Many New Zealand farmers devote very little attention to the feeding of the dry cow-after all they are not producing milk or income and offers a break from the cows

Brassica wintering for dairy cows: Overcoming the challenges

Nichol W¹, Westwood C², Dumbleton A¹, Amyes J¹

¹ Wrightson Research, P.O. Box 939, Christchurch

² Veterinary Nutritionist, 16 Norris Street, Prebbleton, Canterbury 8153

Introduction

For seasonally calved dairy herds, the non-milking winter months are a time for rest and rejuvenation for the cow (and the farmer). For most South Island herds the ‘dry’ period commences from mid May through to early June. Following dry off, cows are wintered on forage crops and / or pasture plus silage, hay and /or straw. Forage brassicas, particularly kale and swedes, are the more common crops used for wintering dry cows. The popularity of forage brassicas reflects the crops ability to produce large tonnages of high quality dry matter (DM) per unit area. Kale and swedes are typically established during early summer and may yield in excess of 12 MT DM / ha. Actual yields will depend on environmental conditions, crop husbandry and management. Winter turnips and forage rape are sown later, either as a crop or sown together with short-term ryegrasses, and produce lower DM yields than kale and swedes. Another important feature of forage brassica is the ability to ‘carry’ the crop forward into the winter without loss of feed quantity and quality. Although other crops such as short-term ryegrasses and greenfeed cereals can be used in a similar manner to brassicas, they don’t typically offer the same DM yields and have only a limited ability to carry through the winter without senescence and lodging.

Forage brassicas offer an excellent dry cow feeding opportunity for South Island dairy producers. To capture the full benefit of a high quality brassica crop (11.5-13.5 MJME / kg DM) it is essential that correct crop and cow management be followed.

The aim of this paper is to identify characteristics of forages brassicas that may limit cow productivity through the winter, and to discuss management practices that will enhance the performance of cows grazing winter brassica crops.

Nutritional requirements of the dry cow

The dry cow feeding period can be split into two periods.

q Period 1 is from dry off to three weeks before calving

q Period 2 is the three weeks before calving and is most often referred to the as the transition, or springer period.

At drying off, South Island dairy cows are often not at the target body condition of 5.0 on the NZ 1-10 condition score scale. The aim in the dry period is to maintain or ideally lift body condition through to calving, targeting an average body condition of 5.5 for two and three year old cows and 5 for older cows by calving.

Energy requirements of dry cows on brassica crops

Period 1 dry cows typically don’t need the same level of intensive feeding and management as cows during the close-up (springer) period. Dexcel’s Feed4Profit ‘Feed Demand Information’ provides guidelines for calculating the energy and dry matter (DM) requirements for your dry cows and should be referred to for more information.

An example of the energy requirements for dry cows is provided below:

Energy requirements on 1^st June for an ‘average’ Friesian dairy cow of 500 kg liveweight, due to calve on the 15^th August, gaining 0.5 of a condition score (or around 20 kg cow liveweight excluding calf or uterine fluids) over the first 6 weeks.

Maintenance 64 MJME

Walking / grazing at 2km per day 10 MJME

Pregnancy (8 weeks from calving) 14 MJME

Condition gain 0.5 of a score in 6 weeks 24 MJME

Total 112 MJME / day

If you’re uncomfortable with interpreting MJME calculations, energy requirements can be roughly converted to DM / head / day by dividing MJME requirements by 11. In this case, the 500 kg cow will need around 10-11 kg DM per day eaten (not offered) to meet her energy requirements.

Cows can’t eat all of the feed on offer. If the utilisation of crop plus supplement is only 70%, a cow needing to eat 11 kg DM / day must be offered 14.7 kg DM / day (11 divided by 0.7).

To meet the energy requirements of the dry cow and to maintain a nutritionally balanced diet, it is recommended that a combination of forage brassica, silage and hay/straw be used (see next section for reasons why). The amount of forage and supplement offered will be determined by age and body condition of the herd, environmental conditions and feeding practice. In most cases 10.0–12.5 kg DM eaten (not offered) per day will suffice, of which 50-60% should be forage brassica, 30-35% silage or baleage and the remainder as hay or straw (see example, Table 1). To increase body condition during the dry cow period, increasing the proportion of brassica in the diet can increase energy intake, provided that the health of the cow’s rumen is maintained. Due to the inherent high-energy characteristics of forage brassica, silage quality only needs to be of ‘average’ quality. Feeding very high quality silage and/or hay may lead to dietary substitution of the brassica crop and will offer fewer advantages for helping to maintain rumen function (see next section).

