (adjusted for windchill) on the coefficient of variation of supplement intake (%; ±SE) by beef cattle grazing dormant northern mixed grass rangeland in December to January 2016 to 2017 and November to January 2017 to 2018 at the Northern Agricultural Research Center Thackeray Ranch, Havre, MT.

Daily visits to the supplement feeders exhibited a T_windchill × cow age interaction (P < 0.01; Figure 4), where there was a linear decrease in visits with increasing age at below average T_windchill conditions (P < 0.01; Figure 4A), and quadratic effects of age at average and above average T_windchill conditions (P < 0.01; Figure 4B,C). However, the quadratic effects of age on visits to the supplement feeder at average T_windchill conditions were limited to yearlings and 6-yr-olds having fewer visits than 2-, 3-, and 4-yr-old cattle (P ≤ 0.05; Figure 4B) and 6-yr-olds having fewer visits than 5-yr-old cattle (P = 0.05). At above average T_windchill conditions 5-yr-old cattle visited the supplement feeder more often than yearlings (P < 0.01) and 2-yr-olds (P = 0.03; Figure 4C). Daily visits to the supplement feeder also exhibited a year × T_windchill interaction (P < 0.01) with quadratic effects of T_windchill in both years (P < 0.01). Visits to the supplement feeders were higher at below average than average and above average T_windchill conditions in year 1 (P < 0.01; below average 1.10 ± 0.03, average 0.92 ± 0.02, above average 0.92 ± 0.03) but were higher at average and above average compared to below average T_windchill conditions in year 2 (P < 0.01; below average 0.97 ± 0.02, average 1.47 ± 0.02, above average 1.48 ± 0.03).

Figure 4.

Influence of cow age and temperature classification (adjusted for windchill) on average daily visits to the supplement feeder (±SE) by beef cattle grazing dormant northern mixed grass rangeland in December to January 2016 to 2017 and November to January 2017 to 2018 at the Northern Agricultural Research Center Thackeray Ranch, Havre, MT.

Discussion

The results of our study suggest that the interaction of cow age and winter weather conditions can have a significant impact on daily supplement intake, variation in supplement intake, and daily visits to the supplement feeder. The few studies that have quantified supplement behavior of mixed age herds of beef cattle have shown that older cows typically visit the supplement feeder more often, consume more supplement and are less variable in their daily supplement intake than younger cows (Bowman et al., 1999; Sowell et al., 2003; Kincheloe et al., 2004). However, these projects were conducted using either liquid supplements (Bowman et al., 1999; Sowell et al., 2003) or hand-fed delivery (Kincheloe et al., 2004), and all three studies measured supplement intake using external markers. External marker intake estimates are based on serial dosing of a marker and a series of fecal collection (5 to 7 d), which limits accuracy and the potential to estimate daily supplement intake behavior in response to environmental conditions.

Our results contradict the conventional idea that older cows consume more supplement as we found the effects of cow age on daily supplement intake behavior to be mediated by weather conditions, where visits to the supplement feeder and daily supplement intake decreased and variability of daily supplement intake increased with cow age at below average T_windchill conditions. Similar results were evident at average T_windchill conditions in year 1, but not in year 2. However, this may be related to average T_windchill conditions in year 1 being much colder than average conditions in year 2, resulting in cattle responding more similarly to that of below average T_windchill conditions. Additionally, as T_windchill increases the effects of cow age on supplement intake appears to decrease.

Chronic cold and wind exposure associated with northern winter grazing environments often expose beef cattle to conditions below their lower critical temperature (LCT) resulting in animals increasing their resting metabolic rate and overall energy expenditure in an effort to maintain homeothermy (Webster, 1971; Christopherson et al., 1979; Keren and Olson, 2006). However, the majority of previous research conducted to determine the LCT of cattle is limited to restricted environments or climatic chambers with single-kept animals (Christopherson, 1985). Thus, the current model used to determine LCT includes ambient air temperature, insulation (fat deposits and coat depth), heat production, and heat of evaporation (National Research Council, 1981). Increasing wind speed is correlated to convective heat loss, which reduces the temperature an animal experiences (effective environmental temperature; Baker, 2004; Osczevski and Bluestein, 2005; Graunke et al., 2011). Although it is recognized that wind can have a profound effect on the effective environmental temperature (i.e., windchill) it has not been incorporated in calculating an animal’s LCT (National Research Council, 1981; Christopherson, 1985; Graunke et al., 2011). The results from this study are consistent with others in showing that temperatures adjusted for windchill can have a substantial impact on beef cattle behavior and may need to be accounted for in evaluating LCT (Tucker et al., 2007; Graunke et al., 2011).

Maintenance energy expenditure can vary with age of cattle (National Research Council, 2016). It is generally accepted that maintenance per unit of size decreases with age in ruminant livestock (Blaxter, 1962; Graham et al., 1974; CSIRO, 1990, 2007). Additionally, metabolic heat production is positively correlated to BW of cattle, thus, younger lighter weight cattle are more susceptible to cold environments than their older heavier counterparts (Thonney et al., 1976; Van Soest, 1994). Due to the poor quality of available forage in our study (~70% NDF, 7% CP), younger cattle may have a higher reliance on supplement to meet their maintenance requirements in a winter environment than older cattle. Cold temperature conditions could result in greater energetic needs for young cows to maintain homeothermy resulting in increased visits to the supplement feeder, increased daily supplement intake per unit BW, and decreased variation in intake as average daily T_windchill conditions drop.

Implications

We found that cow age and winter environmental conditions can have a significant effect on supplement intake per day, variation in supplement intake, and visits to the supplement feeders per day by beef cattle offered a free-choice salt-limited protein supplement while grazing winter rangelands. Our research suggests beef cattle with presumed higher maintenance requirements (younger cattle) increase supplement intake when environmental conditions intensify metabolic demands. In general, younger cows ate more supplement and displayed less intake variation during periods of cold stress than older animals. Our research suggests free-choice, salt-limited canola meal-based supplements can be effective when provided to a diverse group of cattle with significant age differences in extensive rangeland environments. By providing information related to salt-limited self-fed supplements that optimizes nutrient delivery and minimizes economic inputs during times of poor forage quality and environmental stress, we can effectively improve the efficiency of beef cattle production systems at northern latitudes.

Glossary

Abbreviation

LCT

lower critical temperature

Funding

This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2015-38640-23779 through the Western Sustainable Agriculture Research and Education program under sub-award number (GW17-040). Appreciation is also expressed to the Nancy Cameron Endowment, the Bair Ranch Foundation, and the Montana Stockgrowers Association for research funding and to the employees of MSU Northern Agricultural Research Center for their assistance with this project. USDA is an equal opportunity employer and service provider.

Conflict of interest statement

The authors declare no real or perceived conflicts of interest.