Protein requirements of dry cows on brassica crops

The protein requirements of dry cows are much lower than for a lactating cow. For most cows, a total dietary protein of 12% crude protein (CP) will provide enough protein for the demands of maintenance and pregnancy (NRC, 2001). For rising two-year old heifers, demands for protein may be slightly higher in late pregnancy, closer to 15% CP (NRC, 2001). Extra protein is needed by heifers to support ongoing growth by the cow and for relatively more udder development in younger cows.

In most situations, a combination of forage brassicas, pasture silage or baleage and hay or straw will meet the CP requirements of dry cows, particularly if there is a high proportion of leaf material present in the forage brassica crop.

It is relatively simple to calculate how much CP cows are receiving from their winter diet.

An example:

q Cows are grazing kale (5 kg DM eaten per day) plus barley silage (3.5 kg DM eaten per day) and barley straw (2.0 kg DM eaten).

q If we know (from feed testing) that the kale contains 15% CP and that the barley silage contains 8.5% CP and the barley straw contains 4% CP, we can calculate protein levels in this diet as a ‘back of the envelope’ calculation.

o Kale; 5 kg x 150 g CP / kg DM (15%) = 750 g CP

o Barley silage; 3.5 kg x 85 g CP / kg DM (8.5%) = 298 g CP

o Straw; 2.0 kg x 40 g CP / kg DM (4.0%) = 80 g CP

q Total CP for the diet = 1.13 kg CP / 10.5 kg DM = 10.8 % CP for the whole diet.

The diet is short on total CP versus the 12% recommendation. This may disadvantage younger cows (heifers and rising 3s) and cows closest to due calving dates. The easiest way to ‘fix’ this potential protein deficiency is to lift the proportion of crop on offer to increase total protein intake, or to seek some high quality silages (e.g. made from pasture, lucerne or forage peas) to feed in place of barley silage. At least aim to feed higher protein silages to younger cows, and cows closer to calving.

Table 1 Daily dietary intake of a dry cow consuming a diet of kale (7kg DM), cereal straw (2 kg DM) and silage (3 kg DM) relative to nutritional requirements.

	Feed Analysis			Dietary Intake			Total	Actual	Nutritional
Nutritional Factors			Grass	Kale	Straw	Silage	Intake	Intake	Requirement
	Kale	Straw	Silage	7 kg DM	2 kg DM	3 kg DM	12 kg DM
Nutritional Factors	(per kg DM)			(per cow per day)				(per kgDM)	(per kgDM)
Dry Matter (% per kg wet weight)	16	86	37.1
Metabolisable energy (MJME)	12	6.5	10	84	13	30	127	10.6	8.5
Crude protein (g)	160	38	139	1,120	76	417	1,613	134	120
Acid detergent fibre (g) (ADF)	190	509	303	1,330	1,018	909	3,257	271	> 210
Neutral detergent fibre (g)* (NDF)	250	811	487	1,750	1,622	1,461	4,833	403	> 330
Major minerals
Calcium (g)	25	3	7	175	6	21	202	17	4.4
Phosphate (g)	3	1	4	21	2	12	35	3	2.2
Sulphur (g)	7	1.7	3.2	49	3	10	62	5	2
Magnesium (g)	2	2	2	14	4	6	24	2	1.1
Potassium (g)	25	23	25	175	46	75	296	25	5.1
Chlorine (g)	4.5	6.7	6.2	32	13	19	64	5	2
Sodium (g)	1	1.4	1.5	7	3	5	15	1	1
DCAD (meq/kgDM) equation one**	120	355	331	840	710	993	2,543	212	> 100
DCAD (meq/kgDM) equation two**	419	415	370	2,933	830	1,110	4,873	406	> 200

Assumptions:
Nutritional requirement = 500kg liveweight cow at day 240 of pregnancy using NRC (2001) recommendations.
Requirements will differ for cows less than 3 weeks from calving
Total Intake 12 kgDM eaten /cow/day = 7 kg of kale + 2 kg of cereal straw + 3 kg grass silage
Key:
* Additional NDF (particularly as high eNDF forages such as long stem hay or straw will benefit cattle on colder Canterbury days)
**DCAD calculations from CPM Dairy; Dairy Cattle Ration Analyzer version 2.0.25a
Reference:
Straw feed values sourced from McDonald et al, 1995 and grass silage values average of Lincoln University Feed Analyses 2001.
Grass silage and straw mineral values sourced from Hill Laboratories Ltd.
NRC 2001 and NRC Feed Formulation Program 2001